With Plates 6—8.
ON SOME POINTS IN THE STRUCTURE OF EUPHROSYNE
IN the circumstances under which the present paper is contributed, it is perhaps fitting that a form should be selected (viz. Euphrosyne) which first came under my notice by the kindness of Professor Ray Lankester, who sent preparations procured in Herm in 1865. When an opportunity occurred a few years later (1868) of becoming familiar with living examples on the same ground, unfortunately time did not permit a detailed investigation of internal structure, and accordingly I have had to content myself with the examination of rather imperfect spirit-specimens.
While the structure of the genus Spinther, Johnston, has been more or less fully elucidated by the labours of R. von Drasch (‘Anatomie von Spinther miniaceus,’ Grube, 1885), and especially by those of Ludwig von Graff (‘Die Anneliden-gattung Spinther,’ 1887), the present genus, so far as I can ascertain, has received attention only from Schmarda1 and E. Ehlers;2 but at the date of their treatises the modern methods of microscopic investigation had not been introduced. The sections were made from examples of Euphrosyne foliosa, Aud. and Ed., and a few from E. cirrata, Sars, a northern species, kindly sent by Canon Norman; and I have to thank Mr. C. H. Williamson, M.A., B.Sc., for preparing and mounting the slides ; and also for aid in this respect from Mr. A. T. Masterman, B.A. (Cantab.).
The cuticle is of considerable thickness, and in the dorsal region and some other parts has externally in the preparations a striated granular coat, which may be connected with the presence of cilia. It is, on the whole, somewhat thicker than in Spinther. The hypoderm beneath has the usual structure, and is very thin in the mid-dorsal line, but increases laterally, and again ventrally. The inner edge is sharply defined, so as to separate the circular muscular coat distinctly, but a definite basement-layer does not appear to be formed. Proportionately this layer is somewhat less developed than in Spinther. The circular muscular coat lies immediately beneath the hypoderm in the form of a continuous sheath, though in the dorso-lateral regions it is modified; the main mass, however, bounding the perivisceral chamber and forming a thick ventral layer, pierced here and there by the fibres of the vertical and oblique muscles. In the lateral regions a radiate arrangement of powerful muscles takes place in connection with the bristles ; indeed, a double radiate arrangement is present in many sections where the upper and lower series of bristles come in the way of the knife.
The longitudinal muscular layer forms dorsally a series of fine fasciculi beneath the circular, and it follows the latter in its course outside the perivisceral chamber, attaining its maximum thickness after the splitting of the circular coat. Ventrally the powerful longitudinal muscles are grouped in large fasciculi separated by the vertical or oblique strands which pass to the circular layer. The fibres from the oblique muscles decussate beneath the nerve-cords, many mingling with the circular (Pl. 7, fig. 4). The latter feature is much less evident in Spinther; indeed, none of von Graff’s figures indicate it, and no allusion thereto occurs in his description.
In horizontal section this highly sensitive tongue-shaped organ presents externally a thin layer of cuticle, the greater part of the area internally being formed of hypodermic cells and granules. Posteriorly, in the middle line, however, is a double fibrous band, with at intervals various lateral fibres, which extend not only into the lateral regions but also into the narrow spaces between the bands. About the middle of the organ one band becomes prominent and its fibres spread more and more outward on each side until the eyes are reached. These fibres are probably contractile. In such a section as in Pl. 1, fig. 1, the pennate arrangement of the median fibres is conspicuous, and they merge laterally into the hypoderm. Moreover, in this section the complex muscular fibres which intercross at the margins, and which apparently are connected with the flattening of the organ, and other changes in its outline are clearly shown.
In vertical section (Pl. 6, fig. 2) the caruncle has a more or less radiate appearance, due to the fibres which diverge from the inferior area into the surrounding cellular stroma of the hypoderm. The area referred to increases in size from behind forward, and in many views has a definite boundary or arch of decussating fibres superiorly. It (that is, the space below the arch) is occupied by a series of vertical fibres which radiate superiorly into the substance of the caruncle, and cause the somewhat arborescent appearance of the organ in section. While, therefore, the lower fibres in the area just mentioned are vertical, the upper are less definitely so. On the other hand, the fibres are almost wholly vertical in the small or undeveloped condition of the area posteriorly, since they pass straight upward to diverge into the tissue of the caruncle. Below the latter the muscular fibres form a powerful band, which by-and-by are interlaced with longitudinal fibres in conspicuous meshes (Pl. 6, figs. 2 and 3). The origin of these fibres would appear to be the raphe above the folds of the mouth. The area diminishes anteriorly, and seems to be occupied chiefly by the cut ends of longitudinal fibres, while the fibres bounding it form a dense fillet. In vertical section (Pl. 6, fig. 2) the radiate arrangement of the fibres from the arch or fillet is well shown, and they constitute a series of loops or meshes between the arch and the hypoderm. The area itself is occupied by vertical and oblique fibres which come from the dorsum in powerful bands, and which have longitudinal fibres here and there in their meshes.
