The two marine genera Rhabdopleura and Cephalodiscus are contained in this group; the former found near the Shetland and Lofoten Islands, and on the Norwegian coast at depths of 90, 200, and 40 fathoms respectively l; the latter taken by the 'Challenger' expedition in 200 fathoms off the Patagonian coast.
Rhabdopleura forms indefinitely branching colonies, attached to various foreign objects. The zooids are all connected by a stem, and are contained in a tubarium, or connected system of hollow tubes composed of a hyaline material, which is secreted by the buccal shield of the zooids. The axial part of the tubarium, both main axis and branches, is adherent. The tubes of the zooids are more or less erect. The latter originate from the axis laterally, and the axial tube is divided by transverse septa into compartments, one to each zooid and its tube. Cephalodiscus buds, but the buds are detached at a certain stage of growth. Its zooids and their descendants inhabit cavities in a gelatinous branched mass with fimbriated edges probably secreted by the buccal shield as in Rhabdopleura. The head of the zooid bears in Rhabdopleura, on either side a flexible arm, the two edges of which are beset each with a single row of about fifteen ciliated flexible tentacles. Arm and tentacles are alike supported by a mesoblastic skeleton. Cephalodiscus is provided with twelve similar arms, six on each side, similarly beset with tentacles, and each of its arms terminates in a knob. Between the bases of the arms in both genera alike, and overhanging the mouth is a mobile disc or buccal shield.
Its surface is ciliated in Rhabdopleura, and its shape extremely changeable. Not only does it secrete the tubarium, but it is an organ of locomotion by means of which the zooid creeps up its tube. The body in both genera is more or less ovoid, and gives origin on its oral aspect, and somewhat posteriorly to a flexible stalk or stem ("gymnocaulus"). This stem ends freely in Cephalodiscusy and bears at its extremity one or two buds. In Rhabdopleura the stem is branched correspondingly with the tubarium. It contains an axial skeleton, similar to that supporting the arms and tentacles, one surface of which is covered by muscle cells. The part of the stem to which the zooid is immediately attached, and which traverses the zooid tube, retains its soft nature. Contraction of its muscle cells retracts the zooid towards the base of its tube, the stalk itself being thrown into coils simultaneously. When the muscle cells relax, the elasticity of the skeleton probably comes into play, and aids the movement of the zooid up the tube. But the ectoderm of the branched part of the stalk, contained in the main axis and the branches of the colony, secretes an external hard brown chitinoid coat.
It is therefore rigid, and is termed 'pectocaulus' by Professor Ray Lankester. It becomes adherent to, and eventually sinks into the substance of the tubarium on its attached aspect. The mouth lies beneath the epistome. The alimentary canal is ciliated, and has a U-shaped curvature as in Polyzoa. The anus is situated on a more or less projecting papilla at the head-end of the body, but on the opposite side to the mouth. There is a small coelome which extends into the stalk. It is lined in Rhabdopleura by fusiform cells, sometimes branched, occasionally stretching from the body-wall to the digestive tract. The ectoderm cells of the arms, tentacles, epistome, and to a lesser degree of the stalk, contain in Rhabdopleura some an orange, others a black, pigment; and there is a special aggregation of black pigment at one end of the epistome, possibly a rudimentary eye. Cephalodiscus has a pair of eyes apparently placed on the oral aspect above the mouth, and hidden by the buccal shield. Rhabdopleura has a ciliated sensory (?) tubercle at the base of each arm on its aboral aspect. No nervous system and no nephridia have been detected. The testis in Rhabdopletira is an elongated sac opening by a special pore anteriorly and aborally.
Ova have been found in Cephalodiscus. Both organisms reproduce by budding. While the buds are detached in Cephalodiscus', they remain connected by the pectocaulus in Rhabdopleura. The young buds in this genus are formed on the soft stalk or gymnocaulus of a zooid, which appears itself to remain for a time in an arrested condition, i. e. its arms are rudimentary, its buccal shield deeply bifid anteriorly, its gymnocaulus in a state of growth. The youngest bud is the one nearest to it. The buds are successively isolated with increasing age from one another by transverse septa which divide the axial tube into compartments (supra).
1R. compacta comes from deep water off the coasts of Antrim (Hincks, Marine Polyzoa, i. p. 581, PI. 72, Figs. 8, 8 a, 9).
The affinities of Pterobranchia are at present doubtful. In the absence of all embryological evidence it is not possible to say whether the buccal shield or epistome is prae-oral as it is in Brachiopoda and Vermiformia, or post-oral, as it appears to be in the entoproctous Polyzoa; nor again whether or no the flexure of the intestine is dorsal or ventral. The class is accordingly treated as independent here.
Ray Lankester, 'Polyzoa,' Encyclopaedia Britannica (ed. ix.), xix. p. 434.
Rhabdopleura, Id. Q. J. M. xxiv. 1884.
Cephalodiscus, McIntosh, A. N. H. (5), x. 1882.