52 ADAM SEDGWICK. 



embryonic development and asks whether that stage has a 

 counterpart in evolution ; the other deals with the subse- 

 quent development of the blastopore and asks whether 

 that subsequent development throws any light on the evolution 

 of the mouth and anus. 



But at the same time I must admit that the fate of the blas- 

 topore is so peculiar, that the doubts which on that account 

 have been expressed as to its phylogenetic meaning are not 

 unreasonable. The case stands thus. The blastopore in 

 Serpula gives rise to the anus; in most other Chaetopoda to 

 the mouth ; similarly in the Mollusc Paludinait becomes the 

 anus, while the general rule among Mollusca is that it should 

 become the mouth. It would seem to follow from these facts, 

 as Lankester has already pointed out, that if the blastopore is 

 in each case homologous, then the anterior end and mouth of 

 Serpula must be homologous with the posterior end and anus 

 of other closely allied Chsetopods. This is manifestly absurd. 

 There are two ways out of the difficulty ; either the homology 

 of the blastopore must be given up, or we must suppose that 

 primitively it gave rise to both mouth and anus, and that its 

 specialisation as a larval organ has caused the 

 variability of its subsequent history. The latter view 

 is obviously suggested by the elongated form the blastopore 

 first assumes in many animals, extending as a slit along the 

 whole of the ventral surface of the embryo.^ The blastopore 

 never retains for long this form, but soon becomes specialised 

 to a round opening, the definite blastopore,^ by the closure of 

 the lip of the slit except at one point. The point at which it 

 remains open must depend on the shape of the larva, &c., and 

 will obviously be determined by the convenience of the larva. 



This hypothesis that the mouth and anus of the Triploblas- 

 tica is derived from a single opening, represented in living 

 animals by the Ccelenterate mouth and, on the assumption 



1 This fact was first pointed out by Lankester, vide this Journal, vol. xvi, 

 1876, p. 326. 



^ A special name is wanted for this structure, to distinguish it from the 

 blastopore of the £>astrula stage. 



