On Certain Points in the Development and Anatomy of some Earthworms

With Plates 2—5.

Much of the literature of the subject has not yet reached me (including the later portions of Vejdovsky’s ‘Entwickelungs-geschichtliche Untersuchungen ‘), and I have been obliged to defer any detailed reference to it until a future occasion. The organs dealt with are the setæ and the nephridia, and I have added a synoptical description of the two new species of worms with which the paper chiefly deals.

The two worms, Mahbenus imperatrix and “Perichæta” pellucid a, both belong to the family Perichætidæ, but neither, of them to the genus Perichæta. For the one I have founded a new genus, being fairly confident as to what constitute its generic characters ; for the other I have abstained from so doing, as it is, I think, allied to some of the new species described by Fletcher, while it is the only representative of the genus which I have come across. I speak of it for the present as Perichæta.

Setæ very numerous. Circles of setæ almost continuous.

Clitellum extends over more than three segments.

Male pores, one pair, very close together, no setæ between the male pores.

Gizzard occupies not more than one segment.

Intestinal cæca absent.

Septa normal.

Micronephridia present.

Testes, two pairs, freely exposed.

Length 650 mm.. Circumference 35 mm.

Segments 200.

Colour rich dark brown, lighter on the ventral surface.

Prostomium encroaches very slightly upon the peristomium.

—Segment II, 52; Segment V, 80; Segment IX, 110; no modified setæ ; setæ remain in the clitellum. The circles of setæ are continuous in Segments II—XVII and XIX, the circle is interrupted ventrally in XVIII by about eight seta gaps, and in all the posterior segments there is a tiny gap recognisable only with care in the median dorsal and ventral lines.

Clitellum not strictly confined to particular segments, may extend over Segments XIV—XIX and part of XXs.

—Male pores very small and quite close together (with not much more than a seta gap between them) ; the region immediately around them becomes raised into an oval papilla after killing in spirit. Two pairs of glands open in the region of the male pores, one pair on Segment XVII behind the circle of setæ, and one pair between Segments XVII and XIX ; their apertures lie about as lateral as seta 6.

The oviducal apertures are paired.

The spermathecal apertures are very small, and placed between Segments VI-VII, VII-VIII, and VIII-IX ; the two pores of each pair are remarkably close together, so close as to be only recognisable with care as separate apertures.

—The most anterior dorsal pore lies between Segments V and VI ; they are present in the clitellar region in the young worm, but become completely obscured when the clitellum is developed.

—The gizzard is in Segment VII; dilated portions of the oesophagus serve as calciferous glands in Segments IX to XV ; they are large in Segments XI to XIV only.

The typhlosole is a simple flap, deep, down to about Segment XIV; there it narrows, becomes a mere ridge, and goes on to. Segment CXVIII, and there ends abruptly, and at the same place the alimentary epithelium changes and becomes rectal.

—The dorsal vessel is double in Segment vn and onwards for a considerable distance.

—Minute micronephridia are present in large numbers.

—The testes lie in Segments X and XI, and are attached to the septa bounding these segments anteriorly, The ciliated rosettes lie in the usual position. Prostates are large and rounded, and are provided with a muscular duct. The seminal reservoirs lie in Segments ix and xn. The spermathecæ are sausage-shaped sacs with a short duct, and a small cæcum lying in the thickness of the body-wall, and, what is very unusual, increase in size from before backwards. A spermatheca of Segment IX is more than three times as long as one of Segment vn. The ovaries and oviducts lie in the usual positions.

Length 450 mm. Circumference 12 mm.

—The body-wall is very transparent, so that the colouring depends on the blood and the contents of the alimentary canal.

Prostomium small, not dovetailed into the peristomium.

—Segment n, 24; V, 44; IX, 36; XX, 36. The number is subject to variations,and they are not very regularly arranged. No modified setæ are present. The dorsal gap is equivalent to about ten seta gaps, and the ventral gap to about three or less.

Clitellum not strictly confined to particular segments ; it extends over Segments XIII—xviii and a little into Segment XII. The dorsal pore XIi-XIII lies well within the clitellum, and posteriorly it extends nearly up to the seta ring of Segment XX. It is not well developed in the ventral region.

—The male pores lie in Segment xviii and thus in the clitellar region, although the clitellum is not developed in their neighbourhood. They lie in a small dumb-bell-shaped pit in preserved specimens. The distance between them is equivalent to about five seta gaps. There are no setæ between them. There are no other apertures in this region.

The oviducal apertures are paired, and lie about in the direction of seta 1, and just in front of the seta ring in Segment XIv.

