On some points in the anatomy of polychæta

Before the year 1880 it had been conclusively shown by the results of various researches that a “segmental organ” was a glandular tube, either simple or convoluted, ciliated through the greater part of its length, having internally a ciliated funnel-shaped opening by which it communicated with the body cavity, externally an opening to the exterior. The function of the segmental organ had been shown to be excretory, depurative, but it had also been proved that in some cases at least the organ served as efferent duct for the reproductive elements. The segmental organs in their typical form had been found principally in the Chætopoda, but it had been shown that the tubular glands known as the “Organs of Bojanus” in Mollusca conformed in all essential features to the plan of a segmental organ, though they did not as a rule act as genital ducts.¹Notwithstanding that these facts had been firmly established as important landmarks in zoology, in the year I have mentioned appeared a memoir bearing the name of L. C. Cosmovici,1 in which the whole question of segmental organs was thrown into confusion. Overlooking or rather wilfully disregarding the more recent investigations which had brought to light the true relations of the segmental organs, and which had resulted in a generally applicable definition of the organs, this memoir takes the original work of Williams as a foundation, and carefully compares every case examined with Williams’ original description. Because Williams did not perceive the glandular nature of the organs, but only their connection with the reproductive functions, Cosmovici defined a segmental organ as a generative duct, while a glandular tube or pouch must be considered and called an organ of Bojanus. As the real segmental organs, or as they are now called nephridia, are usually both glands and genital ducts, they were stated in this extraordinary paper to be usually compound, consisting of a glandular tube, the organ of Bojanus, and a ciliated non-glandular funnel, the segmental organ; while certain cases were described in which the primitive distinctness of the two structures was, as the author believed, retained. Perhaps the most surprising point in this phenomenal paper is that the absence of an internal or coelomic opening is taken as characteristic of the “organ of Bojanus” in the Polychæta, and this in 1880 when to every commencing student of zoology was demonstrated the internal or coelomic opening in the molluscan organ to which the name organ of Bojanus was originally given. It would be best if it were possible to exclude the paper of Cosmovici entirely from the literature of the subject with which it professes to deal, and regard it as of merely psychological interest; but the paper contains detailed descriptions and drawings of the anatomy of several forms of Polychæta, and these have been by some later writers accepted as trustworthy. In a critical paper by R. S. Bergh,¹ published in 1885 which reviews the relations of the nephridia in the various classes of Vermes, the facts concerning the Polychæta are mostly taken from Cosmovici’s memoir. But a great many of Cosmovici’s statements are quite erroneous, and although he has described other facts correctly the false is so mingled with the true, and the actual descriptions of structure are so mingled with false theoretical views, that it is not safe to accept anything in his paper without re-examination of every case.

In 1868, when Claparède’s ‘Chétopodes du Golfe de Naples’ was published the gonads of Arenicola were unknown. In that work it is stated that most authors had taken the nephridià to be reproductive organs, some describing them as ovaries, others as testes. Quatrefages (‘Hist. Nat. des Annelés,’ 1865) called the nephridia simply genital organs. Grube (‘Zur. Anat. u. Phys, der Kiemenwurmer’) had assured himself that the ovaries were not to be sought in these organs, for he had seen (as he thought) the ova formed on the exterior of vascular cæca of the lining of the coelom. He was inclined to regard the nephridia as testicles. This was an impossible view, because, as Claparède says, the sexes of Arenicola are separate, but it seemed very probable to Claparède that Grube was right as to the origin of ova, and in support of the opinion of Grube he gives a figure of one of the cæcal pseudhæmal vessels surrounded by a layer of cells. Claparède then quits the question of the genital organs, and proceeds to give a not very accurate description of the nephridia in Arenicola Grubii, Clap. He says the organs previously described as generative organs are really segmental organs of a very peculiar structure, and are only connected with the phenomena of reproduction as efferent ducts. There are five pairs placed in the fourth to the eighth setigerous segments. Three parts are distinguishable in each organ, the funnel, the gland, and the vascular reservoir. He describes correctly the fringed and ciliated dorsal edge of the funnel; but his account of the shape of the glandular part, which he compares to a comma, does not agree perfectly with what is seen in A. marina. His figures are not good, though creditable for the date at which they were made. Claparède never saw ova in the interior of the nephridia, but once saw spermatozoa in the funnel. Of Arenicola marina, Lin., he says merely that the segmental organs have a great analogy with those of A. Grubii.

Casmovici’s description of the nephridia and gonads in Areniola marina is correct in almost every particular; his figures of minute structure are not satisfactory, but his anatomical figures are clear and accurate; and if it were not for the absurd manner in which he has separated the nephrosto-mata as distinct organs from the nephridia themselves, his description would be worthy of a permanent place in zoological literature. To him, in any case, is due the credit of having been the first to discover and describe the true gonads in Arenicola. He states there are six pairs of nephridia, each with its external opening, and situated in the third to eighth somites. The external opening of each organ is situated close behind the upper end of the corresponding uncinigerous torus. The funnel or nephrostome is correctly described by Cosmovici; it is provided with a free membranous dorsal border, fringed and ciliated. Along this border runs a pseudhæmal vessel, a branch from the branchial artery, which is given off by the ventral vessel. (Cosmovici believes the ventral vessel to be connected with the heart, and to be arterial; it is more probably venous; that is, it probably receives the blood from the branchiæ.) The vessel which runs along the dorsal border of the nephrostome is continued diagonally backwards and outwards across the nephridium, and in this part of its course, posterior to the nephrostome, it runs through the gonad; the tissue of the gonad is, in fact, continuous with the posterior angle of the nephrostome.

There is not, then, a great deal to be added to Cosmovici’s account, but there are one or two corrections to be made, and a re-examination of the subject was necessary because the falseness of his interpretation causes his description to be received with doubt. His account of the position of the nephridia is inaccurate; they are situated in somites v—x (the fourth to ninth chætigerous) inclusive. The first or buccal somite of Arenicola is destitute of bristles: the following six somites bear each a dorsal fascicle of capillary chætæ, and a ventral torus uncinigerus, but no branchiä; the next thirteen somites bear each both fascicle and torus, and, in addition, a pair of plumose branchiæ; the rest of the body, which is of different lengths in different individuals, is thinner, and devoid of fascicle, torus, and branchia j it is cylindrical and covered uniformly with papillae. Behind the first four somites are incomplete septa. The nephrostome of the first nephridium is on the anterior face of the fourth septum. Between any two successive parapodia, behind the third, are five constrictions, of which the fifth is the deepest. The septum, when present, is attached to the body wall opposite the second constriction. Between the fifth constriction and the parapodium is a prominent ridge ending in a sharp edge. The first nephridia of the first pair are somewhat smaller than the rest. The relation of the gonad to the nephridium is shown in Pl. XVII, fig. 1, which gives a view of the internal face of the nephridium. In the natural position the nephridia are covered dorsally by the bands of oblique muscles, which pass from the sides of the nerve-cord to the line of dorsal bristles; and in this condition the dorsal lip of the nephrostome is horizontal, and its edge is directed downwards and inwards.

Cosmovici says he searched in vain for a long time for the genital organs, and discovered the ovary by accident when examining a piece of the nephridium. I discovered the ovary by tracing the origin of the ova in the body cavity. In February last I found two or three specimens which had ova in the body cavity; in these and in others in which mature ova were not present, loose cellular masses were often seen in the neighbourhood of the nephridia. After careful search, several times repeated, these masses were traced to the cord of cellular tissue already described as attached to the nephridium; the cord of cells is merely, as usual, a local development of coelomic epithelium. In most specimens the cells of this cord were so undifferentiated that it was not easy to be certain it was a gonad, but in specimens which contained a few ova in the body cavity young ova could be recognised in the cord. The reproductive cells leave the gonad at a very early stage of development, and reach maturity while floating freely in the body cavity.

