Nicolea venustula Mont. has only two pairs of equal, arborescent gills (Fig. 176, D); the tentacles are comparatively few. The animal, which is about an inch in length, is cinnamon-yellow with white spots, and has seventeen gland shields. 20 fathoms, North Sea and Mediterranean. Pista cristata Müll. is readily recognised by the shape of the gills (Fig. 176, E), of which there are only two pairs. Each consists of a long peduncle, bearing a number of dichotomously dividing, rather spirally-arranged branches, the whole having the appearance of a "bottle-brush." The worm is 2 to 4 inches long, of greyish-red to yellow colour. Atlantic east and west (even at the mouth of the Congo), and Mediterranean. Thelepus cincinnatus Fabr. is about the same length, pale red in colour, marked on its back with clear areas, giving the appearance of lacework. The gills are represented by numerous unbranched filaments arising separately in two transverse rows. The tube, which is adherent to shells, etc., along its whole length, is of thin, transparent, and flexible material like mica, covered with foreign bodies, and even with Polyzoa and Hydrozoa. 30 fathoms, Atlantic and Mediterranean.

Polycirrus aurantiacus Gr. is sometimes placed in a special sub-family, as it has no gills. The numerous tentacles are very long, and arise from a great hood over the mouth; the capillary chaetae commence in the first segment and extend for about half the length of the body; the uncini commence in the ninth segment. The ventral "shields" are paired. The animal is highly coloured; its phosphorescence and its distastefulness have already been mentioned on p. 294. In Terebellides stroemi Sars, four comb-like gills arise from a single common thick peduncle on the back of the second segment (Fig. 176, F). The ventral surface of the body bends upwards anteriorly so as to bring the mouth to the dorsal surface. 13 to 16 fathoms, muddy bottoms, North Sea and Mediterranean.

Fam. 3. Ampharetidae.—This family differs from the Terebellids chiefly in the shape of the head and in the presence of a bundle of strong chaetae (or paleae) on each side of the head in front of the gills, of which there are four on each side. Each gill is a simple filiform process, considerably longer than the tentacles, which are very few in number. Amphicteis gunneri Sars, Ampharete gracilis Mgrn., and Melinna cristata Sars, occur on our coasts.

Fam. 4. Amphictenidae.—This contains the interesting genus Pectinaria, in which the head is protected by great golden chaetae on the second segment; they are flattened, curved, and pointed, and are arranged in a single transverse row on each side, serving as an operculum to the tube. The posterior end of the worm has undergone great degeneration, and is represented by a small leaf-like "scapha" which serves to close the tube posteriorly. The worm is 1½ inches in length and consists of only twenty segments, of which seventeen are chaetigerous. The tube is nearly cylindrical, but wider anteriorly than posteriorly (Fig. 152, p. 288); the sand grains are uniform in size, and are embedded in the secreted mucus in a very regular way, the surface being smooth both inside and out. These tubes can be carried about by the worm, but may be found projecting from the sand at very low tides. P. belgica Pall. forms a straight tube, whilst in P. auricoma Müll. the tube is larger and slightly curved. The former species appears to be confined to the North Sea; the latter occurs in deeper water, and is also present in the Mediterranean.

Sub-Order 4. Capitelliformia.

Fam. Capitellidae.^[389]—Capitella capitata v. Ben. occurs pretty frequently in the sand under stones near low tide-mark. It is a red worm, about 1½ to 2 inches long, greatly resembling a Tubificid Oligochaete. It may readily be distinguished from other Polychaeta by the strong genital chaetae in the male, which replace the notopodial chaetae of segments 8 and 9; those in the former segment are pointed backwards, and in the latter forwards. There is but a single pair of generative ducts in either sex in the eighth segment. North Sea, Mediterranean.

Notomastus latericeus Sars is a longer worm, living in shallow water, off our coast and in the Mediterranean. The anterior twelve segments are wider than the rest of the body. The notopodial chaetae of the anterior segments of the hind body form a ring. Dasybranchus caducus Gr., which occurs in the Mediterranean, but not on our coast, reaches a length of 2 or 3 feet. It has gills on the hinder segments above the neuropodia.

Mastobranchus Eis. is found in the Mediterranean.

Sub-Order 5. Scoleciformia.

Fam. 1. Opheliidae.—Comparatively short, rather ugly worms of a pearly colour, no prostomial processes: parapodia obscure. The family is represented in British waters by four species, occurring in shallow water.

Ammotrypane aulogaster Rathke, is about 2 inches long; the nearly cylindrical body has a ridge running along each side below the chaetae. The conical prostomium is tipped with a small knob, and carries at each side a ciliated pit (Fig. 144, p. 273). Every segment, except the first chaetigerous, is provided with a filamentous gill (dorsal cirrus). The segmentation is very obscurely marked, for internally there are only three complete septa, placed far forwards. The intestine is bent upon itself. In Ophelia limacina Rthk. the gills commence in the eighth segment, and the longitudinal ridge does not extend in front of this segment. The worm is about 1½ inches long, and occurs between tide-marks. Travisia forbesi Jnstn., North Sea. Polyophthalmus pictus^[390] Duj. is very abundant at some parts of the coast. There are two bundles of chaetae on each side of every segment; each bundle contains three chaetae, of which only one projects to any distance. Paired eye-like spots exist on the sides of twelve segments. The worm is about an inch in length.

