362 INVERTEBRATE MORPHOLOGT. 



It seems to be a secretion of the ectoderm of the mantle and 

 visceral hump, the anterior arms contributing only a thin 

 external layer. 



Development and Affinities of the Cephalopods. — The ova of those 

 Cephalopods which have been studied are richly provided with yolk, in 

 consequence of which the development becomes considerably modified, 

 definite traces of the Veliger condition being entirely lost. It seems clear, 

 however, from the marked development of the head, the presence of a 

 radula, and the general arrangement of the viscera, that the ancestry of the 

 group is to be sought for among the primitive Gasteropods, but in forms 

 more primitive than any existing forms. The symmetrical shape of the 

 body and the character of the viscero-pericardial cavity suggests forms 

 intermediate in development between the Amphineura and the Diotocardiate 

 Prosobranchs. 



So far as the various groups are concerned there can be little doubt but 

 that the Tetrabranchs are the more primitive forms, showing as they do 

 less specialization of the foot, what must be considered a more primitive 

 shell, and a more general tendency towards a paired arrangement of the 

 organs than is found in the Dibranchs. The duplication of the ctenidia 

 and nephridia must, however, be considered a secondary acquisition. The 

 Decapods, again, seem to be on the whole more primitive than the Octo- 

 pods, the character of the coelom and the presence of a shell in the former 

 being points to which attention may be called in this connection. 



The Affinities of the Mollusca. — Attention has already been called to 

 the similarity of the typical Gasteropod and Pelecypod larvae to the An- 

 nelid Trochophore, and the evident conclusion has been pointed out that 

 the Annelids and Mollusca are to be traced back to a common ancestor 

 represented by the Trochophore. It is difBcult otherwise to understand 

 the remarkable similarity which exists between the two larvse— similarities, 

 including not only the general arrangement of the locomotor cilia, but ex- 

 tending as well to internal organs, such as the nephridia. In two respects, 

 however, the MoUuscan Veliger differs from the Annelid Trochophore ; it 

 possesses a shell and a foot. These features are, however, readily ex- 

 plicable as a throwing back in the ontogeny of important structures origi- 

 nally developing at a much later period in the life of the animal — a phe- 

 nomenon of by no means unfrequent occurrence. It must be admitted 

 however, that frequent modifications of the Trochophore arrangement are 

 to be found, as has been indicated in the descriptions of the Amphineura 

 and these become especially interesting from the fact that in the former a 

 primitive arrangement of the parts of the body must be recognized. If 

 the Trochophore represents an ancestor, then it might be expected that it 

 would be found more perfectly represented in the Amphineura than in the 

 highly specialized Gasteropods, or even than in the Pelecypods. 



It is important, then, that the possibility of some of the similar struct- 

 ures of the Trochophore and Veliger having been independently acquired 