The spermathecal apertures are placed between Segments VII-VIII and VIII-IX, and in the direction between seta 1 and seta 2.

—The most anterior dorsal pore lies between Segments V and VI. They remain obVIous between the clitellar segments.

—Gizzard is in Segment V.

The oesophagus presents dilatations with vascular walls in Segments VI to XIII. There are well-developed calciferous glands in XIV, XV, and XVI. Saccular intestine commences in Segment XVIII.

There are no intestinal cæca.

There is no typhlosole.

—There is a pair of complex (see below) nephridia in each of Segments VII to XI, and a pair of small simple nephridia in each of the following segments. There are no micronephridia.

—Testes lie in Segments X and XI. and are attached to the septa bounding these segments anteriorly; The ciliated rosettes lie in the usual position. The prostates are long tubular-shaped glands, but remain in Segment XVIII.

The spermathecæ are elongated pyriform sacs with a small cæcum.

The ovaries and oviducts lie in the usual positions.

The setæ which first develop are, in all the worms which I have studied, replaced by others in either all or in the greater number of the segments before the embryo leaves the capsule ; the replacement takes place in regular order from before backwards, and if it has not taken place in the hindermost segments before birth it does so shortly after. I speak of the setæ which first develop as embryonic setæ, and of the setæ which replace them as permanent setæ, although, of course, these may drop out and be again replaced later. The groups of cells from which the embryonic setæ of any segment develop I term secondary setal matrices. The secondary setal matrices develop from a primary setal matrix in each segment on each side of the body whether the complete number of setæ in the segment is four on each side or a larger number, as in the Perichætidæ.

To exemplify this I have taken figs. 1—8 from sections of an embryo of Mahbenus imperatrIX. The embryo was the same as that drawn in fig. 33, and was 7 mm. long when removed from the capsule. The sections were cut longitudinally through the tail end after it had been flattened out as in fig. 33. They are ·006 mm. thick. They were drawn with a camera lucida and Zeiss F, oc. 3 (for figs. 1—6 ; oc. 2 for figs. 7, 8), and as drawn are, therefore, magnified about 1100 diameters. The indentation marked x in figs. 1—3, due to a fold caused by the flattening, serves to mark the relative antero-posterior position of those three sections. Fig. 1 passes through the “primary mesoblast “of the right side ; the next section on one side (not figured) is precisely median and passes between the “mesoblasts,’ while the section beyond (not figured) passes through the “mesoblast “of the left side. Fig. 2 is the next section to the right of fig. 1, and fig. 3 the next beyond ; they show the row of cells proceeding from the “mesoblast “of fig. 1 passing slightly outwards and forwards (cf. fig. 33). Fig. 4 is a portion of the section next to fig. 3, and commences about half its own length beyond where fig. 3 leaves off ; it shows the row of cells proceeding from the neuroblast of the right side. The lines A B in figs. 4, 5, and 6 are drawn at the same level to show the relative antero-posterior positions of the figures. Fig. 5 is the seventh section from fig. 1, but the cells a, b, and c are inserted from the eighth and ninth sections, while fig. 6 is the tenth section and the most lateral of the series.

Figs. 5 and 6 show the origin of the primary setal matrices ; they arise on each side from a longitudinal row of cells which is, I have little doubt, the row arising from Wilson’s lateral teloblasts.¹ These cells, at first very superficial, take up as they pass forwards a deep-lying position within the coelom. At first there is one cell on each side in each segment lying close to the nephridial cell (fig. 5). Their further history is shown in figs. 7 and 8; fig. 8 comes from near the median line a little in front of fig. 4, and includes a portion of the nerve-cord ; fig. 7 lies a little behind and to one side of fig. 8. The setal matrices are now seen to consist of several cells. There is not the slightest difficulty in tracing these structures forwards from segment to segment until they are old enough to have developed setæ.

The embryos of Moniligaster (probably M.sapphirinaoides) which I have in these early stages show also very clearly that the primary setal matrices take their origin from continuous longitudinal rows of cells, but neither here nor in Mahbenus, at any rate in the youngest stages which I have examined, are any of the teloblasts except the “mesoblasts “as specially enlarged as they are in Lumbricus.

