PTEROBRANCHIA. The most strikingly novel discovery of the "Challenger" Expedition was a mysterious animal, dredged from 245 fathoms in the Straits of Magellan. It was described as Cephalodiscus dodecalophus, in 1882, and was at first placed in the Polyzoa. In 1903 it was announced that Cephalodiscushad been found by the Swedish Ant arctic Expedition, from the origi nal locality southwards to the Antarctic sea ; and that it had been discovered off Borneo (near low water mark), Celebes and Korea. It was later recorded from new Antarctic or Sub-Antarctic locali ties, and off New Zealand, South Africa and Ceylon. Forgotten specimens were found in the British Museum from Kerguelen ("Challenger") and from the Ant arctic (probably the Ross Expedition, 1839-1843). The study of the anatomy of this surprising animal revealed the unexpected fact that its affinities are with Balanoglosms (q.v.).
A colony of Cephalodiscus inhabits an orange, gelatinous dwelling (coenoecium), from a foot to half an inch long. The branches, from two inches to one tenth of an inch across, are composed of numerous laminae, including Forarninifera or other bodies, sometimes so numerous as to mask the gelatinous walls. The animals occur in large numbers, and are from 7 to r mm. in length, without the stalk, which is very extensible. They can wander on the surface of their dwelling, the lower end of the stalk remaining at tached to the wall of the tube, to facilitate retraction.
The body consists of the three regions characteristic of Balanoglossus:—(1) the "proboscis" or "buccal shield" (fig. 2), used as a crawling organ. A crescentic red line, near its lower edge, is distinctive of the genus; (2) the "collar," its front edge produced above into 3-9 pairs of plumes, each with two series of tentacles, and at the sides and below into a large "oper culum"; (3) the third segment or "meta some," with the stalk. Each segment has its own body-cavity, unpaired in the pro boscis, paired in the other two segments (figs. 2, 3). The first body-cavity opens externally by two "proboscis-pores," on the buccal shield, near its junction with the collar. The second cavities extend into the operculum and the plumes, each opening by a "collar-canal," immediately behind which is a "gill-slit," leading from the pharynx to the exterior (fig. 3). From the upper wall of the pharynx a "noto chord" projects into the proboscis-cavity, where there is a "peri cardium," corresponding with that of Balanoglossus. The central nervous system is in the dorsal region of the collar and of part of the proboscis.
The third body-cavities contain the alimentary canal and re productive organs. The anus is on the upper side, the mouth being below, between the proboscis and the operculum. The young ali mentary canal (fig. 2) is shortly U-shaped, but in the adult the U is much longer, passing ventrally down the body, the long axis of which is dorso-ventral. The two ovaries or testes open dorsally,
in front of the anus, and the oviducts (at first mistaken for eyes) are highly pigmented. The eggs have much yolk, and development commences in the tubes. In certain species the colonies are either male or female, while in others both sexes occur, and an animal may have an ovary on one side and a testis on the other. The males resemble the females except in C. sibogae (fig. 1, B), in which they occur with neuter individuals without functional re productive organs. These males have a single pair of plumes, with no tentacles, the alimentary canal is vestigial and the operculum has been lost. The three segments, their body-cavities, the collar canals and the central nervous system are normal, but the third segment is mainly occupied by the two testes. The males are probably nourished by vascular connection with the neuters; and blood-vessels have been observed in this and other species. The buds, often in considerable number, remain connected by their stalks with the parent stalk until a late stage, but ultimately become free.
The species of Cephalodiscus are most easily recognized by the form of their coenoecium. The number of plumes is not always quite constant, but is a useful character. About 13 species are known, referable to three sub-genera :—Demiothecia, with a single cavity extending throughout the colony ; and Idiothecia and Or thoecus, in which each animal, with its system of buds, occupies a separate tube, nearly always isolated from its neighbours. The coenoecium is branched in Idiothecia, discoidal or plano convex in Orthoecus. Rhabdopleura, the only other Pterobranch, resembles Cephalodiscus in form and structure, but is almost mi croscopic. It was first described from Norwegian and British seas (in rather deep water), but it occurs from Greenland to the Antarctic sea, including the Azores, the Malay archipelago, Ceylon, New Zealand, Australia and Tristan da Cunha. Several species have been described, but it is not at all certain how many are valid.
Rhabdopleura has no doubt been simplified, in correlation with its reduction in size. There is only one pair of plumes and gill slits are not present. In most other respects it agrees with Cepha lodiscus, to which it is clearly allied. The elongated, cylindrical tubes are about 2-3mm. long and a quarter of a millimetre in diameter. Each arises vertically from a tube closely attached to a solid object; and all the tubes are composed of laminae, which form distinct rings in the erect parts. The budded individuals do not become free, their stalks remaining connected by a cord. Its jet-black colour, in the adherent part, and the ringed tubes are features easily recognized.