78 ADAM SEDGWICK. 



his in taking a simpler diploblastic form as the starting point 

 for all the Triploblastica discussed. 



Hubrecht in his recent paper on the " Ancestral form of the 

 Chordata '^ has explained Lang's views and instituted some 

 important comparisons between Vertebrates and Nemertines. 

 I differ, however, from Hubrecht in taking a simpler form as 

 my starting point. 



I have purposely refrained from referring to the Turbel- 

 laria and other flat worms in this essay, because I cannot 

 make up my mind as to whether they are degenerate Entero- 

 coela or highly specialised Coelenterata (without a separated 

 coelom). I am, however, very much inclined to the view that 

 they are degenerate Enterocoela. 



I have also avoided discussing the Echinoderms because, 

 while their early development is easy to understand, the later 

 stages and metamorphosis are not so intelligible. 



My hypothesis with regard to the origin of the mouth and 

 anus has, so far as I know, not been suggested before. I agree 

 with Haeckel in regarding the blastopore as homologous with 

 the primitive mouth of the gastrsea. 



I have attempted to explain the peculiar behaviour of the 

 blastopore in a general way, in the first part of my paper. In 

 the second part I again consider this question in connection 

 with the Vertebrate blastopore. I dissent most strongly from 

 the view that the Vertebrate mouth and anus are both secon- 

 dary perforations, and not homologous with those of Inverte- 

 brates, e. g. Annelids. I regard them both as homologous with 

 the corresponding structures in the other Triploblastica dis- 

 cussed. 



But I have not been able to do justice to this part of my 

 subject. I could only do so by reviewing critically the exten- 

 sive literature on this subject, and by making a special investi- 

 gation of the behaviour of the blastopore in animals with a 

 prolonged larval life, and of the structures classed as primitive 

 streaks, and this I have unfortunately been unable to do. I 

 think that any such investigation would have valuable results. 

 I agree with Balfour in his view that the '^ concrescence " 