I gather from Bergh (‘Zeit. f. w. Zool.,’ Bd. 1, p. 523) that Wilson has, in a paper which I have unfortunately been unable to see, shown that “the outer setigerous glands arise from the lateral cell-cord,” and that he recognises a “setiblast.” My observations, which were originally made without any knowledge of the previous literature, corroborate Wilson’s, but go further, as I show that all the setal matrices, however many there may be, arise on each side of the body as a cell-cord. My observations on the development of other organs are still very incomplete, but so far as they go they have led me to the conclusion that all the organs which express a metameric segmentation arise from the cords of cells which grow forwards in the germ bands. The mesoblastic bands give rise, I believe, to the muscles of the septa and coelomic epithelium and blood-vessels only, while the muscles of the body-wall which do not exhibit metamerism arise in some other way. I find no evidence that they arise directly from epiblast, but a certain amount that they arise at an early stage from the primary mesoblasts, and perhaps also from the mesoblastic cords before these have become segmented, growing outwards in all directions, and not in that direction alone which is taken by the mesoblastic cords. Certain it is that at a time when the germ bands are still in their infancy muscles are to be found underlying the whole of the epiblast. Besides the muscles of the body-wall, the epidermis, and the alimentary tract, the only other organs which do not arise from the germ bands exhibit no metamerism in the embryo. The occurrence of such organs as the gizzard in such varying segments in different worms would be explained if we can show that what segmentation the alimentary tract possesses has nothing to do with the metameric repetition of other organs. I have further, like Bergh, come to the conclusion that the nerve-cord arises from two distinct matrices, and that the ganglia are the only structures which arise from the neuroblastic cords. The above theory will, I foresee, land us in great difficulties when we consider organisms other than worms, and I put it forward with great diffidence and in a purely tentative manner.

The primary setal matrices grow in each segment laterally, and also to a lesser degree towards the ventral median line, as soon as they have taken up their position in the cœlom and become covered by coelomic epithelium.

The primary matriciel cells become segregated to form the secondary matrices. The number of these formed in each segment varies ; in Moniligaster each seta couple arises from a single matrIX, which accounts for the fact that the two setæ of the couple are always so close together. This is also the case in Lumbricus.

In Acanthodrilus sp. the setæ are slightly “separated,” and each seta develops from a separate secondary matrIX. In Urochæta there may be three or two secondary matrices on either side, according as the setæ are “scattered “or not in a particular segment on that side (fig. 16). The exact arrangement which obtains in regard to this matter in “Perichaeta” pellucidais described below, but, speaking generally, in this form and in M. imperatrIX and in Perionyx saltans each seta arises from a separate secondary matrIX.

All the secondary matrices on each side of the body remain connected together for a longer or shorter time by a band of tissue which stands out freely into the body-cavity, and is composed of the coelomic epithelium cells which covered the primary matriciel cells as they grew out to form the secondary matrices, and may be termed the intermatricial band (figs. 20, 24, &c., im. ; cf. Vejdovsky, ‘Entwickelungsgeschichtliche Untersuchungen,’ Heft 3, Tab. xXIii, fig. 19, i.f., and Tab. xxVIII, fig. 7, im.).

Fig. 9 is a portion of an embryo of Moniligaster flattened out, drawn with a camera lucida and Zeiss BB, oc. 3, magnified about 150 diameters. At p. m. is seen a series of primary setal matrices, the more anterior ones commencing to grow outwards ; in the four segments (im.) the formation of the two secondary matrices is taking place, these being connected by the intermatricial band. This band disappears very early in Moniligaster, and in the anterior segments are seen inner and outer secondary matrices, as at s. m. i. and s. m. o. In the most anterior segment drawn, a seta (set.) is appearing in the inner secondary matrIX. The embryo from which this figure is drawn has about fifty segments in front of those drawn, but it is not possible to count any segments behind those drawn,i.e. they are not distinguishable as segments in such a preparation ; longitudinal sections enable one to count them further back.

Figs. 10—13 show the development of the secondary matrices in Moniligaster studied in sections. They are taken from a series cut transversely through a portion of an embryo of about the same age as fig. 9, and which had been flattened out in the same manner. They represent the matrices of four consecutive segments ; three or four sections are omitted between those figured. The upper part of each figure lies near the nerve-cord. They are drawn with a camera lucida to the same scale as figs. 1—7. Figs. 10 and 11 show primary matrices, that in fig. 11 projecting freely into the coelom, while that in fig. 10 does not ; and in these segments there is quite clearly no other setal matrIX on either side of the nerve-cord. In the next segment in front, the primary matrIX has given rise to two secondary matrices on each side, those of one side being shown in fig. 13 ; four inches of the drawing have been cut out in the middle to place it upon the plate. In the next segment in front, the inner and outer secondary matrices are quite separated from one another -, the inner one alone is figured. The epidermic thickening marked x is one of a series of such structures with regard to which I can at present give no further information. I figure it to show that it is not a “seta follicle.” Fig. 14 is a surface VIew of a secondary matrIX, showing the development of the seta couple (set., set.) ; the coelomic epithelium shown all round is omitted from the surface of the matrIX. Fig. 15 shows in a slightly older stage, from a longitudinal section, one of the setæ of a couple.

