Chapter XVIII: Introduction: Nematoda—anatomy—embryology—classification—ascaridae (8)

FAM. 6. _Sternaspidae._—The single genus, _Sternaspis_, has not been recorded on our coasts, but is of so peculiar a structure as to deserve a description.[391] _S. scutata_ Ranzani, occurring in the Mediterranean, is rather less than an inch in length, and derives its name from the possession of a pair of peculiar "horny" plates or shields on the ventral surface posteriorly. Around their margins are set about thirty bundles of long capillary chaetae. There are three half rings of stronger chaetae on each side near the anterior end of the body. The mouth is overhung by a very {336}small rounded knob (prostomium), which in _S. spinosa_ Sluiter, is prolonged outwards on each side to form a grooved palp-like organ, recalling that of _Bonellia_. The anus is placed posteriorly; and in front of it, on the dorsal surface, are two bundles of many long thread-like gills. On the ventro-lateral surface, in front of the middle of the body, is a pair of finger-shaped processes containing the genital ducts. The anterior segments of the body can be withdrawn into the body, as in the _Chlorhaemidae_. Further examination leads to the conclusion that the body of _Sternaspis_ consists of about thirty segments, most of them provided with paired bundles of capillary (neuropodial?) chaetae, distributed as follows:—Each of the segments 2, 3, 4 has a half ring of strong chaetae on each side; segments 5 to 7 are without chaetae; segments 8 to 14 have chaetae embedded in the body-wall, but not projecting. The shields cover the remaining segments; and along the outer edge of each are some ten bundles of chaetae, and along the hinder edge some five or six bundles, representing as many segments. Thus the worm consists of about thirty segments whose outlines are nearly obliterated (as in the _Chlorhaemidae_), and whose chaetae, except those which are specially developed, are disappearing: while posteriorly a great shortening of the body brings the bundles close together. A continuation of this process, involving a further disappearance of chaetae, leads readily to the condition met with in _Echiurus_, one of the chaetigerous Gephyrea. Internally, further evidence of the relation between _Sternaspis_ and the Chlorhaemids with the Gephyrea is afforded by the absence of septa, by the coiled alimentary canal, and by the presence of a single pair of nephridia, which in the latter group act both as excretory organs and as genital ducts.

BRANCH B. CRYPTOCEPHALA.

SUB-ORDER 1. SABELLIFORMIA.[392]

FAM. 1. _Sabellidae._—The branchial crown consists of a usually considerable number of filaments arising from a semicircular base. The peristomium may be reflexed to form a collar, {337}which is frequently notched, so that a lateral and a ventral lobe on each side may be distinguished (Fig. 133, B, p. 261). The thorax consists of nine segments, and is provided with ventral "gland shields," which are continued along the abdomen, where they are subdivided into two by a ciliated "faecal groove," which sometimes bends to one side on reaching the thorax, and may extend forwards along the dorsal surface to the head: this groove serves to carry the faeces out of the tube. The gill filaments are always provided with secondary processes, and may be provided with compound eyes.[393] The tubes of the Sabellidae are always of fine mud or of sand.

_Sabella pavonia_ Sav. is about 10 to 12 inches long and about ¼ inch across; the tube of fine mud is considerably longer and embedded deeply in the mud, with its free end projecting to some 2 or more inches, where it serves for the attachment of seaweed, Polyzoa, Hydrozoa, etc. The colour of the animal is orange-brown; the gills, which are about 1½ inches long, are green (due to contained blood) marked with more or less extensive brown or purple-brown spots, which may even hide the green tint. There is a pair of dark filaments arising between the dorsalmost gill filaments, which have been erroneously regarded as "prostomial tentacles"; they are, in fact, prolongations of a peculiar membrane or lip round the base of the gills, which bounds a groove leading to the mouth. These lip-processes (Fig. 133, B, _l_) occur in other Sabellids. Atlantic, North Sea, and Mediterranean.

_Branchiomma vesiculosum_ Mont. forms a sandy tube near low tide-mark. The animal, measuring 6-7 inches, is rich brown, darker anteriorly, abundantly speckled with white; the ventral surface is pink; the gills are green or {338}olive-brown, marked with white bands in a fairly regular fashion. Each gill filament has, just below its tip, a compound eye, consisting of several lenses and retinae. North Sea and Mediterranean.

_Dasychone bombyx_ Dalyl. is a short, comparatively stout worm usually 1 to 1½ inches long; reddish-brown in colour, with a darker spot on each side of every segment. The gills are lighter with greenish marks. This worm may readily be recognised, for each of the gill filaments carries some six to ten pairs of dark compound eyes at intervals along its length, and near to each pair there arise two short processes from the outer side of the filament, which are known as "dorsal appendices." The worm forms a tube of mud, more or less mixed with sand. It occurs at low water and to some depth round the coasts of the Atlantic, North Sea, and Mediterranean.

