REPORT ON CEPHALODISCUS DODECALOPHUS. 33 



size and firm nature of the simple branchial filaments of Phoronis are sufficiently 

 diagnostic, as also is the fine, double, convoluted arrangement seen in the Philippine 

 and Australian forms. The skeleton and circulatory system of these organs is much 

 more highly developed than in either Cephalodiscus or Rhabdopleura. In Balano- 

 glossus, again, considerable divergence has happened, for the branchise are now arranged 

 in lateral series along the second region of the body, and are supported by an 

 elaborate skeleton of chitinous elements ^ and furnished with numerous gill-slits. Such 

 a modification, however, does not seem very far fetched when a section of the bases of 

 the filaments after entering the axis of the plumes is made in Cephalodiscus. In 

 connection with the arrangement of the plumes it is also interesting that in the 

 Eupolyzoa {e.g., as described by AUman^ in Paludicella, Nitsche^ in Flustra mem- 

 branacea, and Haddon * in Flustra carbasea) the growing tentacles in the bud present 

 bilateral symmetry. 



The Circulatory System is evidenced only by the lacunse (nuchal or collar-spaces) 

 and their connections with the bases of the plumes in Cephalodiscus, but it would 

 appear to be more largely developed than in Rhabdopleura, for its presence has not yet 

 been indicated in that form. The circulatory system in Phoronis attains a much higher 

 degree of complexity, since its large vascular ramifications with the well-marked nucleated 

 corpuscles have no parallel in either. Cephalodiscus, however, agrees in that its collar- 

 spaces are in connection with the reticulated or lattice-like centre of the main stem in 

 each plume. The circulatory system in Balanoglossus, again, is also largely developed, 

 especially in connection with its branchial system, though the contents of the vessels are 

 less conspicuous. On this head all the foregoing widely diverge from the ordinary 

 Polyzoa. 



The Digestive Apparatus in both Cep>halodiscus and Rhabdopleura closely agrees 

 with the type in the Polyzoa, all being characterised by the flexure which causes the 

 close proximity of mouth and anus. Moreover, the pyloric difi"erentiation indicated in 

 Cephalodiscus is prevalent in the Polyzoa and also in Phoronis, and though it has not 

 been described in Rhabdopleura, traces of it may yet be found. The environment of 

 the mouth in the latter and Cephalodiscus is related, but while the post-oral lamella is 

 connected with the buccal shield in Rhabdopleura, it forms a special structure in 

 Cephalodiscus. In Phoronis the general plan of the digestive system is the same, 

 though the pyloric region of the stomach attains much greater size. In regard to this 

 system all the foregoing closely approach the Eupolyzoa, the oesophagus, stomach, pyloric 

 vestibule, intestine and rectum showing a similar arrangement. When Balanoglossus, 



1 Vide Monograph on the Brit. Nemeiteans, Ray Society, 1872-73, p. 146. 



* Fresh-water Polyzoa, p. 36. 



3 Zeitschr. f. wiss. ZooL, 1871, Bd. xxi. p. 457. 



* Quart. Journ. Micr. Set., 1883, vol. xxiii. p. 518. 



(ZOOL. CHALL. EXP. — PART LXII. — 1887.) Qqq 5 



