138 CONKLIN. [Vol. XIII. 



another a faint red, and still another sepia or black. Each 

 velar lobe contains many stellate mesoblast cells, and the 

 whole structure is highly irritable and contractile. 



This structure is much larger and more complex than the 

 annelid prototroch ; and perhaps nowhere, with the possible 

 exception of the echinoderms and Enteropneusta, is there a 

 larval locomotor organ which will compare in size and com- 

 plexity with the velum of many gasteropods. 



Comparisons. 



Reference has already been made to the origin of the velum 

 in Neritina (p. 94). Blochmann was able to trace the tip cells 

 of the transverse arms (" Urvelarzellen ") to this structure, but 

 he does not indicate what other cells enter into it. Of these 

 velar cells he says (p. 162): " Schon wahrend das Ektoderm 

 anfing sich nach der ventralen Seite hin auszubreiten, sind die 

 Zellen vz und V3\ an die beiden Seiten geriickt, und in einer 

 dieselben verbindenden Zellreihe werden dieselben lichtbrech- 

 enden Kornchen bemerkbar (Fig. 66), wodurch eine weitere 

 Ausbreitung des Velums angedeutet wird. Dasselbe erscheint 

 jedoch noch nicht kontinuirlich, sondern von den urspriinglichen 

 Velarzellen vz und vsi ausgerechnet sind jederseits nur zwei 

 oder drei Velarzellen sicher zu erkennen. Auch ventral ist das 

 Velum noch nicht geschlossen." 



As I have already indicated, it is very probable that these 

 same cells form the lateral portions of the velum in Crepidula. 

 I was not inclined to accept this view at first, because on the 

 anterior side the first velar row lies beyond the tip cells of the 

 anterior armband because the tip cells of the right and left 

 arms seemed at first sight very far removed from the velum. 

 Accordingly, I said in my first preliminary ('91) : <' In Crep- 

 idula it seems that no part of the transverse arms forms the 

 velum." However, a more prolonged and careful study of the 

 velum shows that these terminal tip cells, increased to four on 

 each side, very probably form the lateral portions of the first 

 velar row. There is thus the most exact agreement between 

 these two animals in the origin of this portion of the velum. 



1 See Note p. 204. 