In Euphrosyne cirrata, Sars, from Norway, a conspicuous strand of fibres passes from the nerve-masses (now at the ventral surface) to the caruncle (Pl. 7, fig. 1), while other fibres converge from the neighbouring region to the same organ. Immediately over the cephalic ganglia in this species the area presents only fine fibres at the sides of the median space, and a large amount of opaque granular matter occurs at the sides of the organ, partly mixed with the hypodermic tissue and partly in a special capsule at each side, and apparently represents a pair of glandular organs. The granular substance has not been stained. Posteriorly (before the termination of the cephalic ganglia) well-marked median and radiate fibres appear in the caruncle ; then bands pass from the lateral regions both into the latter directly and into the median longitudinal band in front of the descending nerve-mass. A very complete raphe is formed, and the distribution of the fibres is so arranged that the organ and the region below it can be directly pulled. The caruncle can thus be elevated or depressed. After the nerve-mass has become wholly ventral, strong muscular fasciculi pass from the ventral wall to the base of the caruncle, and spread out on each side of it; then the band is chiefly median and attached to the base and to the small knob representing the organ posteriorly. The caruncle in this species differs considerably in external form from that of Euphrosyne foliosa, being more or less elongate and free, and the tentacle is like-wise filiform. The latter merges into the base of the former anteriorly (Pl. 7, fig. 1).
In Euphrosyne foliosa a pair of closely approximated eyes lie at the anterior part of the caruncle (Pl. 6, fig. 3, ocd.), and another even more closely approximated pair on the ventral surface of the snout in front of the two prominent pads of the mouth (ibid., ocv.) and thus separated from the dorsal pair by a considerable interval. Functionally thus the one pair serve as organs of vision dorsally, the other for use ventrally. The eyes have a distinct capsule with a broad margin of pale columnar cells, within which is the dense black pigment.
In Spinther the tentacle is apparently homologous with the caruncle in the present genus. It lies over the cephalic nerve-mass, is supplied with two large nerves, four eyes, and considerably developed hypoderm, but the latter and the muscular strands are much more largely developed in Euphrosyne, and the organ is more complex. Vertical muscles pass from the nervecords inferiorly in Spinther, and other muscles from the lateral regions outside the pharynx, but they are less developed than in Euphrosyne. The situation of the eyes in Spinther, however, diverges, since both pairs are dorsal in position, being located at the anterior and posterior margins of the tentacle. The figures of Drasche1 and von Graff,2 as well as my own sections of Spinther miniaceus, Grube,3 show that the minute structure of the eyes is the same in both genera.
In Euphrosyne cirrata the dorsal eyes lie on each side of the anterior region formed by the fusion of the tentacle with the caruncle. They have a somewhat radiate arrangement of the clear vesicles with a dense ring of pigment, and lie in the hypoderm — with the cuticle externally. The ventral pair are more widely separated than the dorsal, but have the same structure and relations to the hypoderm and cuticle.
Branchiæ.—Externally are the cuticle and its cilia—with the thick hypodermic layer beneath—both layers being continuous with those of the body-wall. Moreover, vertical bands of muscular fibres pass through the circular and other layers to the bases of the branchial stems, enter the latter, and form the central longitudinal fibres. The shortening and elongation of the organs is thus explained. In vertical sections (Pl. 6, fig. 6) many transverse fibres occur between the longitudinal, often crossing at right angles to the latter. Though in such sections the fibres appear to be characteristically transverse, yet in transverse sections of the basal region (Pl. 6, fig. 4) they present a radiate aspect, passing in front and behind the lumen of the blood-vessel on each side.
These radiate fibres will readily alter the calibre of the stems, expansion again occurring probably by the elongation of the longitudinal fibres and the distension caused by the blood. The inner margin of each blood-vessel is well defined, apparently by a special coat, while the coagulated blood occupies the central space. Externally the connective tissue and other fibres of the branchial stem are closely united with the vessel, so that no separation other than what has been mentioned exists. In longitudinal sections in which the two vessels are slit symmetrically, the lateral regions are occupied by the latter (vessels), the median by transverse and longitudinal fibres. The constricted region below the tip is circular in transverse section, with the hypodermic cells radially arranged around a central point, for the blood-vessels are now absent (reaching only to the commencement of this narrow region, as in Pl. 6, fig. 5), and the fibres pass into the centre of the dilated terminal region almost to the tip. In the terminal part the large cells of the hypoderm (which give it a vesicular appearance) are also somewhat radially arranged—often sloping from the central axis outward and upward in vertical section, or placed regularly around the central axis in horizontal sections. The tissue in these dilated regions is therefore much more lax than in the constricted region beneath, as Ehlers observed. He also describes a slender, circular muscular coat externally (that is, within the hypoderm), but the preparations did not satisfy me on this point, though in Euphrosyne cirrata certain transverse wrinkles were seen at the base in longitudinal sections. Besides, the arrangement of the radial fibres would indicate that the functions of such a coat are fulfilled by other means. Shortening and elongation of the organs may occur without the presence of circular fibres.
The absence of specially developed branchiæ in Spinther is interesting, but the perivisceral fluid as well as the blood-vessels are in this form nearer the surface, and the great hypodermic membranous flaps on the dorsum may in some respects also subserve this function. The intimate connection of the lateral lamellæ with the bristles may also prove of importance, since the muscles of the bristles must cause extension of the membranous lamellæ.
Externally is the somewhat granular layer covering the cuticle, a condition probably due to the cilia, which Ehlers describes as being largely developed. A thick layer of hypoderm occurs beneath, with large cells here and there. Both layers are continuous with those on the body. The hypoderm at the extremity of the cirrus is finely granular and longitudinally streaked, and the cuticle of this region is very thin. Vertical fibres from the body pass upward into the base of the cirrus and thence to the tip of the organ. Only a few circular muscular fibres were observed at the base of the cirrus (under the hypoderm), the elasticity of the cuticle probably sufficing to restore the shape of the organ on relaxation of the longitudinal fibres. These organs are not represented in Spinther.