Keferstein¹ in 1862 gave a slight description of the segmental organs in Cirratulus filiformis, Kef., bioculatus, Kef., and borealis, Lam. He made out the relations of the organs most completely in the first-mentioned species, in which he describes them as a single pair of ciliated tubes, each bent on itself, extending through segments 1 to 5, and having an internal and an external opening. He gives a figure of the organ as seen in the living animal, and the figure shows both the external and internal openings in the first setigerous or post-buccal somite. Less complete descriptions and figures are given of the organs in the other two species. No other nephridia are mentioned by Keferstein except this pair at the anterior end.

Claparède also saw but a single anterior pair of nephridia in species of Cirratulus. He says (‘Ann. Chet. Naples’): “C. chrysoderma, like all the Cirratuliens, has only a single pair of segmental organs, opening at the second segment (first setigerous) by an oval aperture situated on the inner side of the ventral bristles. The organ is rolled in an angular spiral. Its external part is narrow, but soon enlarges suddenly into a wide ciliated tube. The internal opening has escaped me.” Cosmovici (loc. cit.) was unable to see the large anterior nephridia described by Keferstein and Claparède, but states that in C. filiformis, Kef., segmental organs are present in pairs in nearly all the segments, especially of the middle and posterior region: that they are attached to the anterior face of each diaphragm: the figures of this species which he gives do not show the organs mentioned. The Errantia, among which he places Cirratulus, according to Cosmovici’s peculiar views, have only segmental organs and no organs of Bojanus, and he denies altogether that the internal opening of the segmental organ communicates with the cavity of the somite in front of the one which contains the organ itself.

In Cirratulus cirratus both the large anterior pair of nephridia described by Keferstein and Claparède, and the series of pairs in the middle and posterior region mentioned by Cosmovici, are present. The nephrostome of the first nephridium opens into the cavity of the buccal somite, being situated on the anterior face of the first septum, somewhat ventrally, in the angle between the septum and the lateral body wall. The proximal part of the bent tube passes backwards from the nephrostome till it reaches the second septum (Pl. XVII, fig. 3), then passes upwards to the dorsal body wall, where it opens into the wider distal part of the tube which opens to the exterior beneath the neuropodium of the second somite. The posterior nephridia are smaller and simpler; they appear first in the twelfth somite, and are repeated hence to the end of the body. Each of them has a nephrostome of the typical form, an elongated funnel with its aperture directed forwards. The nephrostome has a similar position to that of the large anterior nephridium, that is to say, it is placed in the lower external corner of the anterior face of its septum. The lips of the funnel are composed of a columnar ciliated epithelium resting on a thin fibrous membrane; this membrane is continued on the one hand into the transverse septum, on the other into the body wall; the lips of the funnel project inwards and forwards into the cavity of the somite. The nephridial tube when traced from the nephrostome (in a series of horizontal sections) is seen to pass obliquely backwards and downwards, curving over the dorsal edge of the ventral longitudinal muscle, and opening beneath the neuropodial bristles. The internal openings of the simple nephridia are shown as seen in a horizontal section in fig. 4, while fig. 5 shows the external opening in a transverse section of a female specimen.

The simple nephridia act as efferent ducts for the reproductive elements in both sexes. I found a number of specimens distended with the genital products at the end of March in the current year, lying under stones on the banks of Granton Quarry, where they are pretty abundant at all seasons. These, when placed in a basin of sea-water, commenced to shed eggs and spermatozoa. The ova were fastened together, after their escape, by transparent gelatinous mucus, which formed a soft mass without any definite shape adhering to the stones and mud among which the worms were lying. The escape of the ova could be observed without much difficulty.

Fig. 6 shows the appearance presented by two somites of the worm viewed under a low power by reflected light, as the ova were escaping; the apertures seen are the openings of the simple nephridia previously described. In sections of a male specimen these nephridia are seen full of spermatozoa. The larger nephridia of the second somite do not, as far as I have been able to ascertain, transmit the sexual products; indeed, no ova (or spermatozoa) are produced in the first eleven somites where simple nephridia are absent.

I have not discovered in my sections any satisfactory indication of the places where the germinal cells are developed; the position of the gonads is doubtful. The reproductive cells undergo the greater part of their development in the body cavity. The presence of a complete longitudinal vertical septum above and below the intestine in Cirratulus is remarkable.

This form has not hitherto been recorded as occurring in the North Sea, either on our own coast or other parts of north-west Europe. It is, however, common enough between tide marks in the sand at Granton. Claparède1 does not give any description of the segmental organs, he merely mentions that the ovaries are attached to them, and render their study difficult. There is, of course, no great difficulty in making out the relations of the nephridia in longitudinal and transverse sections. These relations are in some small points exceptional, and longitudinal sections are the most instructive. The nephrostome is a wide ciliated funnel, the lips of which are simple and entire, not produced into lobes. The aperture of the nephrostome is large and gaping as in Cirratulus. The lower lip of the nephrostome projects into the cavity of the somite, while the upper is continuous with the transverse septum, on whose front face the nephrostome lies. The nephrostome is situated at the lower and outer corner of the septum, and leads into a narrow tube, which pierces the septum and then dilates into a small spherical vesicle, which on the median side is produced into a point, and to this point the gonad is attached (fig. 7). A little more to the exterior side the vesicle gives off a long duct, which passes upwards to the body wall and opens to the exterior (fig. 8). As a general rule the efferent duct of the nephridium in Polychæta passes downwards ventrally, and opens below the level of the neuropodium. In Terebellidæ the external aperture is near the upper end of the uncinigerous torus, but in this case the ventral band of muscle extends some distance upwards, and thus, although the nephridial aperture has an exceptional position with regard to the neuropodium, it has its normal relation to the ventral muscles. In Nerine there is no such reason for the extremely dorsad position of the nephridial aperture; the ventral band of longitudinal muscles on each side is folded in at its borders, and the two bands occupy only the ventral surface of the transverse section; the dorsal bands in like manner occupy only the dorsal wall; the whole of the lateral region of the transverse section, which approaches a parallelogram in shape, is destitute of thick muscular bands, and the fascicles of bristles are widely separated. The efferent duct of the nephridium is therefore not confined in a narrow space between the edges of the dorsal and ventral muscle-bands, as is usually the case, and there is no apparent reason why the duct should not pass between the ventral muscle and the neuropodial bristles as it does in most Polychæta.

It is certain that in this species the nephridia act as sexual ducts in both sexes. I have frequently seen these organs in the male distended with spermatozoa, and the ova doubtless pass out in the same way.

In this species the nephridia have the same character and somewhat similar relations to those described in the preceding, but the efferent duct is by no means so long, and the external aperture is therefore more ventral in position; it is on the same level as the upper neuropodial bristles, and lies in front of the neuropodium in the constriction between adjacent somites (fig. 9). Sections of ripe males show the lumen of the nephridium full of spermatozoa.

Several accounts of the nephridia of Terebella conchilega have been given. H. Milne-Edwards (‘Ann. d. Sei. Nat. (2) Zoologie, x,’ 1838, p. 220), in a paper published in 1838, on the circulation in Annelids, describes the vascular system in a species to which he gives this name, and gives a figure of the animal opened along the dorsal median line. In this figure four looped nephridia are distinctly shown, situated behind the branchial region. The representation of the position and character of these organs is perfectly correct, so far as it goes; they are the upper parts of the four nephridia belonging to somites vi—ix. But the paper I refer to does not describe the nephridia, as it deals with another subject: they are shown in the figure, and that is all; and in the description of the figure the organs are referred to as organs of generation.