Fig. 16 shows in a diagrammatic manner a late stage in the development of the secondary matrices in Urochæta. In all the five segments figured, except the most posterior, the primary matrIX has completely separated into two secondary matrices. In the segments marked a and c the outer secondary matrIX is further diVIding into two to place seta 4 in the very dorsal position it occupies in alternate segments.

The investigation of the development of the secondary setal matrices from the primary ones in the Perichætes is attended with great difficulties. To work out the question thoroughly satisfactorily it would be necessary to obtain a series of very thin sections, so accurately transverse to the long aXIs of the embryo that in each segment one section passed through the whole of the region of the future seta ring. No amount of care would with certainty secure such a series, and my efforts in this direction have not been attended with any special luck. I have, however, sufficient eVIdence to prove that the only important difference in this respect between the Perichætes and Moniligaster is in the number of secondary matrices which are produced. In the preparation shown in fig. 33, from the tail end of which the sections (figs. 1—8) that show the origin of the primary matrices were obtained, these primary matrices may be traced growing gradually outwards; older embryos show that they do this until they have grown right round the segment on each side. The cells meanwhile segregate to form the secondary matrices, the segregation commencing at the ventral ends of the matricial bands, and proceeding gradually towards the dorsal region.

The later stages in the process of production of the secondary matrices I have most conveniently studied in Perichæta pelluc ida, but the small size of the embryo prevented my seeing the earlier stages in that form. I think we may fairly assume, considering the close agreement that obtains between two such different forms as a Moniligaster and Mahbenus imperatrIX, that the origin of the primary matrices and the proliferation of their cells to give rise to secondary matrices takes place as in those forms. The production of the secondary matrices in P. pellucida does not, however, take place in regular succession, beginning at the ventral and ending at the dorsal end of the band. It is quite clear that in P. pellucida new secondary matrices are produced between eXIsting ones, either by matriciel cells which separate from the eXIsting matrices and travel along the intermatricial band, or by the diVIsion of any matrIX into two. There is obVIously no essential difference between these two methods, nor is it possible to draw any definite line of demarcation between them, but the latter method prevails in the earlier and the former in the later stages of the production of the full number of secondary matrices.

Fig. 17, from a transverse section of an embryo of P. pellucida, shows a secondary matrIX diVIding into two. Fig. 18, from the same series, shows two secondary matrices connected by the intermatricial band ; one of these is the most ventral of the series belonging to one side of a segment, while the other one shows the band connecting it with the next matrIX beyond.

Figs. 19—25 are VIews of portions of matricial bands of embryos of P. pellucida which had been slit open and flattened out ; they are arranged, with the exception of fig. 23, in the order of their age. Fig. 22 shows the entire matricial bands on one side of three consecutive segments.

In P. pellucida the two secondary matrices first produced from a primary matrIX come to lie at the two ends of an entire band, all the other matrices in the band being subsequently produced. I have deduced this law from the order of appearance of the setæ, assuming, as we fairly may, that the matrices form these setæ on arriVIng at similar ages.

When all the secondary matrices have been developed they come to lie at nearly equal intervals from one another, and the intermatricial band gradually disappears (fig. 25).

In this, as in all other matters relating to the segmentally arranged organs, any segment presents during development a slightly further advanced condition than the segment behind it.

I have studied the order of development of the embryonic setæ in a very large number of embryos of P. pellucida, and figs. 26—28 are given as three typical stages ; they are diagrammatic, but accurate in respect of the number of setæ present. The embryo is supposed to have been slit open and flattened, and all the setæ on one side of the body are shown. V. V. and D. D. represent the median, ventral, and dorsal lines.

These figures show that the setæ develop, as a rule, in couples, that the most ventral couple is the earliest to develop, that the couple which become the most dorsal follow next, and that then with considerable regularity ventral and dorsal couples appear alternately ; further that from a very early stage the appearance of the setæ becomes retarded in Segment n, a retardation which afterwards extends to the next two or three segments (in stages later than those figured).

All the setæ in figs. 26—28 are embryonic setæ, no permanent haVIng yet appeared even in fig. 28.

In Mahbenus and in Perionyx the order in which the setæ appear is different; the most ventral appear first, then those next to them, and so on, the most dorsal appearing last. I expect that this represents a more modified condition; in P. pellucida each segment passes in the condition of its setæ through a stage which remains as the permanent condition in an octochætous form.

