526 A. M. CARlt SAUNDEUS AND MAliGAKET TOOLE. 



only the deeper central portion is rendered visible by this 

 method, and does appear somewhat as he figures it, while the 

 processes which spread and pass among the organs can only 

 be reconstructed from sections. 



General Considerations. 



Although a large amount of Avork has been done on 

 molluscan embryology, there still remain a number of im- 

 portant questions about which there is no general agreement. 

 One reason for this is that, of the many cases investigated, 

 only a few stages of each are as a rule known. In the 

 majority of cases where the cell lineage has been worked out 

 there is no account of the later stages ; and conversely where 

 these stages are well known, the cell lineage has not been 

 traced. Such is the case with Paludina, about the later 

 stagesof which form more has been written than aboutany other 

 molluscan genus. No satisfactory conclusion is likely to be 

 reached until our knowledjfe of the earlier stay:es of l^aludina 

 is more complete. It was with the object of completing the 

 account of the development in a single genus, in which the cell 

 lineage is known, that we undertook this work. As yet it is 

 not complete for reasons that have been mentioned. But the 

 results that we have so far obtained make it necessary that 

 we should consider their bearing upon certain theoretical 

 points in connection with molluscan ontogeny. 



The most important facts in our description of the develop- 

 ment of Aplysia are the large extent of the coelom and 

 the ectodermal origin of the kidneys. And the first of these 

 points bears directly on the question of the relation of 

 Annelids to Molluscs. The resemblance of the trochophore 

 to the veliger larva has been long recognised ; there 

 cannot, indeed, be shown to be any essential difference 

 between them. But there are two points in the development 

 of both trochophore and veliger which concern us here — the 

 type of segmentation and the development of the mesoderm. 

 That form of segmentation which is known as spiral cleavage 