On viewing the alimentary canal from the dorsum in a spirit preparation of Euphrosyne fo’Jiosa (Pl. 7, fig. 2) the proboscidian region (Schlundkopf, Schmarda) has a somewhat ovoid outline, and is slightly narrower in front than behind. It is divisible into two regions, for anteriorly a glistening whitish layer (a) envelops like a sheath rather more than the anterior third, splitting in the middle line and curving outward on each side, so that its outline resembles that of a bivalve shell. Inferiorly this sheath preserves an unbroken transverse border posteriorly. The enlarged posterior region is free beneath, but dorsally terminates in a canal (b) connecting it with the intestine at d. The outline given in Pl. 7 differs considerably from the organ in Sch m ard a’sEuphrosyne p o 1 y b r a n ch i a, in which the region presents a series of frills posteriorly and only two papillæ or bosses in front.
Behind and above the rounded muscular mass of the protrusible pharynx is a chamber with thinner walls, and having laterally a series of well-marked rugæ (e). This chamber may represent the stomach, as Ehlers states, and it is connected with the intestine at d as above mentioned.
The mouth (Pl. 6, figs. 3 and 7, w) opens on the ventral surface between the third and the fifth segments, as Ehlers describes in E. racemos a, and the walls of the buccal chamber are thrown into many complex folds (w,f) which externally have a cuticular coat. These folds are continuous with the walls of the proboscis. In transverse section the latter presents in front a closely interwoven series of muscular fibres chiefly circular and oblique, though vertical also occur. Moreover, large blood-vessels are visible ventrally at the sides. The eversible portion of the. organ has both inner and outer surfaces coated with the cuticle (which is stained), and the finely granular and streaked hypoderm beneath is well marked, besides certain muscular fibres passing into the bases of the papillæ. In all probability it is the latter processes which Schmarda describes as horny teeth in Euphrosyne poly-branchia.
The buccal chamber gradually enlarges into a flattened canal above (i. e. dorsal of) the posterior portion of the great muscular “stem “of the proboscis. The latter (stem) is composed of a complex series of fibres, the ventral being chiefly arranged in parallel and vertical bundles, and bounded by a definite investment, a few longitudinal fibres being clasped in the interstices. Moreover, this region is cut off by a thin cuticular septum from the part above. Then the median region is occupied by a dense mass of glandular tissue (Pl. 6, fig. 7, gl.), the glands being large and granular and extending to the hypo-dermic coat which, with the cuticle, bounds the chamber now present in the organ at this part. These glands apparently perform an important part in the functions of the region. The upper arch again has transverse muscular fibres close to the hypodermic border. A powerful muscular mass exists above, ending in the thin hypodermic and dense cuticular layers bounding the floor of the upper canal (Pl. 7, fig. 3). The opposite or upper face of the canal has a very thin cuticular coat and a thick glandular (hypodermic) layer. Folds or ridges next appear at the sides (Pl. 6, fig. 7, e) where the dorsal arch joins the muscular stem of the proboscis, and they pass dorsalwards on this arch in the form of an extended ridge on each side in transverse section, the surface being hypodermic, while beneath is areolated glandular tissue. Proceeding backward these lateral ridges increase in size, and instead of an even surface show prominent ridges, while by-and-by a belt of this folded tissue passes entirely across the upper chamber. Inferiorly the stem of the proboscis now presents a somewhat regularly interwoven field of cross-fibres like mesh-work, bounded ventrally by a rim of longitudinal fibres, the upper edge being defined by a cuticular investment. Beyond the longitudinal belt at the ventral border of the decussating fibres is a broad belt of vertical fibres.
The chamber soon shows a regular series of transverse ridges of hypodermic glandular tissue, and then terminates in the intestine. Externally is a layer of longitudinal fibres, internally a coat Of circular fibres, and the glandular lining has cilia on its surface.
In longitudinal section (Pl. 6, fig. 7) the mouth has various prominent folds of the lining membrane in front, but the anterior region of the proboscis in this species did not present the regular series of papillæ indicated in the figure of Professor Ehlers. The main channel passes dorsally, and reaches the thin-walled chamber marked bs. in fig. 7, Pl. 6. The dorsal wall agrees with that shown in Pl. 7, fig. 3, though the cuticular layer is less marked, and extends to the opening into the intestine. In this chamber the ventral surface is formed by a portion with bold ridges (br., fig. 7, Pl. 6, where a part only is shown, as the section is not median). In the ordinary or retracted condition of the parts the food would thus be acted on by the area with the ridges and the powerful muscles beneath, so that—between muscles and glands—a considerable alteration probably ensues. There are grounds, therefore, for thinking that this chamber represents the stomach, since its posterior end leads by a short canal directly into the intestine.
The intestine follows the short canal just mentioned, and consists of a wide passage, usually thrown in the preparations into deep folds—with a few shallow diverticula of its walls ; but, so far as spirit-preparations can be relied on, the diverticula do not attain the dimensions shown by Ehlers in his E. race-mos a.1 The canal has a thick wall of densely granular glandular tissue, papillose internally, and of circular fibres externally. It continues backward and terminates posteriorly in the vent, which is situated between two prominent lips just in front of the caudal papillae. A differentiation of the gut occurs at the rectum, so distinct that in transverse sections it appears at first sight that an independent channel exists posteriorly. The continuity of the mucous membrane is, however, easily made out. In this region, which is somewhat triangular in transverse section, the thin membranous investment has beneath it a layer of longitudinal fibres, and the adjacent granular cells more resemble those of the hypoderm than the homologous cells of the intestine. Portions of food and sand are occasionally observed in the centre of this portion of the canal.