Keferstein (‘Zeitschrift für wiss. Zoologie/Bd. xii, 1862) mentions that the structure and number of the nephridia in T. conchilega are the same as in T. gelatinosa, Kef.; in both cases he says there are six pairs, each organ consisting of a tube bent on itself, of which one half is darker, the other lighter; the organs belong to segments 3—9.

Cosmovici¹gives an erroneous description of the organs; he says there are two pairs without internal openings, which he calls “organs of Bojanus,” one of these situated in front of the cephalic diaphragm, the other immediately behind it, each organ having an external opening; and two other pairs, each of which has an internal as well as an external opening, and is shaped like an urn; the internal opening is large, and surrounded with a circular lip. The gonad is attached to the posterior part of each of these latter organs, which Cosmovici calls segmental organs, and which he says serve as efferent genital ducts.

The species referred to by these three authors is the Nereis conchilega of Pallas, Terebella conchilega of Gmelin; and this is called Lanice conchilega by Malmgren. My specimens were identified from Malmgren’s description, and there is no doubt of their identity with the species of that author; but there is room for some uncertainty regarding the specific identity of the specimens referred to by the authors I have mentioned. For instance, Cosmovici identified his species by means of Quatrefages’ ‘Histoire des Annéles,’ 1865, and there it is stated that the tube of Terebella conchilega possesses no hollow fringes at its mouth; these fringes are always present in the tube of Lanice conchilega, Malmgren. This species is distinguished by some marked characters; two of them are the presence of a large vertical lobe on the third somite (second branchiferous), and the coalescence of the ventral scutes usually present into a continuous ventral plate.

The fact that in Lanice conchilega the nephridia of a side communicate together so as to form a longitudinal tube, has been observed by Edouard Meyer of the Zoological Station at Naples; and his discovery is mentioned by his permission in a single sentence in Lang’s 1 Monograph on the Polycladen,’ published in 1884.1 But the accessible information concerning these organs in this species is so inadequate, that R. S. Bergh,2 in a general review of the excretory system in worms, cites both Lang’s mention of Meyer’s observation and Cosmovici’s account of the organs as if they were both equally correct.

The true relations of the excretory system are as follows:— Enumerating the somites from before backwards, and counting the buccal as the first, we find that the branchiæ belong to somites n, in, and iv: the first notopodial fascicle of capillary chaetæ is on the fourth somite, the third branchiferous; the first neuropodial uncinigerous torus is on the fifth; the neuropodial tori are repeated on every succeeding somite to the end of the body; the notopodial fascicles occur only on seventeen consecutive somites. There are traces of transverse septa behind the first, second, third, and fourth somites, but none in the rest of the thoracic region, which bears the notopodial fascicles. On dissection, four long double nephridial tubes are seen projecting dorsalwards into the body cavity; the lower parts of these tubes are covered by strands of the oblique muscles which pass from the nerve-cord to the neighbourhood of the notopodial bristles; careful examination shows that these tubes belong to somites vI, vII, vIII, and Ix. Their internal openings can be found immediately behind the fascicle of bristles belonging to somites v, vI, vII, and vIII respectively, but their efferent tubes are seen to pass down beneath the fascicle of somites vI, vII, vIII, and Ix. The lower parts of these efferent tubes are very wide, and it is impossible to separate them from one another. Beneath the fascicles of the following four somites (x to xIII inclusive) are seen membranous nephridial sacs, which externally at least are inseparable from one another. These sacs are simple, that is, they are not composed of a tube bent on itself like the anterior nephridia; they scarcely extend above the level of the oblique muscles, and no internal opening or nephrostome can be found in them. In front of the most anterior nephridium, that belonging to somite vII, are seen traces of a rudimentary nephridium (see fig. 10). In order to trace out the relations of these nephridia more accurately, the anterior part of a specimen was cut into a series of horizontal longitudinal sections commencing with the ventral surface, and the reason why the successive nephridia could not be isolated from one another was seen on examination of these sections; the lower parts of the efferent limbs of the four anterior normal nephridia, in somites vi to ix, and the whole of the nephridial sacs in somites x to XIII are in open communication, forming a wide continuous longitudinal tube extending from somite vi to somite xIII (see figs. 11 and 12). Openings to the exterior from this tube were found in somites vi to ix inclusive, corresponding to the four large looped nephridia; each of these openings was close behind the upper end of an uncinigerous torus. The internal openings of the same four nephridia could be traced with ease and certainty; they are attached to the body wall close behind the notopodial fascicles of somites v to vIII. These openings are wide, and are overhung dorsally by a longitudinal lip furnished with a series of small ciliated digitate processes; lower down, the anterior and posterior lips of the opening are simple, thick-walled, and ciliated. The aperture leads into a thin tube, which passes inwards and backwards, curving round the inner end of the fascicle of bristles behind the aperture, and then, crossing the continuous tube, passes up on the inner or mediad side of the loop, at the apex of which it is continued into the efferent wider limb of the loop, which passes down on the outer side to open into the longitudinal tube. Neither internal nor external openings could be found in that part of the longitudinal tube which is behind the loops; it seems evident that this part of the tube represents four somewhat reduced nephridia which have coalesced, but whose openings have disappeared. Anteriorly to the four looped nephridia are traces of three others; the longitudinal tube extends forwards into somite v as if it included a nephridium belonging to that somite, but I could find no external opening in this somite; at the angle between the septum behind somite iv and the body wall is a very obvious nephrostome, which ought to lead into the longitudinal tube, into that part of it corresponding to somite v, but the connection could not be traced. Nephrostomes were also present attached to the anterior face of the septa behind somites II and III (the first and second branchiferous), and leading into tubes seen in somites in and iv, but I could find no external openings in these somites. I could find no nephrostome in somite I(the buccal), nor any trace of a tube in somite II. Gonads are present in the form of clumps of deeply-staining, small, indifferent cells, attached to the exterior of all the nephrostoma mentioned, seven in all (see fig. 13). The germinal cells, when still quite undifferentiated, separate from the gonads, and undergo further development in the cœlom. But I found no reproductive elements in the cavity of the nephridial system, though the body cavity contained them in quantity, and it is probable that at the right season they are expelled through the nephridial cavities. The body cavity contains, besides the reproductive elements, a large number of spherical, vacuolated, nucleated cells. This is the only case in which a communication between successive nephridia has ever been discovered in any adult invertebrate. It is true that in the development of Polygordius, according to Hatschek, each nephridium gives off backwards a prolongation of itself, from which the next nephridium is formed, and the two remain in communication for a time; but the connection is soon severed, and in the adult the successive nephridia are isolated and independent. In Lanice conchilega the nephridia have coalesced together after coming in contact from before backwards, the separating membranes having disappeared. The case is extremely interesting in the fact that we have in it an approximation to the condition of the excretory system in Vertebrata; the presence of a metameric series of nephrostomata in vertebrate embryos has long ago been seen to constitute a resemblance between them and Chætopoda, but no other Chætopod is known which resembles the vertebrate in having a number of nephridia coalesced to form a continuous longitudinal tube.

It is surprising to find that, as far as I have been able to discover, no resemblance to the condition seen in Lanice conchilega occurs in any of its near allies. The only species of the genus Terebella as defined by Malmgren that occurs in the Firth of Forth is Terebella Danielsseni, but of this I have only one specimen, and have not examined its nephridia. Of Amphitrite there are two species in the Forth; Amphitrite cirrata I have not examined anatomically; in Amphitrite Johnstoni there are a large number (fifteen to seventeen) of nephridia forming long loops projecting dorsalwards into the body cavity in the anterior region -, each has its own internal and external openings, and is isolated and independent. In Terebellides Strœmi there is one pair of large dark-coloured nephridia in the anterior end, and three pairs of small rudimentary ones posterior to this.

In Pectinaria belgica there are three pairs, all independent; they are described in the following section. In Melinna cristata there are several pairs all separate.