_Chone infundibuliformis_ Kröyer may be recognised by the absence of lobes on the collar, the presence of a membrane connecting the gill filaments, and the passage of the faecal groove along the dorsal surface of the thorax. The worm is 6 inches long, with purple gills, spotted with yellowish-white. The tube is formed of yellowish membrane covered with sand, and is fixed to stones and other objects. _Potamilla reniformis_ Müll. is about 3 inches long, with about twelve brown gill filaments, some of which have eyes near the base. The tube is transparent and horny, with sometimes a slight covering of sand. Found in old oyster shells. North Sea, Atlantic, Mediterranean.

The genus _Spirographis_ contains one of the largest European Sabellids, _S. spallanzanii_ Viv., which occurs off the Channel Islands and in the Mediterranean. The two gill plumes are unequal; the large rone forms an upright, spirally-coiled column.

FAM. 2. _Eriographidae._—_Myxicola infundibulum_ Mont. has its gill filaments connected by a membrane reaching nearly to their tips. Each gill plume forms a semicircle; there are no eyes; the peristomium does not form a collar; no gland shields. The worm requires neither of these structures, since it is practically a free-swimmer, envelopes itself in mucus, and moves tail first. The faecal groove is not well marked, though continued dorsally. In the abdomen the _tori uncinigeri_ extend dorsally and ventrally {339}(beyond the neuropodial chaetae) and nearly encircle the body. The animal is 4 or 5 inches long, dull green, with purplish gills. Between tide-marks. North Sea and Mediterranean.

_Amphiglena mediterranea_ Leyd. is only about ¼ inch long, hermaphrodite, and has eyes on the peristomium and on the anal segment. It is a very elegant little worm, and as a living object under the microscope, with the cilia on the gills, is very beautiful. The gills consist of six filaments on each side, provided with the usual double row of ciliated processes.

FAM. 3. _Amphicorinidae._—Small hermaphrodite Sabellids in which each gill tuft contains only a few branching filaments. The simplest form is _Haplobranchus aestuarinus_ Bourne,[394] which occurs in the rather foul mud at low tide in the estuaries of the Thames, the Liffey, and other rivers. The animal is about ¼ inch long, with four finger-shaped processes on each side, and a pair of larger, vascular processes on the ventral surface. These five branches are gills (palps), although, owing to the small size of the worm and simple vascular system, the four lateral filaments have no blood-vessels. The animal consists of only eleven chaetigerous segments, and lives in a tube made of mud particles.[395] _Fabricia sabella_ Ehrenb. (_Amphicora fabricia_ Müll.) has three gills on each side, each with a number of secondary branches of different sizes, but so arranged as all to reach the same level. It has eyes in its tail and swims backwards.

FAM. 4. _Serpulidae._—The thorax is provided with an undulated membrane on each side, chiefly employed in smoothing the inside of the tube; it represents the dorsal and ventral cirri of these segments. The gland shields are confined to the thoracic segments. In many genera the dorsalmost gill filament on one or both sides is terminally dilated and serves as an operculum. The tube is calcareous, and attached to rocks, shells, etc., for a greater or smaller part of its extent.

_Serpula vermicularis_ L. forms a pinkish tapering tube about 3 inches long; the narrower fixed end is coiled. It is marked at irregular intervals with encircling ridges, indicating cessation {340}in formation, and has a circular aperture. The worm itself is about 1½ inches long. The horny operculum is conical, with its base upwards, fringed with short processes. 20 fathoms in the North Sea and Mediterranean; also from 275 fathoms off the west coast of Ireland.

_Pomatoceros triqueter_ L.—The white shell is adherent, with a distinct keel along its upper surface; the aperture is overhung by a spine. The tubes are abundant everywhere, attached to rocks, stones, shells, etc., between tide-marks and down to 18 fathoms. The animal is very handsome, the thorax being deep blue, the abdomen red in the female, whitish in the ripe male. The branchiae are barred and spotted with blue, orange, and white; the operculum is calcareous, and furnished with a couple of horn-like processes.

_Filigrana implexa_ Berkeley is a small worm, but the slender white tubes intertwine and adhere together in masses 3 or 4 inches high, occurring at low tide and down to 18 fathoms in the North Sea and Mediterranean. The animal has only eight gill filaments on each side, one of which on each side is slightly expanded to serve as an operculum. The worm multiplies by transverse division.

_Spirorbis borealis_ Daudin is a still smaller worm, the tube of which is coiled in a flat spiral about 1/16 to ⅛ inch across; it is {341}common, adhering to _Fucus_, shells, and other objects. It is represented by fossils in the Palaeozoic rocks. Cosmopolitan.

_Protula_ (_Psygmobranchus_) _tubularia_ Mont. is a Serpulid without an operculum; it forms a straight or slightly and irregularly curved tube. Atlantic and Mediterranean. _Salmacina dysteri_ Huxley has no operculum; it is a small worm incrusting seaweeds, or forming masses like _Filigrana_.

