712 



PROCEEDINGS OF THE ACADEMY OF 



[Oct., 



verse ciliated bands characteristic of the embryo are acquired, among 

 these is one which passes around the head just in front of the mouth, 

 and occupying precisely the position that the row of trochoblasts 

 would be expected to assume after the growth changes which have 

 taken place. It must either be supposed that the ciliated ring in 

 question is derived from the trochoblasts or else from other cells 

 occupying a very similar position. Of the two hypotheses the latter 

 appears to me much the more probable, especially in view of the per- 

 sistence of the prototroch in many annelid larvse, where it encircles the 

 head, passing ventrally just anterior to the mouth, its position thus 

 essentially coinciding with that of the second preoral ciliated band of 

 Dinophilus. 



The cell origin of the annelid prototroch has been determined in 

 Nereis (Wilson, 1892), Amphitrite and Clymenella (Mead, 1897), Areni- 

 cola (Child, 1900) and Podarke (Treadwell, 1901). Among the mollusks 

 the precise cell origin of the velum has been determined in but two 

 forms, Ischnochiton (Heath, 1899) and Trochus (Robert, 1903), 

 although in two other forms, Crepidula (Conklin, 1897) and Planorbis 

 (Holmes, 1900), the origin of the velum has been determined with con- 

 siderable, if not absolute, certainty. Below is given, for the sake of 

 convenience in comparison, a table of the components of the proto- 

 troch and velum in the A quadrant of those forms in which it has been 

 most carefully worked out. The components of the prototroch of 

 Dinophilus are added for comparison. 



Nereis 



Amphitrite ] 

 Clymenella ! 



Lepidonotus , ] 



Arenicola J 



Podarke 



Thalassema 



Ischnochiton 



Trochris 



Dinophilus la 



2ai-2+ 

 2a='-i+ 



lai---+ 



