Notes on the Anatomy of Sternaspis

It was, therefore, with a view to either confirm or correct these descriptions that I began a study of Sternaspis thalassemoides, Otto, during a recent visit to Naples. I may say at once that they both proved to be erroneous.

To these observations have been added some notes on the cuticle and muscular system.

Fig. 1 represents, somewhat diagrammatically, a ventral view of the ovary, or ovisac, as it would be more correct to call it,¹ removed from a female Sternaspis, the oviducts having been cut through near their attachment to the body-wall. These ducts pass forward and open to the exterior at the end of the processes seen in fig. 16, in front of segment 8.

When viewed from the dorsal surface the two ducts appear to be simple tubes, uniting and passing into, or expanding to form, the tabulated genital sac.

When viewed from the ventral surface, however, we see that on either side a small blood-vessel, lat. v., fig. 1, comes off from the large ventral vessel, v. v., passes along the inner surface of the wall of the sac for a short distance, and then emerges on the outer surface of the duct down which it runs to the body-wall. Now this blood-vessel passes out from the genital sac by an open canal formed by the folding of a ciliated membrane, op. The edge of this membrane forms a sort of ciliated funnel, oil. mb., opening into the cœlom, and is produced as a ciliated ridge down the outer and ventral side of the duct for about two thirds of its length.

The exact conformation of the coelomic opening of the genital sac will be better understood on looking at the series of sections represented in figs. 2—8. Fig. 2 is taken through the narrow region connecting the sac with the ducts ; fig. 3 is through the same region, but nearer their point of origin. Although the blood-vessels lie in the wall of the sac, the ciliated epithelium never covers them. In fig. 4, a section immediately after the bifurcation, the beginning of a ridge is visible projecting into the lumen of each duct. In fig. 5 this ridge is seen to project far inwards in the upper section, whilst in the lower section it has cut the lumen completely into two. The blood-vessel, lat. v., has passed into the ventral and smaller lumen. The sections represented in fig. 6 show the bloodvessel lying near the edge of the ciliated membrane, so as to close the aperture of the funnel, which is seen to be widely open farther forward in figs. 7 and 8. More forward still, the membrane, which is for a considerable distance attached to the tube by one edge, becomes quite free. The membrane and the wall of the duct are ciliated on one surface only, and this is continuous from one to the other. This description applies to both sexes.

As for the structure of the wall of the tabulated sac itself, previous observers do not seem to have noticed that it also is ciliated internally,—if not over its entire surface, at all events along extensive tracts reaching up the tabes. Like the ducts, it is covered on its outer surface with flat coelomic epithelium, shown in fig. 10, a drawing of a fragment of the wall treated with silver chloride and stained. In certain regions, separated from the outer epithelium by a very thin connective-tissue layer with a few muscular cells, is the ciliated internal lining, cil. epith., formed of elongated cells with oval nuclei. The cilia are short and closely set along narrow longitudinal tracts directed towards the base of the organ where the ducts come off (fig. 9). The direction of the ciliary current is from the tip of the lobes to the base, and down the ducts to the external openings. The cilia on the membranous funnel, and on the wall of the narrow canal leading from the coelom into the genital sac, produce a current running inwards towards the cavity of the sac. In this way, although the ducts and the sac may be full of ova or spermatozoa, none are allowed to stray into the body-cavity.

These are two lobed sacs of a yellowish-brown colour, situated in front of the genital organ (fig. 11). Their general form and relations have been well described and figured by Rietsch (3)¹. The main mass of each organ lies closely applied to the oesophagus, and is connected with the body-wall at the level of the intersegmental groove between segments 6 and 7 (fig. 20) by a rapidly diminishing stalk, with a narrow lumen, apparently ending blindly ; for, like Vejdovsky and Rietsch, I failed to find any external opening.

After a careful search I found a small ciliated funnel opening into the cœlom, and situated on the narrow stalk a little way above its point of attachment (figs. 11, 12, 13, cil. fun.). The blood-vessel which accompanies the stalk (figs. 12 and 13, bl.v.) sends a branch into the lip of the funnel, running immediately below the ciliated epithelium, as in many Polychæte nephridia. Fig. 13 is a view of a funnel in a still living condition, whilst fig. 14 shows the edge of the lip of another funnel. The cells of which it is composed are vacuolated and slightly granular, bear numerous cilia, and occasionally irregular processes. Flat coelomic epithelium covers the outside of the whole nephridial organ. The internal epithelium is entirely composed of large cells, protruding far into the lumen, and loaded with “granules “of peculiar structure, to be described farther on. In fig. 21 is represented a fragment of the wall of the nephridium, showing the outline of the cells with the nuclei situated at their base.