Mr. Harvey Gibson¹ has by carefully neglecting the distinguishing differences between this species and Amphitrite auricoma, Müller, attempted to prove that the two forms are identical. He points out that in the original figures of Pallas, of Nereis cylindraria, var. bélgica, “the stiff golden comb shows one continuous and uniform series of teeth, not two series as in P. auricoma,” and that figures by subsequent authors, e. g. M ‘Intosh and Malmgren, show the two combs in P. bélgica with perfect distinctness. Moreover, certain references in Pallas’s text imply that his species had two distinct combs. Mr. Harvey Gibson concludes that “either Pallas’s draughtsman made an error in most of the figures of P. belgica, and failed to represent the comb with sufficient accuracy, hence leading Müller into error when comparing his form with that of Pallas; or Pallas’s figures are correct (although his references in the text are wrong), and his species is distinct from that of Müller (for the condition of the comb appears to be the only important difference between the two). Looking at the inaccuracy of the drawings as compared with var. capensis in Pallas’s work, and taking into account the indistinctly double series of teeth shown in figs. 5, 8, and 9 of var. belgica, I think that probably the former view is most likely to be the correct one. In that case P. auricoma of Müller disappears, and becomes P. belgica of Pallas.” How a zoologist, after actually referring to the description and figures given by Müller of Amphitrite auricoma, and to the description and figures of both species given by Malmgren in his ‘Nordiska Hafs-Annulater,’ could suppose the condition of the comb to be the only important difference between the two, it is difficult to understand. The two distinguishing features given by Müller are (1) the curvature of the tube; (2) the serration of the margin of the flattened area behind the palmulæ. Malmgren mentions both these characters and figures them, and he examined specimens of both species; only Malmgren made the two characters generic instead of specific, and called Müller’s species Amphictene auricoma. We can state with certainty that in our specimens the tube is perfectly straight, and the margin of the post-palmular area perfectly entire. The presence of two distinct palmulæ, as Mr. Harvey Gibson would have seen if he had studied the Latin descriptions of Malmgren, is common to the whole family Amphicteuea.

There are three pairs of nephridia inPectinaria belgica, of which the first are the largest; all the organs are of the usual type, each consisting of a tube bent upon itself and provided with a nephrostome and an opening to the exterior. There is a transverse septum separating the buccal somite from the following; the nephrostome of the first nephridium is on the anterior side of this septum. The nephridia are brown or black in colour. The tube of the first reaches dorsalwards above the intestine, and its external opening is a little ventral to the origin of the first branchia. Between the nephridial opening and the root of the branchia is the aperture of a peculiar glandular organ whose function we have been unable to ascertain. On dissection of a fresh specimen this gland is seen as a milk-white, opaque, cylindrical body about one eighth of an inch long, free everywhere except where it is continuous with the body wall round its opening to the exterior. The efferent duct of this gland is lined by a high columnar epithelium, of which the component cells are solid and columnar; throughout the rest of the gland there is a layer of long solid nucleated cells next to the basement membrane, but these are covered by other layers of large vacuolated cells whose walls form a network nearly obliterating the lumen of the organ. The wall of the gland is well supplied with pseudhæmal vessels. The third somite (i. e. second branchiferous) and the fourth are unprovided with nephridia, but the latter contains a nephrostome belonging to the nephridium of the fifth somite; the sixth somite is likewise provided with a nephridium whose nephrostome is in the fifth somite. The nephrostomata are simple elongated funnels with their apertures directed forwards; they are not provided with such a series of digitate processes as is seen in Lanice and Arenicola. The gonads are of the usual type, masses of undifferentiated cells attached to the exterior of the nephrostomata on the mediad side. The reproductive cells become detached at a very early stage and pass through the rest of their development in a free condition in the cœlom. It is certain that the spermatozoa reach the exterior by passing through the nephridia; in a series of sections of a ripe male I saw the nephrostomata and nephridial tubes distended with them. Between the two posterior nephrostomata and the body wall pass membranes, which are rudiments of transverse septa. There is also a rudiment of a septum between the second and third somites (first and second branchiferous). The external apertures of the two posterior nephridia are ventral and posterior to the notopodial setæ of their somites.

Claparède did not study the nephridia in the Nereidæ. Ehlers1 has given a description of the organs as he saw them in Nereis cultrifera, Grube, and other species. He says the segmental organ lies close behind the entrance into the cavity of the parapodium, on the inner surface of the ventral body wall, near the lateral border of the ventral muscular band: that it consists of an easily noticed, almost spherical body, and an efferent duct, which runs on the ventral body wall towards the hinder border of the segment, where it opens to the exterior: that the body of the segmental organ in a large epitokous female of N. virens was’189 mm. by’108 mm. in size. In the inside of the body of the organ he states there were a number of clear cavities, which, when the organ was compressed, were discovered to be portions of a continuous convoluted canal which was embedded in the mass of the organ; the inner surface of the walls of this canal was ciliated. On the upper surface of the body of the organ was a slightly curved, cleft-like aperture, with thickened edges, which carried cilia; this cleft was the internal opening of the organ; on the opposite side of the spherical body the thin efferent duct passed off. Ehlers concludes his description thus: “In support of the view expressed by myself, that the segmental organs serve as efferent ducts for the sexual products, I may point to the fact observed by me that these organs in N. virens contained perfectly mature ova, which were found in the efferent duct of the organ as well as in its body, and that not seldom a single ovum lay in the external aperture.” Ehlers gives a figure of an isolated segmental organ which corresponds fairly well with his description.

But that description is erroneous in one important point. The segmental organ or nephridium in Nereis virens does consist of a somewhat spherical mass composed of a number of glandular ciliated tubes, which probably all form a single convoluted tube, and a straight, thin, efferent duct, passing off from the spherical mass to open to the exterior on the ventral side of the base of the parapodium. But the internal aperture, the nephrostome, is not a cleft such as Ehlers figures and describes in the wall of the spherical mass. The nephrostome is situated at the end of a simple thin ciliated tube, which projects out from the spherical body at the side opposite to the efferent duct. The nephrostome is funnel shaped, as usual, but the edges of the funnel are produced into numerous finger-like lobes, and from the lobes project a forest of delicate, pellucid, branched processes, the surface of which is beset with extremely long cilia, which move somewhat slowly. The mouth and neck of the funnel seem to be divided longitudinally by a partition. But it is probable that the partition is incomplete. (See figs. 15 and 16.) The somites of Nereis virens are separated by transverse mesenteries, and at the line along which these mesenteries are united to the intestine the latter is considerably dilated, while in the centre of the somite it is contracted. In order to conform to the typical nephridium in its relations, the nephrostome in Nereis virens ought to be on the front side of the septum, behind which the body of the nephridium is situated. Whether this is so or not I have not been able definitely to ascertain, but I believe it to be so. The septum is close to the front edge of the base of the parapodium, and the nephridium lies slanting across the entrance to the cavity of the parapodium, and above the edge of the ventral band of longitudinal muscles. Thus the distance between the septum and the body of the nephridium is not too great to be bridged by the tube leading from the nephrostome. It is to be noted that the nephridium lies as usual below the oblique muscles.