In Moniligaster, Acanthodrilus, and Lumbricus the earliest seta to develop on each side in each segment is the seta which becomes seta 1 (i.e. the most ventral), the next to develop is seta 4, the next seta 2, and then seta 3.

Adult Condition and Formation of the Permanent Setæ.—The setæ in the adult P. pellucida in any segment lie at irregular intervals from one another, and the number of the setæ in a ring varies to a small extent. The actual number of setæ in a particular specimen lying on one side of the body in Segments xxv—xxVIi are shown in fig. 29, the relative distance between one seta and another is accurately shown, but the setæ themselves are magnified. It is very rare in the adults of this species to find young setæ ready to replace old ones which drop out, and I believe that the. setæ shown in this figure were the “permanent” setæ formed in the embryo. I term them permanent setæ in contradistinction to the embryonic setæ which precede them. The permanent setæ develop at regular intervals from one another, except for the dorsal and ventral gaps, and a greater number are formed than occur in the adult, so that some drop out, leaVIng those which remain at irregular intervals from one another. The number of those that drop out appears to be nearly constant.

The permanent setæ begin to appear on either side of the dorsal and ventral median lines, one in the immediate neigh-bourhood of each embryonic seta (fig. 30) ; the last to appear are the most laterally placed ones. If 50 setæ develop in the ring (the largest number I have found in this species) seta 1 develops first, then seta 25 on each side, then in fairly regular order, 2, 24, 3, 23, and so on. They always develop first in the most anterior seta-bearing segment (n), and then very regularly backwards. They always begin to appear before the embryonic setæ are VIsible in the most posterior segments. I give three examples :

1. Embryo 25 mm. long, 160 segments counted, with a growing tail in which the segments could not be counted when mounted whole, embryonic setæ VIsible down to Segment LXV, but mere dots after xxx, and four pairs only present in the hindermost of the segments in which setæ are already VIsible (say LV to LXV) ; the permanent setæ are just beginning to develop, and are VIsible in Segments n—v (see fig. 31).

2. Embryo 30 mm. long, embryonic setæ VIsible down to Segment cm, the number of embryonic and permanent setæ in the anterior segments are as under :

   

   

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   The seta rings of Segments n, in, and iv of this worm are shown in fig. 32.

3. Embryo 40 mm., embryonic set æ down to CLXXXVI, the number of embryonic and permanent setae as under:

   

   

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This embryo was much more backward in regard to its seta development ; it had not lost so many embryonic setæ nor developed permanent setæ to such an extent as had the others.

I propose to deal here with certain stages only in the development of these organs—the stages which have a special bearing upon the so-called “plectonephric” condition. I deal chiefly with Mahbenus imperatrIX. All my observations on the early stages corroborate those of Vejdovsky, and the later stages now described resemble very closely those described by that author in dealing with Megascolides australis (^f Archiv für mikr. Anat.,’ Bd. XI).

In an embryo such as that from which figs. 5—8 and 83 are taken it is quite clear that the nephridia arise as paired structures, a pair in every segment except perhaps the first. Nephridia 7 and 16—19, from the right-hand side of this embryo, are shown enlarged in figs. 34, 35.

Each consists of a præseptal funnel, a neck connecting the funnel with the glandular loop, and an excretory duct.

The funnel is at no stage well developed, and is probably never functional, and afterwards entirely degenerates.

This neck becomes afterwards a very important structure, and is dealt with below.

The glandular loop arises by budding from the neck region and rapidly enlarges, ductules develop within it, and it becomes a very complicated structure, as shown in fig. 35 and in outline in fig. 39. It certainly corresponds to a macronephridium of Megascolides, but its further development becomes arrested, and I have been unable to distinguish it in the adult from the loops of the micronephridia which subsequently appear.

It is important to note that owing to the imperfect state of development of the septa these loops are by no means confined to their own segments.

The excretory duct also arises from the neck region as a solid outgrowth (fig. 34, neph. 17 ; and fig. 37, ex. d.) ; it very soon acquires a lumen and opens to the exterior. It elongates rapidly, more than keeping pace with the body-wall in its growth, and the aperture comes to lie very dorsally; the excretory pores of all the nephridia in front of nephridium 17 lie outside the preparation in fig. 33.

The series figs. 36—39, taken from an older embryo, traces the further development of the nephridia ; the figures represent the 7th, 55th, 75th, and 86th nephridia respectively. Fig. 39 represents, therefore, a later stage in the development of the nephridium of fig. 35, but figs. 36—38 represent stages in the development of nephridia which did not eXIst in the embryo of fig. 33, and show that the mode of development alters in the nephridia which are late in appearing.

