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



ganglion in the diagram ^ of the latter would nearly correspond with the nerve-centre 

 in Cephalodiscus. 



In Loxosoma the position of the buds is very different, viz., in the region of the 

 stomach, and there are also often more than two. 



Homologies. 



Cephalodiscus approaches Rhahdopleura very nearly in many 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 Cocncecium 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 ccencecium of Rhahdopleura is, however, very different in form from the 

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

 latter it is the secretion of the adults, whereas in RhabdopAeura 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 Cerianthus.^ 

 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. Further, Bateson describes a 

 peculiar odour in the living animals, and the spirit-preparations of Cep)halodiscus 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 Rhahdopleura 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 Rhahdopleura, 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 usually present, while two exist in Cephalodiscus. Further examination is necessary 

 in regard to these organs in Rhdbdop)leura. 



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

 Cephalodiscus and Rhahdopleura, 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 branchise of Branchiomma. The large 



' Quart. Journ. Micr. Sci., October 1886, vol. xxvii., pi. xxii. fig. 19. 

 2 Vide Dr. Haswell, Proc. Linn. Soc. N. S. JV., vol. vii. p. 607. 



