No. I.] GERM-LAYERS IN CLEPSINE. 173 



of the ancestral history being preserved in the foetus or the 

 larva may be summed up in the following way: There is a 

 greater chance of the ancestral history being lost in forms 

 which develop in the egg; and of its being masked in those 

 which are hatched as larvae." 



Balfour's phylogenetic conclusions were based on a compari- 

 son of the various larval forms with one another. No attempt 

 was made to identify larval with foetal features of development, 

 and to verify in this way deductions based on the occurrence of 

 similar larvae in different groups. It cannot be denied, how- 

 ever, that in this direction lies a crucial test of our theories re- 

 specting larval forms. As long as it remains impossible to find 

 a parallel in fundamental features between the foetus and the 

 larva, so long will it be impossible to decide how much is ances- 

 tral and how much adaptive in the larva. 



In spite of volumes devoted to the discussion of the subject 

 the larva of Polygordius still remains a morphological puzzle. 

 After an extended, critical analysis of the leading theories re- 

 lating to this larva, Fraipont closes his magnificent monograph 

 on Polygordius with the following confession : " It is not yet 

 possible, in the present state of our knowledge, to determine 

 what is the morphological significance of the larva of Poly- 

 gordius, the Trochophora (Trochosphere) of the annelids." 



In the history of the teloblasts we find a satisfactory basis 

 for the direct comparison of the foetal with the larval course of 

 development. What, then, is the foetal Trochosphere? and of 

 what importance is it to our theoretical conceptions of the an- 

 nelid embryo? Does it throw any light on the structure of the 

 ancestral Trochosphere, — the Trochozoon? and does it assist 

 us to a better understanding of the nature and extent of the 

 abbreviations and modifications represented in direct develop- 

 ment? 



For the purpose I have in view Clepsine furnishes an excel- 

 lent example of the direct type of annelid development, while 

 Polygordius affords a well-known example of the larval type. 

 These forms may, therefore, serve as points of departure for the 

 few suggestions to be offered here. The development of the 

 Trochosphere of Polygordius is very imperfectly known, but the 

 gap is now bridged by Hatschek's studies on Eupomatus 

 (No. 35). 