The food consists in some of a soft mass in which cells, spicules of sponges, chitinous fragments, and other débris are present ; while in others little else than sponge-spicules can be seen. Posteriorly the cylindrical fæcal mass usually occupies the centre of the gut. In Schniarda’s species calcareous fragments, bristles of Annelids, and sponge-spicules were found. It may readily be concluded, therefore, that it is difficult to secure either perfection or continuity in transverse sections of the body.
The digestive system of Spinther differs from the foregoing in regard to the much more largely developed lateral caeca of the gut, and the less massively muscular proboscis. The presence of an extensive dorsal blind-gut is another important divergence. Both, however, feed for the most part on the same substances.
The central mass of the nervous system lies immediately under the caruncle, and consists dorsally chiefly of nerve-cells, ventrally of fibres (Pl. 6, figs. 3 and 8, eg.).
The position of the eyes (Pl. 6, fig. 3, ocd. and ocv.) in Euphrosyne diverges from that in Spinther, for in the latter the four eyes are confined to the dorsum, whereas in the former two are dorsal and two anterior and ventral.
The suboesophageal ganglia form a single mass behind the mouth, with a fibrous central region and two lateral cellular masses as in E. cirrata.
The ventral nerve-cords (which Ehlers states are non-gangliated) lie on each side of the median line enveloped in their sheath, and partly separated superiorly by a fascicle of longitudinal muscular fibres (Pl. 7, fig. 4, ne.). The oblique muscles pass down by their external borders to decussate inferiorly. With the exception of the capsule of connective tissue, the cords in section present a granular surface from the cut extremities of the fibres. Beneath the fascicle of longitudinal fibres is a curved strand of transverse fibres (Pl. 7, fig. 4, cm.), which forms a commissure between the cords. It is concave from above downwards. In many sections a strand of fibres closes in the space for the longitudinal muscular fibres superiorly. External to the cords are flattened bands of muscular fibres, the decussating fibres of the oblique muscles,. and the circular coat with the cutaneous tissues (Pl. 7, fig. 4).
In comparing these cords with those of Euphrosyne capensis, the same fascicle of longitudinal muscular fibres and the firm investment superiorly are characteristic. In Euphrosyne borealis the muscular parts are specially massive. In E. cirrata, again, the nerve-cords, which lie close to the hypoderm, are flattened in section and much more widely separated. A transverse commissure, however, is present.
In Spinther ganglionic enlargements and transverse commissures similarly placed are shown by von Graff. He also describes optic, pharyngeal, oesophageal, and other nerves from the cephalic ganglia.
Schmarda was of opinion that in Euphrosyne polybranchia the circulation agreed with the general type of the Annelids. In the midventral line a large vessel occurs, with two smaller lateral trunks which supply the ovaries. The unpaired vessel gives many branches to the alimentary canal. On the dorsum of the alimentary canal are two vessels, from each of which large branches go to the branchi æ. He finds that the vessels have two coats, an outer longitudinal and an inner circular (his transverse) ; while the blood is red, and shows corpuscles in size. Ehlers adds nothing to this description except to observe that the blood in Euphrosyne racemosa is colourless.
In E. foliosa the ventral vessels are distended by the yellowish and minutely granular blood (in the preparations), and various branches are observed amongst the reproductive elements, but the sections are less satisfactory as regards the dorsal vessels. All that can be said is that a dorsal trunk was seeu in certain sections over the alimentary canal, and most frequently in a state of contraction. The inner coat of the larger vessels appears to be a homogeneous elastic one, no circular striæ having been observed, though such may be present in other forms. The outer coat certainly has longitudinal fibres.
At the anterior part of the intestine the sections1 of four or five trunks are seen in the mid-dorsal line close to the gut or amongst the reproductive elements. All these are empty, the collapsed vessel having a dotted or cellular appearance, and thus different from those distended with blood in the other parts of the reproductive masses. Proceeding forward so as to bring the duodenal channel with its dense walls and inner surface covered with cilia into view, a grouping of these vessels above the latter channel (which lies over the intestine) takes place, so that a large mass, composed apparently of thick striated walls round a central lumen, is soon formed. It is difficult to say what this is, though it possibly may be a specially contractile region of the vessel. It is shown in its fully-developed condition in Pl. 7, fig. 5, v. g. It then splits into two trunks which gradually separate from each other as we proceed forward, but they always preserve a dorsal position to the duodenal gut, stomach, and proboscis. They diminish in size as they go forward, but they retain the same structure and contractility. Their mode of termination could not be ascertained.
The structure and arrangement of these parts would point to their connection with the vascular system, bút as no vessel contains blood, and as their structure differs from the distended trunks, some doubt exists. Their highly contractile condition may be associated with the functions of the proboscis, which, as formerly shown, has large blood-vessels inferiorly.
Segmental Organs (Nephridia)
Schmarda does not refer to these organs, but Ehlers describes them in his form as reaching to the twentieth segment. They lie under the dorsal wall of the body, and have their external openings in the median line between the branchiae. Each is in the form of an elongated bifid tube, extending over two or three segments. The two inner openings are trumpet-shaped, with orange pigment and internal cilia. The only structures observed in the sections of E. foliosa are the bifid vessels passing over the proboscidian region from the single dorsal tube as described above, and such would appear to pertain to another system.
