No. I.] THE EMBRYOLOGY OF CREPIDULA. 137 



velum follows the posterior edge of the right and left arms of 

 the cross from tip to base. 



I have not observed any cilia on the posterior branch, and 

 believe that it is not functional as a locomotor organ ; the 

 anterior branch, on the other hand, is clothed with long velar 

 fiagellae, and in later stages grows more and more prominent, 

 forming the dorsal part of the velar lobe. 



The posterior branch seems to be a continuation of the entire 

 velum rather than of either the preoral or postoral portions ; 

 the velum is therefore double on the dorsal side of the embryo. 

 The posterior branch occupies the position of the velum in 

 Ishnochiton and of the prototroch of the annelids, and I inter- 

 pret the fact that it bears no cilia as indicating that it is a 

 phylogenetic remnant of the ancestral velum. The anterior 

 branch, on the other hand, is a new aquisition not represented 

 in Chiton, nor even in the more primitive gasteropods. 



It is now known that there is a postoral band of cilia in 

 quite a large number of molluscan larvae. Brooks ('76) was, I 

 believe, the first one to discover this band. Since then Haddon 

 ('82) has shown that it is present in some Nudibranchia; Hat- 

 schek ('80) has described it and an adoral band, together with 

 the usual preoral band, as present in Teredo ; and McMurrich 

 ('85) has described a postoral band of cilia as present in the 

 veliger of Crepidula. 



Judging from position and structure there can be little 

 doubt that the anterior ciliated ridge in all these cases is 

 homologous with the preoral ciliated band in annelids, the 

 posterior ridge with the postoral band, and the ciliated groove 

 with the adoral band. 



In its fully formed condition at the beginning of larval life, 

 the velum of Crepidula is a very large and an extremely com- 

 plex structure, consisting of many rows of ciliated cells run- 

 ning around the margin of the wheel-like lobes which are borne 

 on each side of the head vesicle of the larva. The locomotor 

 fiagellae are very long and powerful, and their movements indi- 

 cate some kind of nervous control. The entire margin of each 

 lobe is surrounded by many regularly arranged pigment spots, 

 which are beautifully colored, one row being a delicate green. 



