The Development of Ischnochiton. 635 



third quartette in Nereis and certainly in Ischnochiton. These are 

 the only animals in which the formation of the storaodaeum has been 

 accurately followed and it may be a little early to formulate any 

 sweeping generalization , yet from the behavior of the same cells in 

 other forms, as far as these have been followed, the resemblance is 

 so close that it leads irresistably to the belief that in several other 

 forms the second and third quartette both contribute to the formation 

 of the stomodaeum. 



As a result of shifting the mouth comes to he immediately 

 posterior to the prototroch, this latter organ separating the first from 

 the second and third quartettes of ectomeres. 



In the velar field there is reason to believe that a portion of 

 the rosette series develops into the cerebral ganglia, and finally the 

 interesting fact appears that the Annelid head is the homo- 

 logue of that portion of the Chiton including the pro- 

 boscis, the first valve of the shell, and the mantle fur- 

 row including the eyes. The remaining portions, the trunk and 

 alimentary canal, have been shown to conform to the same fundamental 

 type although secondary modifications have arisen which tend to ob- 

 scure the fact. 



Thus it is seen that not only in the origin and position of the 

 various quartettes do resemblances appear, but that the early cleavages 

 of these are in many cases cell for cell the same. In the later stages 

 close cell homologies cease, but the relation of the cell groups and 

 their development in giving rise to larval or adult structures follow 

 along much the same path. After passing these facts in review and 

 considering the various structures in detail and the modifications which 

 they undergo, one fact presents itself with the greatest clearness, that 

 between Ischnochiton and the Annelids the resemblances 

 are more fundamental and closer than are the differ- 

 ences. 



2. The Ancestral Form of the Trochophore. 

 It is a fact of common observation that in the development of 

 Annelids, flatworms and Molluscs the embryo in its earlier stages 

 exhibits a radial symmetry. This in some forms is somewhat modified 

 owing to a precocious segregation of the material that will subsequently 

 enter into the first and second somatoblasts , but a 4 cell stage 

 exists and from these blastomeres three quartettes of ectomeres arise. 

 In such cases however the radial symmetry, imperfect from the first, 