I have been unable, owing to the advanced stage of development of all the other structures, to fully trace out the nephridium of fig. 39 ; but there is sufficient to show that the neck has undergone great elongation, so that the glandular loop lies very dorsally. The loop itself has become very complicated, and the excretory duct extends away beyond the portion drawn. The great importance of the stage is that it shows the developments which are taking place in the neck region. The cells here are giVIng rise to the secondary loops (n. 1, n. 2, n. 3).

Fig. 38 shows that in a nephridium developing at this late stage the growth in the neck region commences at a relatively much earlier stage. The neck is here already much elongated, while the primary loop is still very undeveloped and the excretory duct has not yet acquired its proper lumen. It is clear that the primary loop is the earliest to develop of a series, and that the whole structure is the homologue of the nephridium of Lumbricus, and, as these secondary loops form or give rise to all the scattered nephridia on the one side of each segment, these are taken as a group, but not indiVIdually, homologous with the Lumbricus nephridium.

Fig. 40 is taken from an old embryo ; n n is a portion of the original neck region, now much attenuated ; a to f are secondary loops which have arisen from it ; of these a, at any rate, has acquired an excretory pore. These loops show that the same sort of difference obtains between a secondary loop which develops late and one which develops early as between a nephridium which develops late and one which develops early.

The secondary loops give rise to tertiary loops as outgrowths from their own neck region. In this way some fifty or more loops develop which ultimately become separated from one another, while each develops its own excretory duct and becomes a micronephridium.

There is no elongation in the neck region and no development of secondary loops in the nephridia of the most anterior segments ; the whole structure aborts in these segments, so that in the adult no nephridia occur in them.

I find absolutely no trace of proVIsional nephridia from a gastrula stage onwards, though, of course, in a sense all the loops which first develop are proVIsional, and I expect that all so-called proVIsional nephridia will be ultimately explained by the fact that the mode of development and ultimate structure, and even continued eXIstence of a nephridium, depends upon its time of development ; and, further, that the nephridia which develop early are not confined by septa to their own segments. This becomes very clear in large embryos like those of Mahbenus.

Fig. 41 shows as much of the anatomy of a micronephridium of an adult as I have been able to make out ; the whole structure as drawn is 0 · 25 mm. long.

In P. pellucida the nephridia develop in the same way as in Mahbenus, certain details excepted, as that the funnel becomes better developed up to a certain stage, up to such a stage as fig. 35. The loop has then the same long wandering character. At this stage all resemblance ends. The loop becomes broader in proportion to its length, no elongation occurs in the neck, no secondary loops are formed, and each segment in the adult never possesses more than a single pair of nephridia. The five most anterior pairs, after haVIng attained a well-developed condition, degenerate and entirely disappear. The most anterior pair of nephridia in the adult belong to Segment VIi.

The nephridia of Segments VIi—XI undergo further modification until their structure somewhat resembles that of the nephridia described by Benham in Microchæta. From a portion of the ordinary loop a number of outgrowths form, into which the tubules run in a very complicated manner, and this bunch of outgrowths ultimately form by far the largest portion of the nephridium.

Fig. 42 is taken from a fortunate preparation obtained by macerating a portion of a full-sized embryo in nitric acid. I found it impossible to make out all the details. At d. is seen the excretory duct, composed of a very regularly arranged series of drain-pipe cells ; b. is the apex of the lobe ; at c. the arrangement of the tubules is very characteristic,—there is a central tubule and a double set of convoluted tubules, shown in the figure by single lines ; at a. is the bunch of outgrowths, each one of which subsequently elongates to a considerable extent.

In a Madras species of Acanthodrilus which has scattered nephridia in the adult I have found the nephridia to develop as paired organs, one pair to each segment, which bears out Beddard’s observations upon this genus (this Journal, vol. xxXIii, part 4). I have inserted fig. 43 of the 17th nephridium from a 10 mm. long embryo of this species, as it showed with absolute clearness the exact course of the greater portion of the ductule from the excretory duct d. up to b. ; from thence onwards to the funnel duct the ductule was too fine to be traced. I have not yet obtained stages of this species showing the development of the micronephridia, but as the process seems to be so similar in such widely different forms as Mahbenus imperatrIX and Megascolides australis it is probably the way in which all micronephridia develop.

