104 PHYLUM ENTEROPNEUSTA. 



Order 2. CEPHALODISCIDA. 



Enteropneusta in which the collar is prolonged into paired tenta- 

 culiferous arms and the trunk is much shortened in its antero- 

 posterior axis, with at most one pair of branchial apertures. The 

 animals live in colonies and inhabit tubes which are formed by the 

 proboscis. 



The Cephalodiscida comprise two genera, Cephalodiscus Mc- 

 Intosh and Rhabdopleura Allman. They are colonial animals 

 which possess the power of budding and inhabit tubes secreted 

 by the ectoderm of the flattened proboscis. The two genera 

 while agreeing in most points of structure differ in the presence 

 or absence of branchial apertures. In Cephalodiscus there is one 

 pair of lateral openings leading outwards from the pharynx ; in 

 Rhabdopleura branchial apertures are absent. In the following 

 account the two genera are dealt with separately. 



Cephalodiscus* resembles the Balanoglossids in the main plan 

 of its organization, but differs from them in the fact that the 

 trunk region, though exceedingly shortened antero-posteriorly, 

 is much elongated in the dorso-ventral direction, and in the 

 restriction of certain trunk organs, which are repeated in the 

 Balanoglossida, to a single pair, e.g. gill-slits and gonads. More- 

 over, it has the power of reproducing itself by budding ; and 

 several individuals live in association in a single tube system. 



Cephalodiscus was discovered by the Challenger in 1876, at a 

 depth of 245 fathoms, in the Strait of Magellan. It has since 

 been found in other localities (Japan, and from comparatively 

 shallow water in the Malay Archipelago and in the Antarctic). 

 It was at first thought to be a compound Ascidian. Then it 

 was referred to the Polyzoa, and it was not until 1887 that its 

 structure was satisfactorily elucidated by Harmer and its real 

 nature as an ally of the Enteropneusta demonstrated. 



A considerable number of individuals, probably all produced 

 by budding from a single original individual, live together in a 

 system of ramifying, sometimes anastomosing tubes which 



* W. C. Mclntosh, Cephalodiscus, Challenger Reports, vol. 20, 1887. 

 S. F. Harmer, Appendix to the preceding. Id., On the Notochord of 

 Cephalodiscus, Zoolog. Anzeiger. 1897. Id., The Pterobranchia of the 

 Siboga-Expedition, Siboga-Expeditie, vol. 26 bis, 1905. A. T. Masterman, 

 On the Structure of Cephalodiscus, Q.J.M.S., 40, p. 340, and Transactions 

 of Royal Soc. of Edinburgh, 39, 1898. Id., Q.J.M.S., 46, 1903, p. 715. 



