1884.] 159 [Hyatt. 



sarily upon the probability that the somites of the embryo of 

 Amphioxus imply descent from a segmented animal ; whereas, if 

 we are correct, exactly the opposite view may be considered as the 

 more probable, and the very close comparisons made by Semper 

 between what he considers homogenous organs and parts in Ver- 

 tebrata and Vermes can only be considered as evidence of the 

 production of homoplastic effects by means of similar modes of 

 growth and to the similar habits of elongated and necessarily bi- 

 lateral animals. 



We also object to the theory that the Vertebrata may be con- 

 sidered as descended from a coelenterate ancestor because the 

 actinostome probably arose independently and very late in the 

 phylogenetic history of the Hydrozoa, and undoubtedly arose in- 

 dependently in the Porifera. A stomodeum as it appears in the 

 ascula stage or in a S} r con or ascon may be a single opening not 

 due to invagination, merely an enlarged pore or outlet. The 

 cloaca of the more specialized sponges is first an outgrowth of 

 the peripheral parts which becomes inheritable and causes the ap- 

 pearance of the ectoderm as a lining layer extending to an indefi- 

 nite depth into the interior. A stomodeum, also, does not exist 

 in most of the Hydrozoa except in the primitive shape of an out- 

 growth, the hypostome, which is the homologue of the internal 

 actinostome of the Actinozoa. These facts and the late stage at 

 which it arises in the Actinozoa during the gulinula stage shows us 

 that so far as the low types are concerned it is an independent and 

 homoplastic organ in all three. 



There are no exact comparisons between the embryos of Ascidia 

 and Amphioxus and those of the invertebrates which seem to in- 

 clude any stages later than the planula. Those that have been 

 traced between the mesoblastic somites indicate homoplastic or- 

 gans, but they seem to have no phylogenetic meaning. The dis- 

 tinct modes of development of the anterior invaginations of the 

 vertebrates show that they had a different origin from the anterior 

 tube of the actinostome, and cannot be considered homogenous 

 with that organ in the Coelenterata. The medullary invagination 

 is at first a stomodeum arising as a funnel around the blastopore, 

 and then spreads forwards in the shape of two folds, which sub- 

 sequently form a tube, and it is probable that the notochordal 

 tube, and the lateral differentiations of the archenteron ma}^ have 



