14^ THE VOYAGE OF H.M.S. CHALLENGER. 



The arrangement of these numerous tentacular plumes differs considerably from that in 

 Rhahdopleura, ih ^hich only two symmetrical tentacular arms with their pinnse occur. 

 In certain young buds, however, in which the first pair of plumes far surpass the others 

 in length, a striking resemblance is temporarily produced to the condition in Rhahdo- 

 pleura. The latter and Cephalodiscus diverge from the ordinary Polyzoa in this 

 respect, both having mobile plumes that curve gracefully in various directions, instead of 

 the somewhat stiffish corona and straight tentacles of the other forms. In Cephalodiscus, 

 besides in all probability branchial functions, they are apparently of great tactile service, 

 and if covered with cilia, as in all likelihood they are, they must aid in providing currents 

 in the cavities of the coenoecium, and, as Sars and Lankester suggest, may indirectly 

 cause food-currents, that is, bring the minute particles which constitute the nourishment 

 of the species within reach of the currents between the buccal shield and the post-oral 

 collar. The efferent currents again would readily find exit by the gill-slits behind the latter 

 lamella. Both Rhahdopleura and Cephalodiscus differ from the ordinary Polyzoa in the 

 absence of the tentacular web at the base. Both have very long pinnse ; but Cephalo- 

 discus excels the other in this respect, and is further characterised by the remarkable 

 glandular tips to the arms. The plumes are whoUy absent as such in Balanoglossus, and 

 this constitutes a marked distinction externally. As formerly stated in regard to 

 Phoronis^ however, there are certain evident homologies between the several forms. 



Post-oral Lamella. 



In Rhahdopleura Sars described " a strongly projecting, nearly semilunar border of 

 skin, ciliated on its edges," and extending from the base of the tentacular arms down- 

 wards on each side, thus forming with the buccal shield a narrow half-tube or channel 

 leading to the mouth, through which the nourishment is probably conveyed to the mouth 

 by the ciliated tentacles. The condition in Cephalodiscus, however, considerably 

 diverges, since the post-oral lamella (PI. II. fig. 1) forms a flattened apron-Hke process, 

 fixed anteriorly to the ventral surface behind the eyes, and sloping along this margin 

 backwards to the mouth, the surface gently merging into the mucous membrane of the 

 oral cavity. Moreover, a central space — more or less distinct according to the bne of 

 section — occurs between its layers. Laterally and posteriorly it forms a somewhat free 

 lamella. In minute structure this lamella presents two layers of hypoderm, each with a i 

 fine basement-layer, and having intermediate fibres, chiefly muscular. A strong series 

 of these passes out from the basement-tissue of the post-oral mucous membrane, and 

 radiates to the outer (ventral) layer of the lamella, for the posterior or dorsal has 

 merged into the mucous membrane at the sides. The hypoderm of the two surfaces just 

 mentioned offers certain differences, especially in the free part of the lamella (PJ. VI. 



1 Proc. Roy. Soc. Edin., 1880-81, -vol. xi. p. 217. 