My own observations, those of Vejdovsky, and in a less direct way those of Bergh, Wilson, and others who have dealt with development of the nephridia in meganephric forms, and even those of Beddard himself (on the development of A. multiporus), throw grave doubt upon the conclusions arrived at by Beddard and Spencer with regard to this matter. Apart from this I have for some years, upon anatomical grounds, doubted the eXIstence even, of such a condition of the nephridium as was described by Beddard for P. aspergillum. For one thing, Beddard’s figs. 7 and 10 (this Journal, vol. xxVIII, Pl. XXX) do not seem to me to prove what he would have them prove; fig. 10 contains an impossible blood-vessel, branching and returning blood into itself, which looks very much as though there had been some confusion between nephridial tubule and blood-capillary, while fig. 7 hardly shows a continuity of nephridial tubule from segment to segment. Further, both Beddard’s and Spencer’s figures, especially the latter, clearly show that the tubules possess a different character in different regions, and that they are therefore much more specialised than the tubules of Pontobdella; and in no case can one be certain of the continuity of such fine tubules from an examination of sections alone.

For my own part, (1) in spite of repeated and most careful search in preparations from so-called plectonephric worms made in all sorts of ways, I have never seen any connection between one nephridium and another in the adult. If, however, the mode of formation of the micronephridia is always such as I describe for Mahbenus, it is very possible that there are forms in which all or any of the micronephridia on the same side of any segment may remain connected together ; but I very much doubt whether there is ever any continuity from side to side or from segment to segment.

(2) I have in a large worm like Mahbenus imperatrix been able to count the micronephridia belonging to a definite area, and then to mount the cuticle which showed a corresponding number of pores.

(3) The micronephridia have always a complicated structure, similar to that of a meganephridium. This is the fact which first led me to doubt the plectonephric condition, and I have worked out with great trouble as much of the structure of a micronephridium as is shown in fig. 44. This nephridium belongs to Perichæta mirabilis. The micronephridia in this worm are very numerous, and have, or some of them at any rate have, funnels ; these funnels are not præseptal, and present eight marginal cells, ciliated on their centrally directed faces and arranged in a horseshoe fashion, and. one central cell, but no other cells. The funnel is only 0 · 05 mm. in diameter. The coils of tubule are shown in the figure. Fig. 41 shows a similar complicated condition for Mahbenus, and I have obtained similar results in all the micronephridia which I have examined, so that even if they were all connected together into a network and did not develop, as they appear to do, we should have a condition very different from that of Pontobdella, and much more nearly connected with a meganephric condition.

(4) I knew that in Perichæta pellucida and some other species which, although not to be placed, strictly speaking, in the same genus as P. aspergillum, P. mirabilis, &c., are very closely allied forms, one pair only of nephridia were to be found in any segment, which rendered it, at any rate, unlikely that anything so fundamentally different from the mega-nephric condition as the plectonephric condition would occur. (The eXIstence of Perichætes with meganephridia reopens the question of the systematic position of Perionyx.)

I think that the condition of the nephridium in Perichætes, Acanthodrilus, and many other genera must have arisen from the meganephric condition. That the funnel appears, as in Mahbenus and Megascolides, only to disappear, and that the loop which appears earliest in the nephridia which develop first and attains such great complication, only to be arrested in its development or even to abort, to set aside all other considerations, is very strong eVIdence that the development of micronephridia by budding from it, is a specialised condition.

Taking into account what we know of the development of the nephridia in other leeches and the presence of funnels (if M. Bolsius will allow me to call them so), so seemingly out of keeping with the rest of the nephridium in Pontobdella, I shall not be surprised to learn from a history of its development that that is by no means a primitive structure, and has possibly no genetic relationship with the tubules of Platyhelminths.

I am unable at present to bring forward any direct eVIdence as to the relationship between genital ducts and nephridia in earthworms, but I cannot refrain upon this occasion from pointing out that the demonstration of the fact that a so-called plectonephric condition is not a primitive one removes the strongest objection to the theory brought forward by Lankester in one of his earliest contributions to this Journal,

Illustrating Professor A. G. Bourne’s paper “On Certain Points in the Development and Anatomy of some Earthworms?’

FIGS. 1—8.—From Mahbenus imperatrix, fully described in the text.

Fig. 1. M., m. Primary “mesoblasts.” Ep. Epidermis, al. ep. Alimentary epithelium (the cells fully shown here and in Fig. 8, partially so only in Figs. 2—7). Muse. Cells which give rise to the muscle of the body-wall (their presence here at this stage will be discussed in a subsequent paper), x. marks a corresponding spot in Figs. 1—3.

Fig. 2. M. Row of cells growing from the primary “mesoblast.” Other letters as before.

Fig. 3. Letters as before.

Fig. 4. N. Neuroblast, n. Cells forming nerve-cord. Sept. Septum. Cœl. ep. Cœlomic epithelium. The line A B in this and Figs. 5 and 6 marks a corresponding level. Other letters as before.

