Chap. II. LIMITS OF THE CLASS. 101 



words, that the structural features of the animals represented as belonging to the 

 class of Acalephs are not only homologous to one another, in a general way, for 

 this would only prove that they belong to the same branch of the anmial kingdom, 

 but that they are homologous in the strictest sense, which will prove them to be 

 meml)ers of the same class, if special homologies are really a criterion of class 

 affinities. 



The animals Avhich I have considered above as belonging to the class of Aca- 

 lephs are the Ctenophoraj, the Discophorae, the Hydroids proper, and the Siplio- 

 nophorsB, within the limits ascribed to these groups by most authors. I have no 

 hesitation in referring all these to the Acalephs ; nor do I think there can be 

 any doulit left that Hydra and the Tabulata, still referred to the class of Polyps 

 by Milne-Edwards, also Ijelong to that of Acalephs. To these I would further add 

 the Rugosa, a type of Coi-als fii'st recognized as distinct by Milne-Edwards, and 

 referred by him to the class of Polyps. Respecting these last, some uncertahity 

 still remains, since they are all fossil, and their affinities can only be inferred from 

 the structure of their solid parts. As to all the other groups, the evidence that 

 they belong to the class of Acalephs seems to me satisfactory, though it is not 

 throughout of the same kind. For instance, the evidence that the Ctenophoroe are 

 Acalephs is altogether anatomical, and chiefly based upon the special homologies 

 of their parts : it receives no additional confirmation from Embryology, as the 

 young at birth are already very similar to the parent, and do not exhil)it those 

 complex relations which we observe in other Acalephs. The affinities of the Dis- 

 cophorte, Siplionophorte, and Hydroids, on the contrary, are established upon embry- 

 ological as well as anatomical evidence. 



Beginning with the Ctenophoraj, we have first to sift the arguments brought 

 forward to support their connection with the Mollusks. The idea that the Cteno- 

 phoraj are allied to the Tuuicata, and especially to the Salpa^, was first suggested 

 by Quoy in the Zoology of the Astrolabe (vol. 4, p. 36), and afterwards more 

 fully developed by Vogt in his Zoological Letters (vol. 1, p. 254), where he repre- 

 sents them as a distinct class, intermediate between the Bryozoa and the Tuuicata, 

 which are themselves also considered as distinct classes. The ground upon which 

 they are brought to the branch of Mollusks is chiefly their bilateral appearance ; 

 and it is there stated," that, with the exception of their glassy transparency, they 

 have not one trait of their organization in common with the Acalephs. Such an 

 assertion, from a naturalist to whom science owes important contributions to our 

 present knowledge of an extensive and most intricate group of Acalephs (the 

 SiphonophorK), cannot be passed unnoticed. That Bryozoa and Tuuicata are bilateral 

 animals and truly belong to the type of Mollusks, is unquestionable ; and that the 

 Ctenophoraj share the peculiar consistency of their body as fully with the Salpae 



