29 



formoi* are woll dcvelopGd. However, the velijin in various 

 Lame 1 lib ranch larvae is very much larn:er than the palps in 

 early stages, so that most of it must be cast oil' or ab- 

 sorbed. In the newly-attached oyster I have observed that 

 the cells of the velum are absorbed more clowly, though / 

 the palps are developed somewhat later mo.-ely as ridges on 

 the sides of the mouth. The evidence from X. fimbriata 

 and 0. virr-iniana, it seems to me, shows conclusively that 

 the palps arc not derived from the velum. The loss of the 



velum is an event not confined to the Lamellibranchs. Wil- 



(17) 

 son has observed that the trochal cells of Polygordius 



are suddenly cast off and eaten as in X. fimbriata, and 



Pruvot has described the loss of the test in nondessia. 



These all seem to be one and the same phenomenon, and indi^ 



cate that the loss of a part of the ectodermal covering in 



these and many other forms is a very primitive and general 



occurrence. 



The addition of the "S;^.ip-worms"to the forms which 

 have heretofore been knovm to possess a byssus apparatus, 

 indicates that this structure is pey... iversal in La- 

 mellibranch larvae, though in the adult it may become de- 

 generate. I may add that 1" ni-tr"-' •"i • -i'^* n-vi .'^ byssus 