” Ein innerer Hohlraum, sowie die Bewimperung fehlen hier gänzlich.” There can be no doubt that a cavity of considerable size really exists inside the organ. As to the presence of cilia, which is also denied by Rietsch, I must confess that I feel by no means convinced of their absence even in the main sac. When these soft-walled organs are placed under the microscope the cells of the internal surface, which are so full of granules, and bulge inwards into the reduced lumen, become inevitably pressed against each other, and being very thin-walled, they have a great tendency to burst, so that a few scattered cilia would be very difficult to detect. When teased up the cells break off, and present the appearance shown in fig. 23 a. On the other hand, the cells which line the narrow duct leading towards the funnel are less bulky, and preserve their shape better. In this region I have seen cilia producing a current from the funnel towards the main cavity of the organ.

The “granules “filling the cells of the internal epithelium of the nephridium are of curiously complex and quite constant structure (figs. 23a and ó). They consist of an outer transparent sphere filled with clear liquid, in which some minute granules are occasionally seen floating. In the centre of the sphere is situated a highly refringent yellowish body, which I shall call the concretion. This body is composed of two halves, one of which is slightly larger and darker than the other. In each cell spheres may be found of varying sizes, from a maximum diameter of about 15 p. to a mere speck. But even when quite minute they always contain, as far as I have been able to see, a central concretion of structure similar to that described above. When teased out and brought into some foreign medium the spheres always burst and disappear ; no fixative, as far as I am aware, will preserve them. The concretions, on the other hand, are much more resistent. Treatment with divers reagents reveals the fact that the two halves have different properties, and, moreover, that they are cup-shaped, enclosing a third element, which may be called the central granule. Distilled water, ammonia, alcohol, and ether have no effect on the concretion. Caustic potash (5 per cent.) dissolves the small half, and apparently the central granule, but not the large half, which resists even when heated. Acetic acid, on the other hand, dissolves the large half, and the central granule after prolonged action ; the small half remains unaffected. Weak hydrochloric acid dissolves the large half first, and then the smaller, whilst the central granule remains. Strong mineral acids destroy the whole concretion. Neither osmio acid nor iodine stains the concretion to any marked extent.

Fig. 22 shows the smaller half and central granule (the larger half having been dissolved) of concretions in cells preserved with Hermann’s fluid, and stained with alum carmine, a stain which the concretions readily take up.

Vejdovsky (5) briefly mentions the refringent granules in the cells of the nephridium of Sternaspis ; but Rietsch curiously enough seems to have mistaken them for nuclei.¹

It will be seen from the description here given that these are no ordinary excretory granules.¹ On comparing them with the granules found in the nephridia of other Polychætes, we find in some forms, such as Trophonia and Pectinaria, transparent spheres containing concretions ; but the latter are often numerous, and are composed of irregularly aggregated smaller concretions without constant shape.

Of course, there is no direct evidence that these granules in Sternaspis are of an excretory nature ; they certainly do not appear to be got rid of by the animal, since the nephridium is not known to open to the exterior. It is, therefore, not impossible that they may be stored up in the organ to serve some further purpose in the economy of the worm.

In the foregoing account the anterior paired brown sacs have, for convenience’ sake, been called nephridia. It is quite possible, however, that they are not true nephridia, but peritoneal funnels peculiarly modified, and retaining but a small ciliated opening into the general body-cavity. The question of their exact homology can only be answered by the help of embryological data which we do not yet possess.

The same may be said concerning the homology of the genital organs. Yet in this case the anatomical evidence seems to point more clearly to the ducts and lobed sac being formed by the modification of a pair of ciliated peritoneal funnels. Cases of the overgrowth of the gonad by the funnel of the genital duct (or a fold of the peritoneum), so as to almost or entirely close it off from the general coelomic cavity, are by no means rare amongst the Coelomata. The male organs of Lumbricus offer a familiar example in which the testis and genital funnel become enclosed in a sac surrounded by, yet separate from, the general coelom. The peritoneal sacs of the Eudrilidæ described by Beddard (1) are an instance of the same thing occurring in connection with the female organs. Throughout the higher Vertebrata the male genital cells are shed directly into the ducts ; whilst in certain cases—as, for instance, many Teleostean fish and the mouse— the ovary is likewise shut off from the body-cavity, being enclosed in a sac in direct continuity with the duct to the exterior. It is, indeed, amongst the Mammalia that we find perhaps the closest parallel with the state of things described above in Sternaspis. In the female racoon (Procyon) or badger (Meles) the ovary is overgrown by a fold of the peritoneum (“broad ligament “) and the funnel of the Fallopian tube, leaving only a narrow aperture communicating with the general coelom (cf. A. Robinson, 4). The steps intermediate between the ordinary condition in Polychætes, in which the gonad is situated close to the wide-mouthed peritoneal funnel, and the condition in Sternaspis, in which the gonad is enclosed by the funnel, are easy to conceive.

