32 THE VOyAGE OF H.M.S. CHALLENGER. 



Homologies. 



Cephalodiscus approaches very nearly to Rhabdopleura in almost all the structural 

 features, and it is probable, when more complete investigation of both is carried out, 

 these resemblances will be increased rather than diminished. 



Thus the Cosnoscmm in both is largely developed and wholly independent of the 

 polypides, while it is mainly secreted by the buccal shield or disk. The regularly ringed 

 cylindrical ccenoecium of Rhabdopleura is, however, very different in form from the 

 irregular, much branched and hispid ccenoecium of Cephalodiscus. Moreover, in the 

 latter it is the secretion of the adults, whereas in Rhabdopleura much of it would 

 appear to be the product of the younger buds. Phoronis, again, secretes its simple 

 gelatinous investment in the sand, or in the form of tubes attached to stones or other 

 foreign bodies, while an Australian species betakes itself to the gelatinous case of Ceri- 

 anthus. There is thus comparatively little method in the formation of its isolated 

 dwelling. Balanoglossus, on the other hand, has only a mucous lining to its perforation 

 in the sand, though the secretion of this form is also very abundant. Moreover, BateSon 

 describes a peculiar odour in the living animals, and the spirit-preparations of Cephalo- 

 discus also give evidence of a characteristic odour, though it may differ from that of the 

 former. 



The general form of the polypides of Cephalodiscus and Rhabdopleura diverges 

 very considerably, the former being free, while the latter is fixed by the axial stem. 

 Both, however, are small, while the size attained by Phoronis is a distinctive feature, 

 as also is the absence of a pedicle from its cylindrical body. 



The Buccal Shield is much larger in Cephalodiscus than in Rhabdopleura, and its 

 secreting powers more active. The buccal shield is absent in Phoronis as such, but is 

 represented by the epistome. As will be pointed out by Mr. Harmer, the proboscis of 

 Balanoglossus appears to be the homologue of the disk, though only one proboscis-pore 

 is present, while two exist in Cephalodiscus. Further examination is necessary in 

 regard to these organs in Rhabdopleura. 



The Branchial Plumes have a kind of skeletal system or basement-tissue in both 

 Cephalodiscus and Rhabdopleura, but they are much fewer in the latter than in the 

 former, which, moreover, has a bulbous and glandular tip to the main axis, thus simulating 

 such organs as the large eye at the tip of the branchiae of Branchiomma. The large 

 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 branchiae are now arranged 

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