Ehlers’ observation above quoted, if it were perfectly accurate, would prove beyond a doubt that the nephridia in Nereis virens serve to convey the sexual products, or at least the ova, to the exterior. But that observation is in contradiction to all the evidence that has come under my notice. The breeding habits of the Nereidæ still remain in some particulars mysterious, but it seems clear that the worms of this and other errant families, becoming provided with more efficient swimming organs at the breeding season by the metamorphosis of some of their parapodia, leave their burrows and swim about freely in the water while they are discharging their reproductive elements. I have only met with two forms of the family Nereidæ in sexually mature condition—Nereis pelagica and Nereis virens. Specimens of the latter are not unfrequently thrown up on the shore in the Firth of Forth about the month of April. Solitary female specimens thus cast up have occasionally been brought to me, but I have not examined these very minutely. On May 1st this year we found about 150 specimens among the débris at high-water mark within about a quarter of a mile, close to the Granton Laboratory. Every one of these specimens proved on examination to be male; they were all alive, though some had been half desiccated by the warmth of the sun after the tide had left them, but they were not vigorous, and from their position and condition must have been dead before the tide returned. These specimens were in most cases distended with milt, and when handled they burst or broke into pieces on the least provocation and discharged the milt copiously. I cut sections of some of the nephridia of these in situ, and saw not a trace of spermatozoa within the organs. If the dehiscence is normal it seems most probable that the animals of both sexes die after discharging their sexual products, and the dehiscence is so constant that it cannot be other than a normal process. If the sexual products normally escape by dehiscence, why should they pass through the nephridia, or if they passed through the nephridia why should dehiscence occur at all ?

I have not seen specimens of Nereis pelagia cast up by the waves upon the shore in an enfeebled condition, but we found the epitokous form of the species abundantly at the beginning of February; some we found under stones between tide marks, but the greater number among the roots of Laminaria and under stones in the Laminarian zone. We kept these often for some time in captivity, and whenever they were handled, or even without being touched, they discharged ova and spermatozoa by dehiscence, and they invariably died after being kept some time. Sections revealed no ova or spermatozoa in the nephridia, in fact it seems impossible that the ova should be found in the lumen of the nephridium, for the ovum is many times larger than the tube of the latter in diameter, being nearly equal in size to the whole body of the nephridium.

Dr. R. Horst, of Leyden, in ‘Zool. Anz.,’ viii, published a discussion of this curious and problematic structure. He gave an account of his own examination of the heart in some specimens of the genus Brada, belonging to the family Chlorhæ-midæ, and from the structure of the cardiac body in that genus, and a comparison of it with certain structures in the Oligochæta, draws the conclusion that the cardiac body in Polychæta is originally derived in the embryo from the intestinal epithelium, is, in fact, an evagination from the intestine. Dr. Horst gives a rapid sketch of the history of our knowledge of the heart in the Chlorhæmidæ. The organ was first mentioned by Otto in 1821,¹ and considered by him to be a second oesophagus. Claparède gives an erroneous description of the organ (‘Ann. Chet, de Naples’); he says it is a tubular structure, which appears to open anteriorly in the dorsal wall of the buccal cavity. Delle Chiaje also considered the heart to be connected with the digestive system. Gab. Costa, Dujardin, Max Muller, and Quatrefages have all recognised the organ as a true heart, whose function is to propel the blood into the branchiæ. Claparède, however, seems to have been the first to discover the curious dark-coloured cells containing granules, which occur in the heart, while those zoologists who recognised the true function of the heart were unaware of anything peculiar in its structure. Claparède met with the cells of the cardiac body, and thought the organ was entirely glandular, while others saw that the organ was a heart, and were unaware that it contained a glandular body.

Dr. Horst found that in Brada, as in Serpulidæ, Ammocharidæ, &c., there is a blood sinus round the intestine, and the heart is continuous with this sinus, and therefore a true dorsal vessel, through which the blood is conducted from the walls of the intestine to the gills. It is to be remarked that this is a confirmation of the account given by Quatrefages in ‘Hist, des Annelés,’ 1865, i, p. 54. Max Müller, on the other hand, supposed the posterior end of the heart to be blind. Dr. Horst then points out the agreement between the arrangement described by him in Brada and that described by Vejdovsky in the Enchytræidæ, which also possess a dorsal vessel only in the anterior somites, its place being supplied posteriorly by a blood-sinus in the wall of the intestine; and further says the researches of Salensky on the development of Terebella (‘Arch, de Biologie,’ t. 4) have shown that such an arrangement is in other Annelids only embryonic.

I have examined the vascular system in Trophonia plumosa, and although I find Horst’s statements for the most part correct, there is one feature which he does not mention which might give rise to another explanation. There is, namely, a thin vessel running in the dorsal median line on the inner surface of the body wall, unaffected by the convolutions of the intestine, receiving metamerically arranged transverse vessels from the walls of the latter, and opening into the dorsal side of the heart at a point a third of its length from the hinder end (see fig. 17). It is thus a question which represents the typical dorsal vessel—this separate vessel that I have described, or the blood-sinus in the walls of the intestine. I am inclined to think the former, and that the posterior end of the heart may be taken as representing a vessel passing from the intestinal blood-sinus to the dorsal vessel, while the anterior end of the heart is the direct continuation of the dorsal vessel. These relations are shown in fig. 17.

However this may be, the thin dorsal vessel mentioned above is not represented in Terebellidæ, Ampharetidæ, and Amphic-

Cosmovici (loc. cit.) was unable to see the large anterior nephridia described by Keferstein and Claparède, but states that in C. filiformis, Kef., segmental organs are present in pairs in nearly all the segments, especially of the middle and posterior region: that they are attached to the anterior face of each diaphragm: the figures of this species which he gives do not show the organs mentioned. The Errantia, among which he places Cirratulus, according to Cosmovici’s peculiar views, have only segmental organs and no organs of Bojanus, and he denies altogether that the internal opening of the segmental organ communicates with the cavity of the somite in front of the one which contains the organ itself.

In Cirratulus cirratus both the large anterior pair of nephridia described by Keferstein and Claparède, and the series of pairs in the middle and posterior region mentioned by Cosmovici, are present. The nephrostome of the first nephridium opens into the cavity of the buccal somite, being situated on the anterior face of the first septum, somewhat ventrally, in the angle between the septum and the lateral body wall. The proximal part of the bent tube passes backwards from the nephrostome till it reaches the second septum (Pl. XVII, fig. 3), then passes upwards to the dorsal body wall, where it opens into the wider distal part of the tube which opens to the exterior beneath the neuropodium of the second somite. The posterior nephridia are smaller and simpler; they appear first in the twelfth somite, and are repeated hence to the end of the body. Each of them has a nephrostome of the typical form, an elongated funnel with its aperture directed forwards. The nephrostome has a similar position to that of the large anterior nephridium, that is to say, it is placed in the lower external corner of the anterior face of its septum. The lips of the funnel are composed of a columnar ciliated epithelium resting on a thin fibrous membrane; this membrane is continued on the one hand into the transverse septum, on the other into the body wall; the lips of the funnel project inwards and forwards into the cavity of the somite. The nephridial tube when traced from the nephrostome (in a series of horizontal sections) is seen to pass obliquely backwards and downwards, curving over the dorsal edge of the ventral longitudinal muscle, and opening beneath the neuropodial bristles. The internal openings of the simple nephridia are shown as seen in a horizontal section in fig. 4, while fig. 5 shows the external opening in a transverse section of a female specimen.

The simple nephridia act as efferent ducts for the reproductive elements in both sexes. I found a number of specimens distended with the genital products at the end of March in the current year, lying under stones on the banks of Granton Quarry, where they are pretty abundant at all seasons. These, when placed in a basin of sea-water, commenced to shed eggs and spermatozoa. The ova were fastened together, after their escape, by transparent gelatinous mucus, which formed a soft mass without any definite shape adhering to the stones and mud among which the worms were lying. The escape of the ova could be observed without much difficulty.

Fig. 6 shows the appearance presented by two somites of the worm viewed under a low power by reflected light, as the ova were escaping; the apertures seen are the openings of th e simple nephridia previously described. In sections of a male specimen these nephridia are seen full of spermatozoa. The larger nephridia of the second somite do not, as far as I have been able to ascertain, transmit the sexual products; indeed, no ova (or spermatozoa) are produced in the first eleven somites where simple nephridia are absent.