Fig. 5. a, b, c. Row of cells giVIng rise to primary setal matrices, as s.,s. Neph. Nephridia! matrix.

Fig. 6. s. Continuation backwards of the row a, b, e, of the preceding figure. Neph. Nephridial matrices ; the actual nephroblast does not come into this section.

Fig. 7. Letters as before.

Fig. 8. Letters as before.

FIGS. 9—15.—Moniligaster (probably M. sapphirinaoides).

Fig. 9. Portion of an embryo flattened out. Neph. Young nephridia. n. Portion of the nerve-cord. Sept. Septa, p. m. Primary setal matrices, i. m. Secondary setal matrices in process of formation. S. tn. i. and S. tn. o. Inner and outer secondary setal matrix. Set. Rudiment of a seta. x. Series of large cells of unknown significance.

Figs. 10—13. Transverse sections, described in the text. p. m. Primary setal matrix. S. m. i. and S m. o. Inner and outer secondary setal matrix. Neph. Nephridium. Ep. Epidermis, x. Epidermic ingrowth.

Fig. 14. Surface VIew of an advanced secondary setal matrix, set., set. The two setæ of a couple.

Fig. 15. Similar structure from a longitudinal section.

FIG. 16.—Urochæta sp. Secondary setal matrices of one side of the five segments a—e. v. v. Ventral median line. Ant. and Post, mark the anterior and posterior regions.

FIGS. 17—32.—Perichæta pellucida; Fig. 33. Mahbenus imperatrix.

Figs. 17—21. Secondary setal matrices diVIding before the appearance of setæ. 8. tn. Secondary matrices, i. m. Intermatricial bands. Cost. ep. Coelomic epithelium, ep. Epidermis.

Fig. 22. Portions of the setal matrices of two consecutive segments with developing setæ. In the lower portion of the figure which is anterior two new setæ are shown developing between an older pair.

Fig. 23. The entire setal matrices on one side of the body in three consecutive segments. V. Ventral end. D. Dorsal end. Setæ are seen developing in the most ventral and most dorsal regions, the former being the elder.

Figs. 24 and 25. Later stages in the development of the secondary matrices.

Figs. 26—28. Diagrams showing the exact number of setæ on one side of the body in the segments marked II, m, IV, &c., in three embryos of different ages. V. V. and D. D. mark the ventral and dorsal median lines.

Fig. 29. A similar diagram taken from an adult worm.

Fig. 30. Portion of a matrix in which permanent setæ are beginning to develop alongside the embryonic setæ.

Fig. 31. The complete seta ring of Segment n of an embryo 25 mm. long. The long fine lines represent the embryonic setæ, the short thick lines the permanent ones.

Fig. 32. The complete seta rings of Segments II, III, and IV of an older embryo from which the embryonic setæ have nearly all dropped in these anterior segments ; those left are shown by the small fine lines, of which there are 6,12, and 19 in the different segments.

Fig. 33. An embryo of Mahbenus imperatrix showing the entire series of nephridia. The references Neph. 1, Neph. 2, &c., are connected with the excretory ducts ; the excretory pores themselves are within the limits of the preparation in the case of nephridia 7 backwards. S. m. Setal matrices. Sept. Septa torn during the removal of the alimentary wall.

FIGS. 34—41. Mahbenus imperatrix; Fig. 42. Perichæta pellucida; Fig. 43. Acanthodrilus sp. ; Fig. 44. Perichæta mirabilis.

Figs 34 and 35. Enlarged VIews of some of the nephridia from the prepaparation, Fig. 33. Letters as before.

Figs. 36—39. Series of nephridia from an older embryo of Mahbenus. fun. Funnel. NZ. Neck region. I Glandular loop. Ear. d. Excretory duct. n. 1, n. 2, n. 3. Micronephridia beginning to develop.

Fig. 40. From a still older embryo, n. n. Portion of the neck region of a nephridium such as Fig. 39. a, b, c, d, e, f. Micronephridia developing from it. g. Micronephridium developing from a. Ex. d. Excretory duct. Ex. p. Excretory pore. s. m. Setal matrices, i. m. Intermatricial band.

Fig. 41. A micronephridium from an adult Mahbenus.

Fig. 42. A nephridium from one of the Segments VII—XI of a full-sized embryo of P. pellucida.

Fig. 43. A nephridium from an embryo of Acanthodrilus sp. B. W. Body-wall. Sept. Septum, fun. Funnel, a. Duct from funnel. From b to d the exact course of the duct is shown, d passes downwards to the excretory pore.

Fig. 44. A micronephridium from an adult Perichæta mirabilis. fun. Funnel (not præseptal). Ex. d. Excretory duct.