Schmarda describes the ovaries as funnels or tubes with a blood-vessel in the middle. These tubes end in an oviduct which opens near the vent or near the inner branchiae. The specimens examined by Ehlers had no generative elements. These elements in E. foliosa occur in the perivisceral space around the alimentary canal, and at the bases of the feet (Pl. 6, fig. 1, on., and Pl. 7, figs. 5 and Q,g.p.). In the male the dense masses of sperm-cells are often arranged round the blood-vessels, and with the cells somewhat regularly placed in rows, so as to form long loops, concentric or slightly radiate areas. The linear arrangement of these cells, indeed, is characteristic. The same regions are occupied by the large eggs in the female. The sexual elements were best developed in those obtained in July and August, both in Britain and in Norway, and they probably escape dorsally by the nephridia. Many ova occur amongst the branchiæ and bristles of the dorsum. In regard to the escape of these elements, the most likely channels appeared to be those that passed to the skin near the bristle-bundles.
The ova in Spinther are proportionally as large as in Euphrosyne, but no nephridia have been observed, though it is possible they may yet be found.
ON CERTAIN YOUNG STAGES OF MAGELONA
The occurrence of several stages of the young of Magelona. in the bottom trawl-like tow-net in St. Andrew’s Bay gives an opportunity for taking a brief survey of our knowledge of the subject.
Claparède, in 1863,1 described some young stages of this species which he had procured at St. Vaast la Hougue in the summer of 1861. His youngest form was about 1 mm. in length, and cylindrical. The anterior end formed a wide funnel, and thus differed from any larvæ hitherto found at St. Andrews. The border of the funnel was beset with cilia, and cilia occurred behind it. Four reddish eyes occurred transversely on the dorsum of the head. Between the broad cephalic region and the first body-segment is an expanded region bearing a tuft of long cilia. There are from fifteen to twenty body-segments—which are widest anteriorly. From the projecting process of the first body-segment proceeds the tuft of long smooth bristles, which are almost as long as the body. The following segments had touches of brownish pigment, a pair of small hook-pads, and short bristles. Each segment had a ventral band of cilia, whereas none occurred on the dorsum. The terminal segment had a ring of long cilia. The alimentary canal was roseate. This larva is distinguished, he says, from a Leucodore larva by the dilated anterior end, by the brownish pigment-touches, the smooth bristles, by the ventral cilia on all the segments, and by their absence on the dorsal surface.
It exhibits by-and-by a modification of the head, which becomes elongate heart-shaped, and by the development of lateral papillæ from which the tentacles grow behind the eyes. The provisional swimming apparatus, viz. the bands on the head, the lateral tufts, the ventral bands, and the anal ring are all increased.
When it reaches the length of about 2 mm. it approaches a Spio larva, and swims freely in the water by a rapid wriggling of the body. The heart-shaped snout is narrowed in front, concave below, and convex above. The four reddish eyes are larger, and the middle pair placed in advance. The first body-segment overlaps the head, and from its sides spring the short tentacles, which are borne like a pair of horns, the inner side of each being marked by brownish parallel striæ, and the slender papillæ are 0 · 017 mm. long. The tentacle has a central blood-vessel which ends blindly, and its blood contains corpuscles. The first segment bears the huge bristles, while the last is elongated, cylindrical, and is devoid of bristles. The rows of hooks occur from the ninth segment, some of them in sacs. They are more numerous posteriorly (to the number of fifteen). The posterior end is pointed and bears small, colourless, pyriform (birnformige) papillæ. The digestive canal begins with an oval mouth and a muscular proboscis, which has a pair of opaque (brownish) glands at its posterior end. The canal, which is constricted in front, dilates at this point. The colour of the larva is delicate brownish, with parallel bands of the same hue on the tentacles ; longitudinal and transverse brownish touches on the snout. The translucent anterior region and the three following segments bear similar touches. The middle fifteen segments of the body have brownish pigment-rings. The following segments have lateral touches of the same colour, which posteriorly almost assume the form of rings. The tip of the tail is brownish, and the same tint occurs anteriorly at the lips of the proboscis.
His next stage presents elongated tentacles with long papillae and coiled like ram’s horns, yet the snout is even shorter than in the foregoing stages, so that a certain amount of variation exists. The length is 8 mm., and it swims like an eel. Claparède compares the elongated papillae with the peculiar “stabchen “on the tentacles of Spio as described by Strethill Wright, but the differences are considerable. The first body-segment bears the long bristles. From the second to the eighth segment the lateral bristles have disappeared. From the ninth to the fifteenth long provisional bristles as well as hooks with an elongated shaft are present. The brownish glands behind the proboscis are larger. The blood has a bluish appearance. The end-segment is hoof-like and somewhat resembles that of Leucodore, and it has small coloured warts.
He concludes by mentioning that further stages of Mage-Ion a were not got in his nets, and that they probably took to the bottom to burrow in the sand at the extreme margin of low water. He was inclined to relegate the larva to the Spionidæ.