I have not discovered in my sections any satisfactory indication of the places where the germinal cells are developed; the position of the gonads is doubtful. The reproductive cells undergo the greater part of their development in the body cavity. The presence of a complete longitudinal vertical septum above and below the intestine in Cirratulus is remarkable.

This form has not hitherto been recorded as occurring in the North Sea, either on our own coast or other parts of cannot usually be seen in them; the most internal, those nearest to the lumen, are almost spherical, and they project separately and at various levels into the lumen, just as do the similar cells in a section of a nephridium. Indeed, the whole structure recalls that of a nephridium very forcibly. Usually the lumen of the tube contains débris which is stained by carmine, and among this can be recognised spherical cells similar to those which project from the epithelium in process of disintegration. It is difficult to resist the conclusion that these tubes are glands, but I have been unable to discover any trace of an opening from the cardiac body either to the exterior of the body or into any other organ. In many of the cross sections of the cords no lumen can be seen; in some cases this is obviously due to the fact that the plane of the section is too near the surface of the cord, and has therefore passed only through the epithelium; in other cases the plane of the section is median longitudinal, or transverse, through the widest part of a cord, and yet no lumen is seen, the epithelium on one side being so thick as to come into contact with that of the other. It is probable that the obliteration of the lumen is partly due to the contraction produced by reagents. The sections of the cords in any section of the heart of Trophonia are small and numerous, and the whole cardiac body almost fills up the entire cavity of the heart, from its thick posterior portion to its thin anterior region -, the channels left for the passage of the blood are in consequence very small. The blood contains small oval corpuscles, each showing a relatively large, well stained nucleus. These corpuscles are not numerous.

The cardiac body in Flabelligera affinis (Siphonostoma) presents a great contrast to that of Trophonia in its size relatively to that of the heart; in the former species the organ constitutes an irregular flat, folded band, running longitudinally through the cavity of the heart and occupying only a small portion of that cavity. Between the cardiac body and the wall of the heart is a wide space occupied by blood. The lower edge of the band is in the central line of the ventral side of the heart, whence it rises like a longitudinal partition, its upper branched part coming into contact with the dorsal and lateral sides of the heart. The organ also differs from that of Trophonia in minute structure. In transverse section the band is narrow and branched; that is to say, the main band gives off other longitudinal bands of less extent than itself, so that in transverse section it appears as an irregularly branched narrow tract. No distinct lumen is visible in the centre of this tract, but there is a distinct central line which separates the epithelia of opposite sides where they come into contact. The clear vacuolated cells seen in Trophonia are here absent, the epithelium consisting of elongated columnar nucleated cells only; the granules are smaller and less numerous (fig. 20).

In Amphitrite Johnstoni the cardiac body occupies nearly the whole cavity of the heart, the channels left for the passage of the blood being very small. It is composed of cylindrical cords which generally have a longitudinal direction. In prepared sections no lumen is visible in the cords, each being completely filled with a mass of cells whose outlines are somewhat indistinct, but the nuclei are large, spherical, and deeply stained. The cells are small, so that the nuclei are closely crowded together. In Amphitrite cirrata and Terebella Danielsseni the cardiac body exists, but I have not specially examined it.

In Lanice conchilega the cardiac body is smaller in relation to the heart than in Amphitrite Johnstoni. The cords are thinner, and confined to the immediate neighbourhood of the walls of the vessel, so that a large central space is left for the passage of the blood. In the cords a lumen is frequently but not always visible. The cells have a similar character to those of Amphitrite Johnstoni (fig. 14).

In Terebellides Strœmi there is but a single cord in the cardiac body, which runs longitudinally and fills up very nearly the whole cavity of the heart. In prepared sections a lumen is visible in the centre of this cord, and the cells are so arranged as to form radii passing from the wall of the cord towards the centre.

In Cirratulus cirratus there are three longitudinal cylindrical cords occupying nearly the whole cavity of the dorsal vessel. Two of the cords occasionally anastomose and then separate again. The cells filling the cords are elongated, pale, and nucleated; the nuclei stain but the rest of the cells remains uncoloured in stained preparations. There is no lumen, and the cells are not arranged in regular radii, but are placed so that the longer axis passes from the dorsal side of the cord to the ventral. The cells contain large numbers of the usual granules, which are brown and spherical, and usually more numerous near the exterior of the cord than in the central part. In one specimen I found only two cords present of which the dorsal was the larger. The cords are quite free in the interior of the vessel, and have no connection with the walls of the latter (fig. 21).

The cardiac body is present in the families Chlorhæmidæ, Cirratulidæ, Amphictenidæ, Ampharetidæ, and Terebellidæ. The three last are closely allied, but neither of them has any other points of close agreement with either of the two first mentioned; nor are the Chlorhæmidæ and Cirratulidæ at all connected with one another. The cardiac body is present in every species belonging to the families mentioned, though it varies in details of structure in different species.

In the heart of Polyophthalmus pictus Edouard Meyer1 has described an organ which Horst claims as the homologue of the cardiac body we have been considering. It is, of course, probable enough that Horst is right, but it must be remembered that Meyer’s description is not complete enough to decide the question whether the organ in the heart of Polyophthalmus is similar in structure to the true cardiac body. Meyer says that a peculiar organ having the form of a thick, short tube, which is provided with strong cellular walls, and a canal running through its axis, occurs in the cavity of the heart. The organ projects with its posterior half, at the end of which is the broad entrance opening into the axial canal, into the intestinal sinus, and is here, by means of special small muscular bundles which arise from processes round the opening, fastened on to the intestinal epithelium. The front end of the organ contains the anterior opening of the axial canal and is fastened to the anterior wall of the heart.

From this account it follows that the blood passes in at one end and out at the other of the heart organ of Polyophthalmus, while as yet no opening at all has been demonstrated in the cardiac body in the families above mentioned.

I have been unable to find any facts which go to support Horst’s view that the cardiac body is homologous with an organ which exists in some of the Enchytræidæ, and which arises as an evagination from the intestine. The account given by Salensky¹ of the development of the cardiac body in the larva of Terebella is not very full, but he states that the cadiac body is at first a tube passing from the posterior extremity of the heart and terminating blindly in its interior; and this tube, from his description and figure, seems to have an opening from the exterior posterior surface of the heart. It would seem probable, therefore, that the cardiac body in Terebella is derived from an invagination of the wall of the heart, and not from the intestine. It is quite certain that in the adult Trophonia there is no connection between the cardiac body and the intestinal epithelium, closely as they come into relation. I cut a continuous and complete series of longitudinal vertical sections through the posterior part of the heart of Trophonia, and the intestine it rested upon, and found that there was nowhere any connection between the intestinal epithelium and that of the cardiac body.

If Horst’s view were correct one would be tempted to homologise the cardiac body of Chætopoda with the cellular structure which grows out from the intestine and projects into the heart in Balanoglossus, the structure which Bateson believes to represent the vertebrate notochord. Professor T. J. Parker1 has already pointed out, in opposition to Bateson’s arguments, that the vessel which lies on the same side of the intestine as the notochord in vertebrates conveys the blood from before backwards, while the dorsal vessel in Balanoglossus conveys the blood from behind forwards. In this respect the dorsal vessel of Balanoglossus agrees with the dorsal vessel of all other Invertebrata, and I am strongly of opinion that Balanoglossus is constructed on the same plan as a Chætopod. I would consider the proboscis as the præoral lobe; the nerve-cord in the collar and in the proboscis as the enlarged representative of the supraœsophageal ganglia. The circumœsophageal commissures are present at the posterior region of the collar, and they unite into a well-developed ventral nervecord. The great obstacle to this view is the presence of the dorsal nerve-cord in Balanoglossus. But it may be pointed out that this dorsal nerve-cord is much thinner and more insignificant than the ventral, and that the ventral is in shape and character the real continuation of the nerve-cord in the collar. The absence of nephridia and the meaning of the proboscis pore and proboscis gland are points which cannot at present be explained on the view I have advocated.

