ORIGIN OF METAMERIC SEGMENTATION. 51 



ciliated edge of the body of the supposed radiate ancestor/^ 

 ' Comparative Embryology/ vol. ii, pp. 311, 312. 



It seems to me that nothing can be added to make the case 

 stronger. I only wish to make one addition to the hypothesis, 

 and that is that the type of nervous system from which that of 

 the above-mentioned groups has been derived was a broad 

 ring round the mouth, in fact, more resembled the nervous 

 system of Actinia in its general diffusion over the oral surface 

 than the compact ring of the Medusa; the latter being a highly 

 specialised part of this generalised nervous system, which has, 

 however, in part persisted in the subumbrella plexus of ganglion 

 cells described by Schafer and Claus. If this hypothesis is 

 correct, i. e. if it be true that the oral surface of a Coelenterate 

 is homologous with the ventral surface of the mentioned groups ; 

 and if the nerve ring of the Medusa, the nerve-ring of 

 Peripatus, the nerve-ring and general ventral nervous plexus 

 of Chiton and Proneomenia, the cerebral ganglia and ventral 

 nerve-cords of other Mollusca^ and Annelida and Arthro- 

 pod a are all derived from a general peri-oral nervous system 

 of a Coelenterate-like ancestor, then the relation of the blas- 

 topore to the nervous system is the same in the Annelida, 

 Arthropoda and Mollusca and the same as that of the 

 mouth of Ccelenterata. 



With these facts before us, viz. similarity in development and 

 in relation to other important structures, I think we can hardly 

 doubt the fact that the blastopore in the cases mentioned and 

 the Coelenterate mouth are homologous structures. 



In the above discussion I have avoided referring to the ulti- 

 mate history of the blastopore. The fate of the blastopore in 

 the Tripoblastica is extremely variable, and it is this variability 

 only which has caused the homology ever to be doubted. 



But I think we have here two distinct questions : one deals 

 with the blastopore or mouth of the two-layered stage in 



' The absence of the connection dorsal to the anus in some Mollusca, 

 Annelida, and Arthropoda, will not I think be regarded as a fact of any 

 importance if the hypothesis be accepted with regard to the nervous system of 

 Peripatus and Chiton. 



