It contained at that time the single genus Rhabdopleura, a minute animal dredged by Sars off the Lofoten Islands, and by Norman off the Shetlands.
-Rhabdopleura was at first regarded as an aberrant Polyzoon, but with the publication of the Challenger Report (Cephalodiscus) in 1887, it became clear that Cephalodiscus, the second genus now included in the order, had affinities in the direction of the Enteropneusta.
Rhabdopleura is no doubt of world-wide distribution, since it has been recorded in various localities from Greenland to South Australia, usually in water of not less than forty fathoms. Cephalodiscus, which for many years was known solely as the result of a single dredging by the " Challenger " from 2 4 5 fathoms in the Straits of Magellan, has recently been found in entirely different parts of the world, as for instance between Japan and Korea at ioo fathoms, at about half that depth off the south-east coast of Celebes, and between tide-marks on the coast of Borneo.
No less than twelve species, referred to three sub-genera (Demiothecia, Idiothecia, Orthoecus), have now been described; but it is at present uncertain whether more than a single species of Rhabdopleura is valid, although several specific names have been suggested for specimens from different localities.
In Rhabdopleura each zooid forms its own delicate tube composed of a characteristic series of distinct rings.
(iii.) The collar is produced dorsally into arms (one pair in Rhabdopleura, four to eight pairs in Cephalodiscus), each of which bears numerous ciliated tentacles, the organs by which the microscopic food-particles are conveyed to the mouth.
While in Rhabdopleura the buds remain in organic continuity with the parent, in Cephalodiscus they become free at an early stage, and the coenoecium accordingly contains a number of separate individuals.
3, g.s.), which do not occur in Rhabdopleura, open immediately behind the collar-pores.
The reproductive organs of Rhabdopleura have but seldom been ob served.
The suggestion has been made by Allmann and recently upheld by Schepotieff that Rhabdopleura is related to some of the Graptolites.
Sars, " Rhabdopleura," Christiania Univ.
I.; (9) Schepotieff, " Rhabdopleura," Zool.
553; " Rhabdopleura and Graptolites," Neues Jahrb.
The suggestion to place Brachiopods with the Polyzoa, Phoronis, Rhabdopleura and Cephalodiscus, in the Phylum Podaxonia made in Ency.