SUB-ORDER 2. HERMELLIFORMIA.

The single family _Hermellidae_ is represented by two species—_Sabellaria alveolata_ L., which is littoral, and _S. spinulosa_ Leuck., occurring in 10 to 30 fathoms.

_S. alveolata_[396] is about an inch long; the thorax is purple, the abdomen yellow to red. The narrow caudal region is bent sharply forwards, so that the anus, situated at its tip, comes to lie at the orifice of the tube, which is irregular and sandy. Great numbers of the animals live together, so that the masses of their tubes may be 2 or 3 feet thick and several feet long. They are well seen on the shore, at Paignton, near Torquay, and on Hilbre Island, off the Cheshire coast. North Sea, Atlantic, Mediterranean.

ORDER III. MYZOSTOMARIA.[397]

These animals are parasitic on Crinoids or Asteroids.[398] The {342}single family, Myzostomatidae, contains but two genera, _Myzostoma_ F. S. Lkt. and _Stelechopus_ v. Gr.

Some of them move, more or less actively, on the surface of their hosts, others live in a sessile condition between the joints of the arms or pinnules, causing a greater or less malformation thereof, sometimes leading to the formation of a more or less globular cyst like a plant-gall, due to overgrowths of the joint, as in _M. deformator_ v. Gr. and _M. cysticola_ v. Gr.: while _M. pulvinar_ v. Gr. is endoparasitic in the intestine. Two species occur on our common _Antedon_ (_Comatula_) _rosacea_; one, _M. cirriferum_ Lkt., creeps about the oral surface, especially along the food grooves of the disc and arms; the other, _M. glabrum_ Lkt., lives close to the mouth of the Crinoid, so that its pharynx can be inserted into the oesophagus of the host; this species rarely moves from this position, and carries a young one on its back.

The Myzostomaria are circular or oval, more or less markedly convex dorsally, flat ventrally; _Stelechopus_, however, which lives on _Hyocrinus_, is elongated. The margin of the body is provided with ten or more pairs of cirri, short (_M. glabrum_) or long (_M. cirriferum_), and the general appearance of the animal is greatly changed in some species by the great elongation of the hinder cirri, into which the viscera may extend (_M. filicauda_ v. Gr.). On the ventral surface are five pairs of small conical "parapodia," arranged, like the internal organs, in a radiate manner. Each parapodium carries a couple of chaetae; one a hook, the other serving as a "guide" for this hook. The four "suckers" on each side are either glandular or sensory organs; and Wheeler considers them homologous with the lateral organs of Capitellids; they are usually little developed in those species which live inside cysts.

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The mouth and anus are usually ventral; but in _M. glabrum_ the anus is dorsal, and in a few species both apertures are carried on to the back by the great development of the ventral surface. The alimentary canal is provided with a protrusible pharynx; the intestine is branched; amongst its branches is the coelom, packed with eggs, and functioning as a uterus (usually called "ovary"). The true ovary is a small mass of cells on each side, a proliferation of the coelomic epithelium covering the intestinal wall. A median continuation of the {344}uterus passes backwards above the rectum, and opens either into it or by an independent pore dorsal to the anus. The "lateral oviducts" of Nansen are nephridia with ciliated funnels opening into the coelom (uterus), and with pores leading into the cloaca on its ventral surface; or, in _M. belli_ Wheeler, opening to the exterior. The two testes are branched, and each sperm-duct opens laterally on a papilla, just outside the third parapodium of each side. Wheeler[399] has recently shown that in the young _Myzostoma_ the spermatozoa ripen before the ova, so that it is functionally a male; before the spermatozoa are all discharged the ova mature, and the animal is for a time hermaphrodite; later on, however, when all the spermatozoa are used up, the worm is a female. Beard's "dwarf males" are therefore merely the young of hermaphrodite forms. In cysticolous species each cyst usually contains a large female individual and a small male. In these cases the young one (male) discharges all its spermatozoa before the ova ripen, so that a period of immaturity intervenes and a true hermaphrodite condition is omitted; the animal is at first male, and later female. The Myzostomaria are thus "protandric hermaphrodites."

The affinity of these animals has been much discussed; they superficially resemble the Tardigrada in many anatomical features, and differ greatly from Chaetopoda, but as they possess the characteristic chaetae or parapodia, and pass through a larval stage[400] similar to that of the Polychaetes, there is no doubt that they are closely allied to the group, and indeed may be regarded as degenerate Chaetopods. It has been suggested that they form a passage group between them and the Tardigrada; and von Graff forms a group Stelechopoda, to include the Myzostomaria, the Tardigrada, and the Linguatulida.

OLIGOCHAETA (EARTHWORMS, ETC.) AND HIRUDINEA (LEECHES)

BY

F. E. BEDDARD, M.A. (Oxon.), F.R.S.

Prosector to the Zoological Society

{347}CHAPTER XIII

OLIGOCHAETA (EARTHWORMS AND THEIR ALLIES)