The texts for the few words I have to say on this subject are two papers, one by Professor McIntosh,2 published in 1877, the other by Dr. Emil Rohde,3 published in 1886. The first gives a brief account of the anatomical relations of the ventral nerve-cords in the families of marine annelids, while the other discusses the giant fibres of the nerve-cord in Aphroditidæ. These structures are asserted by Rohde to be nerve-fibres, which commence as processes of certain colossal ganglion cells occurring in definite positions in the brain or ventral cord. He gives the following account of the giant-fibres in the genus Sthenelais. There are three kinds of colossal nerve-fibres: (1) some traversing the whole nervous system from the anterior to the posterior extremity; (2) some running from the posterior to the anterior extremity; (3) some starting from the nervous system on each side, and running to the periphery. He further says that if the nervous system be traced through a series of transverse sections he finds in the posterior part of the brain a colossal ganglion cell on each side which sends off a large process. This process passes first forward for some distance into the brain, then through the oesophageal commissure into the ventral cord. These two nerve-fibres unite into one which runs ventrally on one side of the ventral cord to the posterior extremity of the body. This colossal nerve-fibre is enveloped by a fibrous sheath, which is at first closely applied to it, but in its further course separates from it, and then encloses a cavity which becomes larger posteriorly, and in the middle of the body attains an enormous diameter.

Unfortunately, no figures illustrating these descriptions have yet appeared, and I have therefore had to confine myself to a comparison of my own sections with the above description. I have been totally unable to see the connections which Rohde declares to exist. I have prepared series of sections from different parts of the body of-Sigalion boa, Johnston, which, according to McIntosh (Invert. Fauna of St. Andrews), belongs to Kinberg’s genus Sthenelais. In the middle region a pair of colossal fibres, or as I shall usually call them, neural canals, appearing under a low power like tubes, are conspicuous. One of these is situated on the inner side of each cord, towards the dorsal region, and at the periphery of the cord (fig. 22). The neural canal is internal to the layer of ganglion cells, and is partially occupied by a shrunken homogeneous substance. Processes can be often seen passing off from the ganglion cells transversely, and entering the substance of the cord where they are seen to branch into fine fibrils, exactly in the same way as that illustrated so well by F. Nansen1 in his memoir on the Myzostomidæ. But I have been unable to trace any connection between the neural canal, alias giant-fibre, and a ganglion cell. Indeed, in Sigalion the canal or tube becomes very small long before the brain is reached, and I cannot even distinguish it in the oesophageal commissures, or in the cord immediately behind them.

In the central part of each cord in the middle region of the body are one or two tubes, which are similar in structure to the large neural canals, but much smaller.

A few words as to the character of the nerve-cords in Sigalion boa. The cords are nowhere separated from the epidermis. Ganglion ceils are abundant beneath the ventral cords, both in the ganglia and between the successive ganglia. Above the cords is a striking development of a very peculiar tissue whose function is problematic. In the middle of the body this tissue consists of waved fibres or laminae, which are often arranged in parallel curves. These form a network whose meshes occasionally contain cells with nuclei, but usually are filled with a stained granular substance. Close behind the head this mass of tissue is of very great size, and is much more cellular. In the oesophageal cords it is reduced to a very small quantity, but it forms a thick envelope round the brain. The tissue stains with difficulty. It is in all probability a kind of connective tissue not directly concerned in nervous functions.

With regard to Aphrodite, I entirely agree with Rohde that colossal fibres or neural canals are altogether absent; and in this genus the nerve-cords are quite separated from the epidermis.

Polynöe I have not examined, but in Harmothöe imbricata I find a pair of neural canals corresponding in position to those of Sigalion boa, but I have not seen any in a ventral position, such as those mentioned by Rohde in Polynöe. In Harmothöe the ventral nervous mass is distinctly defined, but not separated, from the epidermis.

Of the Nereidæ I have examined Nereis virens, Sars, and here cannot confirm the account of the neural canals given by McIntosh. In many sections three, or four neural canals are seen, which are not quite symmetrical; these are sections through inter-ganglionic transverse commissures. In the cords between successive ganglia there are seen to be a single pair of canals, one of which is often divided into two. The pair occupy an exactly similar position to that of the neural canals in Sigalion. McIntosh states that in Nereis virens there are several neural canals, viz. two large infero-lateral, a single superior median and a smaller, a little below the latter on each side. This apparently means five in all, two pairs and one median. Probably he examined sections of the ganglia in which several canals are often seen. But these have not a constant relative position, and are, I believe, due to the subdivision of the two canals which are seen in the separated cords.

In Nereis pelagica l find canals placed in the positions ascribed by McIntosh to those of N. virens. There is one dorsal median in the fibrous partition between the cords, a pair corresponding to the typical pair of Sigalion, and another pair consisting of one on the external border of each cord.

In Nephthys, instead of the typical canal on the inner side of each cord, there are two large canals, one above the other, with a smaller one between them. There is also a smaller canal in the external side of each cord, and still smaller ones in the substance of the cords. The nerve area here is not separated from the epidermis.

In Phyllodoce no well-marked neural canals can be distinguished. The cords are widely separated from the epidermis.

We pass now to the examination of the families of Sedentaria.

In the Sabellidæ there is a pair of canals or tubes of much greater size than any seen in the Aphroditidæ. Only a few somites behind the head these tubes reach a thickness equal to or even slightly greater than that of the nerve-cords themselves, and they retain an almost uniform thickness in their course to the end of the body. These tubular fibres have been well described by Claparède1 as they occur in Spirographis Spallanzanii. He found several transverse connecting branches between the two tubes in the thoracic region immediately behind the union of the œsophageal commissures, and he traced the tubes into these commissures, where each divided into two branches. These branches passed forwards into the cerebral ganglion, where they ramified into thinner and thinner branches, but the ultimate terminations Claparède could not discover.

I have endeavoured to trace the anterior extremities of the tubes in Sabella penicillus. I found a transverse connecting tube in the first transverse commissure, like those described by Claparède, but could not find more than one, and it is noticeable that while Claparède mentions a number of these anastomoses in the text he figures only one. I found that the tube, much diminished in diameter, passed up the œsophageal commissure, but could not discover that it branched. In my sections it seems to become smaller and smaller, and simply end blindly.

In Sabella the mass of ganglion cells representing the cerebral ganglion is situated on each side of the œsophagus, and below it is the fibrous œsophageal commissure, which is continued by an arch above the œsophagus into its fellow of the opposite side. The tubular fibre ends below this mass of cerebral ganglion cells, and, as far as I can see, sends no branches towards the mass, nor could I see any trace of a connection between the end of the tube and any ganglion cells.

Although these structures are spoken of as tubes, they are not actually empty. Their interior in the sections is partially filled by a transparent gelatinous-looking mass, which for the most part does not stain; but there are lines in the mass which are stained, and which somewhat resemble a network of fibres. In my opinion these stained lines are due to the unequal coagulation of the gelatinous mass, which during life is homogeneous and semi-liquid. On the dorsal side of the tube is a space between the gelatinous medulla and the wall of the tube, and the edge of the medulla below this space is deeply stained. All this is, in my opinion, the consequence of contraction and coagulation.

