194 CONK LIN. [Vol. XIII. 



and that the velum had a wholly different origin from the 

 annelidan prototroch. 



Lillie ('95) has extended all the above-mentioned resemblances 

 between annelids and gasteropods to the lamellibranchs, and in 

 addition has discovered the following: (5) the first somatoblast 

 i2d), which gives rise to the ectoderm of the trunk, has exactly 

 the same origin and position and a similar history in the anne- 

 lid and lamellibranch ; (6) it gives rise to a growing-point and a 

 ventral plate in all respects essentially like those of the annelids. 

 Lillie shows good reason for believing that in other mollusks 

 the posterior growing-point is derived from these cells. 



To this list of resemblances between the annelid and the 

 mollusk, which I can confirm in the case of the gasteropod, I 

 have been able to add the following: (7) the rosette series of the 

 gasteropod is exactly like the cross of the annelid in origin, 

 position, and probably in destiny. The intermediate girdle cells 

 of the annelid are like the cross of the gasteropod in origin, 

 position, and destiny (at least in part). The differences, there- 

 fore, between the annelidan and molluscan cross which Wilson 

 emphasizes are not real ones; (8) the trochoblasts of the anne- 

 lids are precisely similar in origin and destiny (at least in part) 

 to the turret cells of the gasteropods. In some annelids 

 (Amphitrite, Clymenella), the prototroch is completed by cells 

 of the same origin as in Crepidula and Neritina. The differ- 

 ences which Wilson points out between these two structures 

 do not therefore exist. In both annelids and mollusks the pro- 

 totroch lies at the boundary between the first quartette on one 

 side, and the second and third on the other. In both there is 

 found a preoral, an adoral, and a post-oral band of cilia; (9) in 

 the gasteropod the apical cells give rise to an apical sense organ 

 such as is found in many annelid trochophores ; (10) the sjipra- 

 oesophageal ganglia and commiss2ire apparently arise from the 

 same group of cells in annelids and gasteropods; (11) the fourth 

 quartette in annelids and gasteropods contains mesoblast in 

 quadrant D, but is purely entoblastic in quadrants A, B, and 

 C; (12) a fifth quartette is formed in gasteropods and some 

 annelids (Amphitrite, etc.), and consists of entoblast only; (13) 

 in the gasteropod larval mesoblast arises from the same group 