Fam. 2. Maldanidae (= Clymenidae).—Represented on our coasts by four fairly common species. They form sandy tubes, which are embedded in the sand with a short portion projecting. In some places they are so abundant that at low water the sand has quite a rough appearance. The prostomium is frequently truncated and depressed, and is always fused with the peristomium. A horny plate may be developed on the upper surface of the head, and the skin at the side of the prostomium is frequently raised into a more or less prominent fold. The hinder end of the body carries a funnel surrounding the anus. There are no gills or sensory processes on the body. Some of the segments towards the middle of the body may be longer than the rest. Peculiar serrated hooks of characteristic shape constitute the neuropodial "torus." The buccal region is eversible.

Nicomache lumbricalis Fabr. is a rosy-pink worm with white spots anteriorly; the chaetigerous ridges are red. The worm consists of twenty-six segments, and measures 2 or 3 inches. It is very narrow and readily breaks in pieces. The prostomium is laterally compressed; the anal funnel is fringed with a number of short equal processes. Under stones in the Laminarian zone.

Axiothea catenata Mgrn., which may reach a length of 3 or 4 inches, resembles the above in general colour, though of a deeper tint. There are only eighteen chaetigerous segments. The head has a membranous fold of skin on each side, and the anal funnel is produced into longer and shorter processes. Both these species are also found on the west side of the Atlantic. Clymene lumbricoides Qfg. is about 8 inches long; pink, with a light ring round each segment; the seventh segment is larger and reddish-brown. The prostomium is laterally compressed. Anus on a cone, which rises from the bottom of a funnel, the margin of which is entire. Atlantic.

Fam. 3. Arenicolidae.—Here belongs the common "lug-worm" Arenicola marina L., which occurs all round our coasts between tide-marks, and is so generally used as bait in fishing. The worm, which measures 5 to 8 inches, is of a dark tint, usually brownish-green. It burrows to a depth of some 18 inches or 2 feet, and throws up a considerable quantity of "castings" or "sand-ropes," which are noticeable on every shore consisting of mud or muddy sand. The body of the worm is cylindrical, thicker anteriorly; the segments are indistinct, owing to the secondary groovings and furrows on the skin. The prostomium is in the adult fused with the peristomium; this and the second segment are achaetous. Then follow twenty chaetigerous segments with dorsal bundles of capillary chaetae and ventral rows of short crotchets. The hinder region of the body is achaetous and narrower than the rest, forming a "tail." There are twelve (sometimes thirteen) pairs of arborescent red gills on segments 9 to 20 (21). Internally there are only four complete septa, and six pairs of nephridia, which are of large size: the fore-gut is eversible. Atlantic and Mediterranean. A second and smaller species, A. ecaudata Jnstn., occurs on our southern coasts; it is readily distinguished by the absence of a "tail," the chaetae and gills being continued to the end of the body.

Fam. 4. Scalibregmidae.—Prostomium ill-marked, fringed with small processes. Parapodia represented by slight papillae; two bundles of chaetae; usually cirriform lobes above and below them. Lipobranchius jeffreysii M‘I. has a grub-like body pointed at each end; forms tubes of mud. Firth of Clyde and elsewhere in North Sea. Sclerocheilus Gr. in old oyster shells. Channel, Mediterranean. Eumenia crassa Oerst. has gills on first six segments. North Sea. Scalibregma inflata Rthke. has arborescent gills on segments 4 to 7. The anterior part of the body is dilated. North Sea.

Fam. 5. Chlorhaemidae.—The family derives its name from the green colour of the blood, due to chlorocruorin. The representatives are comparatively short worms, with capillary chaetae on all the segments, the limits of which are not evident. The prostomium carries a pair of long grooved yellowish processes, which are perhaps palps, and several green tentacles, acting as "gills," arranged in a transverse series above the mouth (Fig. 134, F, p. 262). The peristomium is achaetous; the whole "head" can be withdrawn into the body. The chaetae of the anterior segments are especially long, and directed forwards so as to form a "cage" for the head. The body-wall is covered with longer or shorter papillae. Internally, the chief points of interest are the presence of only two septa (Trophonia) or only one septum (Siphonostoma), situated somewhere in front of the middle of the body, and forming a great backwardly-directed pouch, which contains a part of the looped intestine, and the nephridia, of which there are only two or four.

Trophonia plumosa Müll. is about 2 to 4 inches long, yellowish-brown in colour, with a rough skin; the head is usually retracted. It lives in the mud amongst Laminarian roots down to 50 fathoms. North Atlantic. Siphonostoma (Flabelligera) diplochaitos Otto, has a transparent body-wall, so that the coloured viscera are visible. The skin carries long papillae, which traverse a thick jelly-like envelope secreted by it, in which numerous diatoms live (symbiotically?); the surface is covered by particles of mud, etc. This species, which may be found under stones at low tide, occurs also in the Mediterranean.

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 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, 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 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 (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 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 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 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.

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 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.