The cuticle of Sternaspis has been well described by Vejdovsky and Rietsch. It is very thick, and consists mainly of intercrossing fibres. On the outer surface, however, is a thin layer of somewhat different nature, to which are fixed numerous sandy (chiefly siliceous) particles (fig. 24). In this respect Sternaspis resembles the Chlorhæmids, in which the cuticle is also covered with sand. These foreign particles are chiefly grouped round the base of the numberless little papillæ which cover the surface of Sternaspis, and are especially numerous and coarse behind the seventh segment. On the anterior retractile segments the particles are finer and fewer in number.

Both inner and outer layers of the cuticle are insoluble in alcohol and ether. The thin outer layer is insoluble in hydrochloric acid, but soluble in strong solution of caustic potash. On the other hand, the thick inner layer is soluble not only in potash and hydrochloric acid, but also in boiling distilled water. It therefore resembles the cuticle of the earthworm (2).

The thick brown ventral shield is insoluble in boiling water, caustic potash, alcohol, and ether. Placed in cold concentrated hydrochloric acid, it gives an orange-yellow solution ; a transparent colourless portion remains, which dissolves with difficulty on boiling.

The chætæ are insoluble in water, caustic potash, alcohol, and ether. As in the case of the earthworm (2), au outer shell and distal cap remain undissolved when they are boiled in hydrochloric acid.

The shield, then, is probably formed of the same substance as the chætæ—not of true chitin.

The Muscular System.—Since the musculature of Sternaspis has been only very slightly dealt with by previous observers, a somewhat detailed account of the highly modified muscular system of this Polychæte is here given. It is well known that, when irritated, Sternaspis can rapidly withdraw the first seven segments into its hind body ; this introversion is brought about by a complicated system of muscles, derived chiefly from the longitudinal layer.

A side view of an expanded specimen is shown in fig. 16, and of a retracted specimen in fig. 15. In the latter the whole of the anterior region has been withdrawn to the level of the genital papillæ {gen. p.), which remain projecting from the front edge of the body. The branchiæ at the hind end are not withdrawn, but merely contract into close spiral coils. Their contraction is, however, quite independent of that of the body.

Externally we notice the first segment, bearing the mouth (m.) and the rounded prostomium (prost.’), followed by segments 2, 3, and 4, armed with strong chætæ. Each row of chætæ is set on a crescentic and slightly elevated area on each side at the hinder margin of the segment. As has been shown by previous observers, the bundles of chætæ in segments 8 to 14 are sunk in the body-wall, and completely hidden from view. The grooves separating the first seven segments are shallow, and completely surround the animal ; those separating the posterior segments are deeper (in the expanded worm), and do not reach right round, but leave a narrow smooth strip on the mid-dorsal and mid-ventral regions. The posterior bundles of chætæ are set round the lateral and posterior edges of the ventral shield; the lateral bundles are held by small conical parapodia.

The anterior end of the body is extruded (pleurecbolically) by the contraction of the circular layer of muscles in the hind body. These muscles are disposed in a thin layer, interrupted dorsally and ventrally at the smooth strips already mentioned (fig. 16, d. a. and v. a.). The inner layer of longitudinal muscles, passing from one intersegmental groove to another, lines the body-wall within (figs. 17—19, long. m.’). The muscles which serve to move the large anterior chætæ are disposed as follows (figs. 17—20) :—The chætæ are bound together at their inner ends with connective tissue into three bundles on each side. A short thick muscular band joins these-three bundles together, and a similar band attaches them to the posterior end of the pharynx. A larger strand of muscles runs forwards to the body-wall near the base of the prostomium, passing along the side of the pharynx (fig. 20). A slender band of muscle, starting from the inner end of the outer bundle of chætæ, passes almost horizontally outwards to the body-wall, where it is attached at the hinder limit of the fifth segment (lat. m.). This muscle draws the ends of the chætæ inwards and towards the body-wall, whilst the other bundles just described push them outwards and towards the pharynx. Each bundle of chætæ is, moreover, provided with proper protractor muscles (protr.). The retractor muscles are joined together in two large bundles (retr.) (not three, as figured by Vejdovsky and Rietsch), which pass backwards to their point of attachment on the anterior edge of the ventral shield on either side. Not only the chætæ, but also the whole anterior region of the body, are withdrawn by these powerful muscles.