On the present occasion only the more advanced Spio-like forms, approaching Claparède’s two last stages, figured in his pl. x, figs. 10 and 12, will be dealt with. The first was obtained on the 28th May, and had about twenty-five segments exclusive of the head and tail. The animal (Pl. 8, fig. 1) is nearly translucent, faint touches of white pigment appearing on the sides of the proboscis posteriorly, at the commencement of the narrow part of the gut, and at the base of the tentacles. An opaque white mass (the contents) also marks the centre of the gut towards the tip of the tail. Lateral opaque white specks in groups further indicate the segments in the anterior (dilated) region of the body. The latter consists of nine segments, the tenth being opposite the whitish opacity (a, Pl. 8, fig. 1) marking the commencement of the constricted portion of the gut. Besides the anterior long bristles, there are several pairs on the sides posteriorly, as shown by Claparède, who figures four pairs in Magelona (ibid., Taf. x, fig. 12). In a coloured drawing of one at this stage by Mr. J. Pentland Smith, M.A., B.Sc., about seven pairs are present. The tentacles are longer than the body, and contain only a single vessel, as in Claparède’s stage of Magelona of 2 mm., and no circulation is yet visible, though a few stationary corpuscles are observed on the wall of the vessel. In this condition the Annelid stretches itself freely, with the tentacles widely expanded, and apparently draws in water by the mouth at intervals, to judge from the movements of the gullet. When irritated it coils its tentacles like springs, and wriggles rapidly through the water, as noticed by Claparede in Magelona. Only slender spine-like papillæ occur on these elongated organs, as in S pi o, to which the young form has close affinities. No forward growth of the snout has yet taken place, and therefore this example would appear to belong to another form or to be less developed than ClaparèdeJs fig. 12. Moreover, the eyes are black, not red, and much smaller than he shows. The pigment of the body further is white, not brownish, such, perhaps, being due to variation. As in other species, such forms do not follow the younger in regular succession as regards date, for the spawning period is evidently prolonged. Thus, for instance, the foregoing example was much larger than some examples of Magelona procured at the same time, or even than others found in the middle of June. Moreover, the shedding of the long larval bristles takes place at different periods in specimens of the same or nearly the same age. They are absent, for instance, in the example figured in Pl. 8, fig. 2, though in other respects it agrees in structure with the foregoing.
On the 17th of October (similar forms, however, having been seen earlier) a more advanced specimen of Magelona than described by Claparède was obtained in the bottom trawl-like tow-net (Pl. 8, fig. 3). The basal part of the tentacles is now furnished with larger papillæ, while the slender processes formerly mentioned still exist on the terminal region, as, indeed, was observed in some examples in May. The eyes corresponded with those already figured in the Spio-like larvæ (figs. 1 and 2), and are still much less than in Claparède’s representations. The snout now forms a flattened spathulate process of considerable dimensions, though it is less in proportion than in the adult.1 Moreover, no blood yet enters this region (snout), though the circulation in the tentacles is complete. The latter organs can be fixed to the glass vessel by the papillæ, which thus have adhesive properties. Each of the nine anterior segments is furnished with slightly clavate feet and bristles of considerable length. Then follow the constriction of the alimentary canal (which Claparède does not indicate) and twenty segments, the first eight or nine of the series having still longer bristles than the foregoing division. The dorsal vessel (or vessels) sends powerful currents forward by regular contractions, which form a fold of the trunk opposite the fifth bristle-bundles.2 The opaque white glands of the posterior part of the proboscis were distinct on capture, but they became less visible after confinement in the laboratory.
The Spio-like forms figured in Pl. 8, figs. 1 and 2, thus diverge from the unmistakable larvæ of Magelona figured by Claparède and in fig. 3 of the plate just mentioned in the present paper. They show no transverse striæ at the base of the tentacles, and the proboscidian region of the gullet is much shorter. Opaque white glands, however, occur at the sides of the latter posteriorly, and the arrangement of the eyes, the general contour and the number of segments in the anterior region of the body, are similar. Again, some examples agree with the two figured (Pl. 8, figs. 1 and 2), but have at the base of the tentacles indications of the rugæ which foreshadow the papillæ characteristic of the species in its adult condition ; but such have never presented at St. Andrews the long slender larval papillæ simultaneously in the same region, and which Claparède shows in his fig. 12. Though the young stages of allied members of the Spionidæ are not yet sufficiently known, yet the weight of evidence inclines to the view that the Spio-like larvæ pertain to Magelona, at a stage previous to the appearance of the rugose ridges ushering in the thick cylindrical papillæ. An interesting feature is the disparity in size between the latter examples without forward growth of the snout, and others, fully a third less, with a considerable snout.
ON CLAPARèDE’S UNKNOWN LARVAL SPIO
The first notice of a form apparently identical with the larval Annelid which forms the subject of these remarks is given by Maximilian Müller,1 who alludes to a fragment of the posterior end in connection with his observations on bacillary corpuscles.
When sojourning at St. Vaast la Hougue, between July and September, Claparède2 found not unfrequently an unknown larval Annelid (pertaining to a common form) which has also occurred in considerable numbers in the bottom-nets at St. Andrews from July to October. The same form has been met with on the Norwegian coast at Christiansand.
The youngest stage measures about 0 · 045 mm., and has about twelve bristled segments, besides several without bristles. The head is short and is divided into symmetrical lobes. The ridge is richly ciliated, and differs from the arrangement in the larva of Leucodore. The larger bosses have short cilia and a pair of longer ones. The eyes are arranged more or less on a trapezoid (the posterior pair more widely separated), and have reddish pigment. The mouth lies on the ventral surface between two ciliated lobes. Two bands of cilia occur on the ventral surface behind the mouth. The first body-segment has a larger lateral process than the rest, and bears a long tuft of slender bristles, minutely spinose. The succeeding segments have shorter bristles. Each segment has a short tuft of cilia, somewhat resembling the ventral tuft in the Magelona-larva. The terminal segment is ring-like and bears long cilia. The alimentary canal is pale and wide, but constricted at each segment-junction.
