90 ADAM SEDGWICK. 



behaviour of the blastopore in other cases is due to larva^ 

 specialisation. 



The structure and distribution of Peripatus all point to its 

 being an extremely primitive type. We should, therefore, 

 a priori, expect to find that its development showed primitive 

 features. 



In the second part of this paper I shall attempt to show that 

 the very variable behaviour of the blastopore is explicable. 



It is hardly necessary to point out that the stomodseum and 

 proctodseum are, on the above hypothesis, structures of purely 

 secondary importance, and that I am in agreement with 

 Balfour's suggestion that the stomodseum and proctodseum 

 are not in all cases completely homologous. He says (' Comp. 

 Emb.,' vol. ii, pp. 285, 286), "As a rule an oral and anal 

 section of the alimentary tract — the stomodseum and procto- 

 dseum — are derived from the epiblast ; but the limits of both 

 these sections are so variable, sometimes even in closely allied 

 forms, that it is difficult to avoid the conclusion that there is a 

 border land between the epiblast and hypoblast, which appears 

 by its development to belong in some forms to the epiblast and 

 in some to the hypoblast." In other words, the development 

 of certain parts of the alimentary canal may be so much 

 delayed that they appear to arise from the epiblast. 



This view is of special interest in considering the structures 

 classed together as primitive streaks. As is well known, these 

 structures are generally regarded as rudimentary parts of the 

 blastopore (Balfour, Rauber). I would go further and sug- 

 gest that it is an attempted development of that portion of the 

 alimentary canal of the original ancestor which gave off the 

 coelomic pouches ; that the portion which is not wanted in the 

 development of simple larvae of living animals is delayed, and 

 consequently modified. I shall discuss this question at 

 greater length in the second part of this paper. 



I may conclude this part of my paper by describing briefly 

 the ideal ancestor of the Coejenterata and Triploblastic groups 

 now under consideration, so far as the nervous system and mouth 

 are concerned. 



