THE CELL ORIGIN OF THE PROTOTROCH. 131 



cells — in Hydroides of eight, and in Capitella of more than 

 sixteen. Nereis limbata differs from these forms in that 

 four of the sixteen descendants of the trochoblast do not 

 enter the prototroch, and Nereis Dtimmerillii is the only 

 annelid, so far as I am aware, in which the primary proto- 

 •troch has been thought to arise from other than the primary 

 trochoblasts. 



Passing now to other representatives of the branch Trocho- 

 zoa, we have at present among mollusks more or less complete 

 accounts of the cell-origin of the velum, in Neritiiia by Bloch- 

 mann, in Crepidnla by Conklin, in Planorbis by Holmes (prelimi- 

 nary), and in Isehnochiton by Heath (as yet unpublished). The 

 most elaborate published account is given by Conklin in his 

 beautiful work on the gas- 

 teropod Crepidnla, and for 

 this reason we may con- 

 sider this form first. 



Crepidnla lays its eggs 

 in capsules, which are pro- 

 tected under the shell of 

 the animal. In two species 

 " it is about four weeks 

 from the time the ova are 

 laid until the fully formed 

 escape from the ^^^ cap- 

 sules," and probably much 

 longer in other species. 

 Hence it is not surprising 



that the velum becomes Fig. 23. — Cr(7//n'«/«, i6-cell stage, shaded nice previous 



figures, viewed from anterior end. 



functional at a late period 



compared with the annelid prototroch, for the annelid begins 



to swim usually in a few hours after fertilization. 



The general type of cleavage in Crepidnla is the same as 

 that in the annelids. Fig. 23 of the i6-cell stage of Crepidnla 

 recalls in particular the same stage in Scolecolepis, whose eggs 

 are protected like those of Crepidnla. In both, the cells of the 

 subumbrella, or posterior hemisphere, are very large and full of 

 yolk, while those of the anterior hemisphere are very small and 