An older stage with from eighteen to twenty-four segments showed dorsal and ventral foot-papillæ, and a pair of rudimentary tentacles on the head, the posterior part of which is elevated (as in Pl. 7, fig. 7).
When the larva reaches to 3 mm., and has from thirty-five to fifty segments, the tentacles are longer and show an internal cavity, and the rows of cilia are longer, though shorter than in the larvæ ofLeucodore. The lips are richly ciliated, and a tuft of cilia occurs on each side behind the head. The anterior pair of eyes are blackish, as in the older larvæ, whereas the posterior pair are reddish. The foot-processes are larger at this stage and somewhat conical. Moreover, a brownish speck occurs between them, whereas in the examples at St. Andrews it was whitish or yellowish white. The bristles have the same rough aspect (from minute spikes), and the first segment bears longer bristles than the following. In all the segments the ventral row of cilia is present, while posteriorly is the anal ring of long cilia. The pale alimentary canal contains sea-water. The larva wriggles through the water for some time and then settles on the bottom. It is translucent like Tomopteris, the only pigment being the coloured specks on the sides (between the feet).
When supplied with sea-water, development proceeded, the dorsal and ventral divisions of the feet from the seventh to the eleventh becoming longer, thicker, and with a slender tip, all the others remaining as before. Further, the dorsal branch of the foot showed reddish pigment, whereas the ventral remained pale. The opaque speck (black?) remains between them. By-and-by the long cilia of the lateral regions of the head and the anal ring disappeared. Moreover, the presence of the longer feet from the seventh to the eleventh segments was somewhat inconstant, for in one in which only thirty-five segments existed they were well developed ; whereas in another with forty-five to fifty segments they were not longer than the others.
The geographical distribution of this larva is extensive, and Claparède gives a figure of one from Christiansand which for the most part corresponds. He does not know any adult Annelid with longer feet from the seventh to the eleventh, nor with the peculiar spinose bristles, which are probably provisional organs The peculiarity is that they existed so long.
The most advanced larva procured by Claparède was thus only supplied with short tentacles, that is, little more elongated than in Pl. 8, fig. 4, of the present paper; whereas several procured at St. Andrews in October had these organs considerably elongated—stretching backward to the fifth (Pl. 8, fig. 5) and even to the eighth bristled segment. They are large and comparatively massive organs resembling those of the Spionidæ. The unpaired process in front is considerably shorter than that figured by Busch,1 and Claparède and Mecznikow,2 in the larval Nerine cirratulus. Perhaps it only develops in the later stages, for in Pl. 7, fig. 7, it is not visible in a lateral view. Moreover, several with long tentacles had shorter bristles than in the earlier stages. The tail terminates in a somewhat ovoid tip with a dimple in the centre and a ring of cilia towards the tip (Pl. 7, fig. 8). In lateral view, however (ibid., fig. 9), the tip occasionally assumes a conical condition. A dorsal and a ventral blood-vessel (Pl. 8, fig. 6) are evident in the same view (lateral). The conspicuous pigment-speck between the bases of the feet is either opaque white or yellowish white, and is often finely ramose. Towards the end of October (23rd) an advanced example had peculiar globules which refracted the light like oil along the lateral region (Pl. 8, fig. 7) from the tenth foot backwards. They were absent from both dorsal and ventral aspects till near the tip of the tail. Their nature is doubtful, but it is possible they were due to degeneration, though the animal appeared to be active and healthy, the only feature of note being the great length of the bristles flanking the sides and the comparative shortness of the tentacles. Some of the advanced specimens were three-eighths of an inch long and had thirty-two segments behind the head.
The bristles of the long larval tufts in front seem to be more or less smooth in spirit-preparations. The minute spikes on the stronger bristles of the feet are readily seen. The bristles are generally in groups of three or four on each foot.
In section the cuticle is found of considerable thickness, and beneath is a feebly developed circular coat, then a boldly marked layer of longitudinal muscular fibres, arranged in two dorsal and two ventral bands. The long processes characterising the feet from the seventh to the eleventh segments have large hypodermic cells internally, with their long axes parallel to that of the process. The nerve-cords form two flattened granular bands on each side of the middle line ventrally. They have the hypoderm externally, and apparently a space over each internally. The oblique muscles pass to their outer edges, and probably go beneath them.
No further light has been thrown on the relationships of this form except that the tentacles in the most advanced confirm the opinion of Claparède that it pertains to the Spionidæ. It is apparently the larva of a species not uncommon at St. Andrews’.
EXPLANATION OF PLATES 6—8,
Illustrating W. C. McIntosh’s paper, “A Contribution to our Knowledge of the Annelida.”
FIG. 1.—Longitudinal (horizontal) section of the caruncle of Euphrosyne foliosa, Aud. and Ed., behind and on a level with the eyes, × 350.
FIG. 2.—Vertical section of the caruncle, showing a somewhat radiate arrangement of the fibres in the organ, while inferiorly strong fibres pass from the trunk into it. × 55.