Myxicola in the greater part of its body has a single tubular fibre similar in structure to one of the pair which exist in Sabella. According to Claparède (loe. cit) the tubular fibre of one side, at a point a little behind the oesophageal commissures, opens into that of the other side, and the latter proceeds as the unique fibre, while the nerve-cord corresponding to the first tube joins the other cord without fusing with it and finally terminates. My series of sections of this animal is not quite perfect, but, as far as I can judge, Claparède has been somewhat deceived. It is true that only one tubular fibre persists, but it seems to me that the other terminates and does not open into its fellow. Some distance behind the œsophageal ring the right hand cord is seen thicker than the other and destitute of a tubular fibre; the tubular fibre is seen on the left-hand side of the ventral median mesentery, which is pushed considerably to the right. Farther back the tubular fibre becomes central, and the two nerve-cords, one on either side of it, are equal in thickness. I have seen no indication of the disappearance of one nerve-cord. In my opinion, all that has happened in Myxicola is that one of the large tubes has disappeared except in the extreme anterior region, and the other has increased in size, and in the greater part of the body become central. One very interesting point to be seen in Myxicola is that the two tubes are continuous with one another in the lower part of the cerebral commissure; the tubes in each œsophageal commissure, which are of considerable diameter, can be seen to become continuous with one another in the section of the cerebral commissure. In my sections of Myxicola the tube in the posterior part of the body is entirely empty, except in the ventral part, where a thick stained band occupies the cavity; this is due to a greater shrinking of the contents of the tube in the process of preparation. It may be pointed out here that the nerve-cords in Sabella are not separated from the epidermis, while in Myxicola they are completely so.

We pass now to consider another family in which colossal tubes or neural canals are greatly developed, the Spionidæ. In Nerine coniocephala, Johnston (fig. 23), the nerve-cords are differentiations in a thickened epidermis, less distinctly defined from the surrounding cells than is the case in Sabella. In the median line between the two cords is an enormous neural canal, larger in sectional area than the two nerve-cords together, but having a structure similar to the tubes in Sabella. The appearance of this canal in section is seen in fig. 23. The canal contains a shrunken, gelatinous-looking mass as, in other cases. On tracing this canal forwards it is found to become smaller near the anterior end, and to cease altogether much sooner than is usually the case. I have found no indication of its anterior division into two canals. The last trace of it seen in approaching the head is shown in fig. 24.

In Scolecolepis vulgaris, Malmgren, there are two neural canals, one on the inner side of each nerve-cord.

In Magelona, which forms a family by itself, there is a very large median neural canal resembling that of Nerine, but lying below the nerve-cords instead of above them (fig. 25).

In the Ariciidæ I need only confirm the account given by McIntosh, that in the middle of the body the nerve-cords are thrust inwards by the great ventral longitudinal muscles, which contain between them a narrow lamina preserving the connection between the nerve-cords and the epidermis. A single median neural canal runs above the nerve-cords as in Nerine.

In Arenicola (Telethusidæ) the nerve-cords are separated from the epidermis by the layer of circular muscles; there is a small neural canal, entirely filled with a homogeneous mass, at the dorsal and inner side of each cord.

In Trophonia (Chlorhæmidæ) McIntosh does not mention the existence of neural canals, but one of these exists in each cord on its inner side dorsally. The cords are entirely free from the epidermis. The neural canals are similar in size and appearance to those in Sigalion boa.

Among the Terebellidæ I find a median neural canal in Lanice conchilega, Malmgren: it is of considerable size, but has not such well-defined fibrous walls as are usually present. In Amphitrite Johnstoni I have been unable to detect any canal, nor in Terebellides Strœmi.

In the Ampharetidæ, however, which are, so to speak, on the way towards the Terebellidæ, the neural canals are large and conspicuous, and have the typical structure. In Melinna cristata there is one on the inner side of each nerve-cord in the thoracic region.

In the Capitellidæ (Capitella and Notomastus) the nerve-cords lie in the epidermis posteriorly, while in a few of the anterior somites they are entirely separated from it by both the circular and longitudinal muscles. In this anterior region there is a large median neural canal on the dorsal side of the double cord in Notomastus. In Capitella the canal is absent.

Among the Maldanidæ I have examined Nicomache lumbricalis and Axiothea catenata. In the former the nervecords are not separated from the epidermis, and there is a considerable single median neural canal above the cords in both species.

Among the Hermellidæ, in Sabellaria, as pointed out by McIntosh, the two cords are at a considerable distance from one another; they are completely separated from the epidermis and lie on the upper and inner side of the ventral longitudinal muscles. Each has a large neural canal, similar to that of Sabella, on its inner side (fig. 26).

In Serpula the neural canals are similar in structure and position to those of Sabella.

McIntosh mentions a small and indistinct neural canal in Ammotrypane aulogaster, H. R. In some of my sections it can be made out, but always with difficulty, as it is exceedingly ill-defined. In Cirratulus cirratus, also, neural canals are absent.

It would be very startling, if not even absurd, to maintain that such neural canals as are seen in Sabella, and in Nerine are colossal nerve-fibres, and that their contents form a nervous medulla which commences as a process from a ganglion cell. I have entirely failed to trace any connection between these canals and any ganglion cells. At the same time it is difficult to refuse to admit that the neural canals of the Sedentaria are completely homologous with those of the Errantia.

In my opinion, in both cases they are supporting structures which serve to prevent the nerve-cords being bent at a sharp angle, causing them always to remain in curves, and so to escape injury during the wriggling and burrowing of the worm. It is noticeable as a support of this view that the canals always reach their greatest development in worms which are extremely long in proportion to their thickness. Sabella and Nerine are both extremely long, as compared, for instance, to Ophelia or Cirratulus. Another fact, which seems to have some significance, is that where the neural canals show their maximum development the nerve-cord is not separated from the epidermis, and is therefore more exposed to the danger of being injured than where they have reached a more internal position. The origin of the vertebrate notochord from the hypoblast seems so well established that a comparison of it with the neural canals of the Chætopoda will scarcely be regarded seriously by morphologists. At the same time, seeing that the notochord at a later stage is separated from the intestine by the aorta, it is very difficult to understand how the former structure could have been derived phylogenetically from the intestine. The neural canals are remarkably constant throughout the Chætopoda, those in the Polychæta being obviously homologous with the three giant-fibres in the earthworm and other Oligochæta. They have a position in relation to the nerve-cords and ventral blood-vessel which is similar to that of the notochord in relation to the neurochord and aorta. Their origin in the embryo has not so far as I know been investigated, so that it is doubtful whether they are intercellular or intracellular in origin. I hope to devote further attention to the subject, at present I can only say that the evidence adduced in favour of their specifically nervous nature is quite inadequate, and that the possibility of a phylogenetic connection between these neural canals in the Chætopoda and the notochord of the Chordata, cannot at present be altogether dismissed.

In concluding this paper I have to explain that my attention was attracted to the points discussed in the course of a systematic examination of the Polychæta, which I carried on at the Granton Marine Station, in collaboration with my friend Mr. G. A. Ramage, Vans Dunlop Scholar in Edinburgh University.

For the facts and views I have set forth I am alone responsible, but the collection and identification of specimens were chiefly carried on by Mr. Ramage, and he rendered much valuable assistance in preparation and dissection. The drawings for the paper were all executed by myself.

Illustrating J. T. Cunningham’ s paper (< On Some Points in the Anatomy of Polychæta. “

Fig. 1.—An entire nephridium of Arenicola marina, seen under a low power, in the fresh state. The membranous funnel has been turned back, so that the ventral side is seen. A. Anterior, P. Posterior end. bl. Blood-vessel, which joins the branchial vein. d. b. Dorsal fringed border of nephrostome. ne. Nephrostome, go. Gonad.

Fig. 2.—Optical section of a portion of the wall of the nephridium of Arenicola, after treatment with osmic acid. bl. Blood-vessel. E, Zeiss, oc. 3.

Fig. 3.—Horizontal section of 2nd and 3rd and part of 1st somites of Cirratulus cirratus. tie. Nephrostome of anterior nephridium, opening from buccal somite, a. n. Ascending part of the nephridium. d.n.Descending part. se. Transverse and longitudinal septa. A, Zeiss, oc. 2.