Large strands of longitudinal muscles extend dorsally from behind the prostomium to the median dorsal region, from the anterior seven segments to segments 8 to 14 (figs. 17 and 18, d. retr.). Similar strands extend ventrally from beneath the pharynx to segments 8 to 14, and the front edge of the shield (v. retr.) on either side of the nerve-cord. A retracted specimen cut in half vertically (fig. 18) shows these muscles plainly ; dorsally and ventrail y they are attached to the smooth areas interrupting the intersegmental grooves on the outer surface of the worm. They are the chief retractor muscles of the anterior region of the body ; the sharp bend in the dorsal retractors (fig. 18) is due to the pressure of the viscera. This figure also shows the position of the prostomium and pharynx in a retracted specimen, and the nerve-cord bent back at a sharp angle. It will be noticed that owing to the length of the nerves running to the body-wall from the ventral surface of the nerve-cord, the latter organ does not closely follow the curve and folds of the body-wall, and thus is possibly saved from injury during the rapid process of retraction.

The fourth set of retractor muscles are seen in fig. 17, and in fig. 19 a retracted Sternaspis cut horizontally above the introvert. These are narrow muscular ribbons running from the posterior margin of the fifth segment to the grooves between the 7th, 8th, 9th, 10th, 11th, and 12th segments (lat. retr.).

It is obvious that all these muscles, by co-ordinate and successive action, form a very perfect apparatus for the acrembolic introversion of the first seven segments.

Posteriorly the rectum is provided with paired dorsal retractors (figs. 17 and 18, d. red. m.), and with paired ventral retractors (figs. 17 and 19, v. red. m.), attached to the ventral shield.

One of the functions of the ventral shield seems to be to act as a sort of fulcrum for the attachment of the main retractor muscles.

The ten large lateral bundles of chætæ set round the sides of the shield are provided with protractor muscles, and with peculiar slender retractors attached above the nerve-cord (retr. tat. ch., fig. 19). No such muscles belong to the small posterior bundles of chætæ as figured by Vejdovsky. They have small retractors attached to the shield.

It has been shown in the foregoing account that the cavity of the genital sac of Sternaspis communicates with the bodycavity ; that the nephridium is provided with a small ciliated funnel, possesses a lumen, and in one region, at all events, is ciliated internally. The complex granules of the nephridial cells have been described in detail ; experiments have been recorded as to the solubility in certain reagents of the granules, the cuticle, the ventral shield, and the chætæ. A detailed account of the muscular system has also been given.

 
• a.

  Anus.

 
• bl.v.

  Blood-vessel,

 
• br.

  Branchiæ.

 
• ch.

  Chætæ.

 
• cil.

  Cilia.

 
• cil.epiih.

  Ciliated epithelium,

 
• cil.fun.

  Ciliated funnel,

 
• cil.mb.

  Ciliated membrane,

 
• cil.ri.

  Ciliated ridge,

 
• cod.epiih.

  Cœlomic epithelium,

 
• d.a.

  Dorsal area.

 
• d.red.m.

  Dorsal rectal muscles,

 
• d.tetr.

  Dorsal retractors.

 
• extr.

  Extremity of the nephridium attached to the body-wall,

 
• gen.d.

  Genital duct.

 
• gen.p.

  Genital papilla, yr. Granule,

 
• ini.gr.

  Intersegmental groove.

 
• lai.retr.

  Lateral retractors,

 
• lat.m.

  Lateral muscle,

 
• lat.par.

  Lateral parapodium.

 
• lai.v.

  Lateral vessel.

 
• l.gen.s.

  Lobes of the genital sac.

 
• long.m.

  Longitudinal muscles.

 
• lu.can.

  Lumen of the canal leading from the body, cavity to the cavity of the genital sac.

 
• m.

  Mouth,

 
• n.

  Nucleus,

 
• n.c.

  Nervecord.

 
• neph.

  Nephridium, o.

 
• cut.

  Outer layer of cuticle,

 
• op.

  Opening leading into the genital sac.

 
• ov.

  Ovum.

 
• ph.

  Pharynx.

 
• post.ch.

  Posterior chætæ.

 
• prost.

  Prostomium,

 
• protr.

  protractors,

 
• red.

  Rectum,

 
• retr.

  Retractors.

 
• retr.lat.ch.

  Retractors of lateral chætæ.

 
• sil.p.

  Siliceous particles.

 
• sph.

  Transparent sphere,

 
• st.

  Stalk of nephridium.

 
• v.a.

  Ventral area.

 
• v.rect.m.

  Ventral rectal muscles.

 
• v.retr.

  Ventral retractors.

 
• v.v.

  Ventral vessel, w.

 
• gen.d.

  Wall of genital duct.

 
• w.gen.s.

  Wall of genital sac.

Illustrating Mr. Edwin S. Goodrich’s paper “Notes on the Anatomy of Sternaspis.”

PLATE 15.

PLATE 16.