FIG. 3.—Vertical longitudinal section of the anterior region of the same species on one side of the median line, showing the muscular fibres, m., passing to the caruncle, car. ocd. Dorsal eye. ocv. Part of a veutral eye. c. g. Cephalic ganglia. v. Section of blood-vessels, w. Mouth. wf. Anterior folds of the lining of the mouth. From the slightly oblique nature of the section certain of the anterior bristles, &c., are seen in front. × 55.
FIG. 4.—Transverse section of a branchial stem, towards the base. × 350.
FIG. 5.—Transverse section of the same organ, towards the tip. Zeiss, obj. D, oc. 1.
FIG. 6.—Vertical (transverse) section of a branchial stem as it leaves the surface of the body, showing fibres entering the region. × 350.
FIG. 7.—Vertical longitudinal section of the anterior end of E. fol io sa. 6s. Gastric chamber, br. Transversely ridged region, only a portion being seen in this section, e. Muscular ridges, gl. Glandular area. w. Mouth. wf. Folds of the buccal membrane. × 24.
FIG. 8.—Slightly oblique section (though more or less vertical and transverse) of the cephalic ganglia of Euphrosyne foliosa in the region of the dorsal eyes, ×55.
FIG. 1.—Vertical (transverse) section of the region of the caruncle in Euphrosyne cirrata. a. Vertical fibres from the ventral region, above nerve-trunks, car. Caruncle, t. Tentacle. The knife has passed through the caruncle after the central space has disappeared. The section is probably more or less oblique. × 55.
FIG. 2.—Slightly enlarged view of a softened.example of Euphrosyne foliosa, with the alimentary canal exposed, a. Anterior glistening region of the proboscis, b. Dorsal continuation of the canal, c. Striated or ridged muscular region, d. Pyloric part of the passage b. e. Intestine.
FIG. 3.—Vertical transverse section of the proboscidian stem near the glandular region. A thin line cuts off a lateral area at each side. bs. Alimentary canal, × 80.
FIG. 4.—Vertical transverse section of the anterior ventral region, ba. Wall of intestine, n. c. Nerve-cords, c. m. Circular muscular coat; the same letters mark the nerve-commissure. The oblique fibres decussate beneath the nerve-trunks, and join the circular coat. Sections of vessels are seen at v.
FIG. 5.—Transverse vertical section of the anterior region, where the canal, bs, from the gastric region joins the intestine, ba. vd. One of the dorsal trunks, ng. Central dorsal vessel. gp. Reproductive elements—in this case male. × about 60.
FIG. 6.—Transverse vertical section, anterior to Fig. 5. The dorsal vessel has now split into two trunks, vg. Zeiss, obj. A, oc. 1.
FIG. 7.—Head of Claparède’s larval “Spio “of 16th July, x 52.
FIG. 8.—Posterior end of the foregoing form, in a specimen procured in October (?). × 52.
FIG. 9.—Lateral view of the tip of the same example. × 52.
FIG. 1.—Young form resembling Magelona papillicornis in which the head has not yet extended beyond the tentacles, and in which the long larval bristles are present anteriorly, a. Constricted region of the alimentary canal. gl. Glands of the pharyngeal region. The anterior region of the alimentary canal is distended with water. 28th May. × 55.
FIG. 2.—A similar specimen after the long larval bristles have been shed, and viewed from the ventral surface with the tentacles fully extended. × 55.
FIG. 3.—A more advanced example, in which the basal part of the tentacles has larger papillæ, and the snout has considerably increased in size ; yet the long larval bristles are still present. 17th October, × 50.
FIG. 4.—Dorsal view of the anterior region of Claparède’s larval “Spio “of 16th July. ph. Pharynx, ×52.
FIG. 5.—Anterior end of the most advanced form of the same species yet found. The tentacles are considerably larger. 23rd October. × 52.
FIG. 6.—Lateral view of an example of the foregoing, showing the chromatophores and feet. 16th July, × 52.
FIG. 7.—A similar view of a specimen (23rd October), in which peculiar refracting bodies, like oil-globules, occurred along the lateral aspects. × 52.
‘Wirbellosen Tiñere,’ 1, ii, p. 136, pl. 33, figs. 264, 287.
‘Die Borstenwürme,’ i, p. 67, Taf. i and Taf. ii, 1864.
’Anat. von S. miniaceus, Grube,’ Wien, 1885, p. 10, Taf. ii.
I am indebted to Professor JJ-von Graff for various examples of this species.
Op. cit., Taf. ii, fig. 1.
Which, unfortunately, are less complete than I could have wished, and no additional examples could be procured.
‘Beobachtungen üb. Anat. u. Entwicklungs. Wirb. Th.,’ p. 74, Taf. x, figs. 9—14, and Taf. xi, figs. 1, 2.
Claparède (op. cit., fig. 10) figures the younger stage with a considerably larger snout than the later stage, a feature perhaps due to individual variation.
In a dying specimen, somewhat smaller than the present form, a large pinkish oil-globule occurred in the anterior region—the product of decomposition of the blood or other fluid.
‘Observât. Anatom, de vermibus quibusdam maritimis,’ 1852, p. 29, pl. ii, fig. 29.
Op. cit., p. 77, Taf. vi, f. 1—11.
Beobachtungen üb. Anat. u. Entwicklung., &c.,’ p. 65, Taf. viii, figs. 1—4.
“Bcitr ä ge zur Kenntniss der Entwickelung. der Chætopoden,” ‘Zeit. wiss. Zool.,” Bd. xix, separ. Abdr., p. 11, Taf. xii, fig. 4.