THE GASTR^A THEORY AND ITS SUCCESSORS. 95 



Pelagia. Too many forms with the characteristic 

 coelenterate soUd planula intervene phylogenetically, 

 to say nothing about the individual differences of the 

 gastrulae. If the Sponges are included among the 

 Coelenterata it is necessary to homologize the osculum 

 of such a form as OlyntJms or Sycajidra with the mouth 

 of a Cnidarian. This being the case the gastrulae of 

 Sycajidra and Oscarella cannot logically be considered 

 homologous with the gastrula of Pelagia, since, whereas 

 in the latter the blastopore becomes the mouth of the 

 adult, in the Sponges it closes, the embryo settling down 

 upon it, and a new mouth (the osculum) breaking through 

 at the opposite pole of the embryo. 



The relation of the gastrulae of Pelagia and NaiisitJioe 

 to the invaginate gastrula of the higher Metazoa is 

 also highly improbable. Whatever scheme of Coelen- 

 terate ancestry for the higher Metazoa we accept, 

 whether the Actinozoan, the Ctenophoran, or the Me- 

 dusan, it is improbable that such a form as one of the 

 higher Discomedusae ever came into the series. 



Is it probable too that the invaginate gastrula of 

 EiipODiatus is without genetic relation to that of Ar- 

 bacia, of Sagitta, and of AvipJiioxiisf Let us try the 

 phylogenetic method of solving this question also. 

 What form of gastrulation is most prevalent among 

 the lower forms, and how far can it be considered an- 

 cestral .-* 



Some years ago, while studying Molluscan embryology, 

 I was struck with the similarity which exists between 

 the segmentation and gastrulation of certain forms of 

 that group, and what has been described as occurring 

 in the marine Turbellarians. • The conclusion suggests 



