Cephalodiscus dodecalophus[[45]] was found in the Straits of Magellan, during the "Challenger" voyage, at a depth of 245 fathoms, and has recently been rediscovered in shallower water in the same neighbourhood by the Swedish Antarctic Expedition. Another Cephalodiscus, at present undescribed, has been obtained by Dr. Levinsen from 100 fathoms off the coast of Japan; while the Dutch expedition carried out by the "Siboga" has resulted in the discovery of two other specimens, one from a reef close to low-tide mark on the coast of Borneo, the other from 41-52 fathoms off Celebes. These three specimens differ markedly from one another and from the "Challenger" specimen of C. dodecalophus, and it is probable that they all belong to new species. The occurrence of a deep-sea animal at a great distance from the locality at which it was first found is not in itself a matter for great surprise; but in the present instance two of the newly discovered forms are from shallow water, and one of them is actually littoral. The occurrence of so many species of Cephalodiscus in Oriental waters suggests that the Pacific or the Indian Ocean may be the headquarters of the genus, which may prove to be far less of a rarity than has hitherto been supposed. There is evidence derived from the results of the "Siboga" expedition that abyssal animals may migrate into comparatively shallow water in the Malay Archipelago.
Cephalodiscus and Rhabdopleura are remarkable for their power of producing buds. In the former these arise from the apex of a stalk which is given off on the ventral side of the body, and they break off when they reach a certain age; in the latter they do not become free, and a colony results, which consists of a creeping "stolon" from which vertical branches are given off at intervals, each ending in an individual of the colony. Cephalodiscus forms a gelatinous "house" (Fig. 9, A), in the passages of which are found large numbers of the free individuals, together with their eggs and embryos. Rhabdopleura (Fig. 12) is protected by cylindrical tubes, one of which corresponds with each individual.
Fig. 10.—Longitudinal median section of Cephalodiscus dodecalophus. a, Anus; b.c1, b.c2, b.c3, first, second, and third body-cavities; int, intestine; m, mouth; nch, notochord; n.s, central nervous system; oes, oesophagus; op, operculum, the ventro-lateral part of the collar; ov, ovary; ovd, pigmented oviduct; ph, pharynx; p.p, proboscis-pore; ps, proboscis; st, stomach; stk, stalk.
Cephalodiscus, though no more than two or three millimetres in length, is provided with practically all the important organs possessed by Balanoglossus. Its proboscis or "buccal shield" (Fig. 10, ps) is a large flattened structure, which overhangs and entirely conceals the mouth. The anterior body-cavity opens to the exterior by two symmetrically placed proboscis-pores (p.p), just in front of the tip of the notochord (nch). The collar, which has paired body-cavities, is produced dorsally into 4-6 pairs of plume-like arms, which bear an immense number of pinnately-arranged tentacles. The arms, which may end in a swollen bulb,[[46]] have ventral grooves along which food doubtless travels to the mouth by ciliary currents. The anterior edge of the ventral half of the collar is drawn out into a narrow flap or operculum (Fig. 11, op), in front of which is the mouth, and behind it the gill-slits (g) and collar-pores (c). The central nervous system (n.s) is a thick mass of nerve-tissue in the dorsal epidermis of the collar; it is not sunk beneath the skin as in Balanoglossus. The details of the nervous and vascular systems, and the development of the buds, have been described by Masterman. In the dorsal region of the collar the alimentary canal has a slender diverticulum, the notochord, which passes into the base of the proboscis; it is believed by Masterman to have a function similar to that of the neural gland (cf. p. [52]) of Tunicates.
The next part of the alimentary canal, the pharynx,[[47]] has a single pair of simple gill-slits opening to the exterior immediately behind the collar-pores. The short oesophagus (Fig. 10, oes) is followed by the wide stomach (st), and this by the intestine (int), which opens by the anus (a) near the front end of the body.
Fig. 11.—Longitudinal section through Cephalodiscus dodecalophus, passing through the two sides of the body; a, tentacular arm; b.c2, collar-cavity; b.c3, trunk-cavity; c, collar-pore; g, gill-slit; i, intestine; n.s, central nervous system; o, oesophagus; op, operculum; p, pharynx; s, stomach.
The trunk contains paired third body-cavities (b.c3), the septum between which and the collar-cavities is slightly behind the line of origin of the operculum. Two ovaries (ov) are situated between the pharynx and the last part of the intestine, each opening to the exterior dorsally between the central nervous system and the anus. Each oviduct (ovd) contains dark pigment, which is seen through the dorsal skin on removing the tentacular arms. Eggs, each enclosed in a stalked membrane, occur in numbers in the cavities of the gelatinous house. The early stages of the development are passed through inside the tubes; but there is at present little other information with regard to the embryonic development of the Pterobranchia. The specimen obtained by the "Siboga" from Celebes is a male colony with dimorphic individuals, the reproductive organs being confined to two-armed zooids with vestigial alimentary canal.
