82 ADAM SEDGWIOK. 



On the Homology of the Mouth and Anus with the 

 Mouth op the Ccelenterata. 



It will be generally admitted that the mouth and anus of 

 the Annelida, Arthropoda, and Mollusca are homologous struc- 

 tures — i. e. that the mouth of an Arthropod is homologous 

 with that of an Annelid, and with that of a Mollusc, and that 

 the anus in each of these groups is homologous with the anus 

 of the other groups. It is well known that the blastopore in 

 these groups presents considerable differences in its relation to 

 the mouth and anus. In one form it is directly converted into 

 the mouth, in another into the anus ; while sometimes it 

 entirely closes up and gives rise to neither (for summary of 

 facts vide Balfour ' Comp. Embryology,^ vol. ii, pp. 281, 382). 

 This variability, in the fate of the blastopore was first pointed 

 out by Lankester.^ It is very puzzling, and has led some 

 morphologists to regard it as a structure which is not homolo- 

 gous in the different animals, and of no particular phylogenetic 

 significance. It seems to me, however, that a little considera- 

 tion shows that this view of the blastopore must be given up, 

 and that there are very strong grounds for regarding the 

 blastopore as homologous in every case,^ and also as homologous 

 with the mouth of the Ccelenterata. Before proceeding to 

 discuss the main point of this section of my paper, I must 

 definitely examine this question about the blastopore. 



On the Blastopore. — Either the blastopore has an ancestral 

 meaning or it has not. It seems to me that we have no right 

 to assume that this or any other embryonic structure or process 

 is without a phylogenetic significance until all other views 

 have been shown to be untenable. 



Itis often said when any peculiar embryonic process is discussed 



' " On the Coincidence of tlie Blastopore and Anus in Paludina." 

 'Quarterly Journal of Mic. Sci., 1876. 



' It must be distinctly understood that the only groups referred to in the 

 following paper are the Vertebrata, Annelida, Arthropoda, Mollusca, Balano- 

 glossus, Brachiopoda and Sagitta. For the present, 1 leave the Platyelminthes 

 and Echinoderms out of consideration. The special case of the Vertebrata 

 will be considered in Part II. 



