68 LILLIE. [Vol. X. 



hooks are forced into the tissue of the fin. Now were the 

 original position of the thread-gland to be retained, i.e., at the 

 anterior angle of the valves of the shell, the larva would not 

 be likely to "land" on the fin in so favorable a position for 

 attaining a secure footing. We have seen before that the 

 primitive function of this gland was probably excretion ; the 

 change in function, then, has brought about a corresponding 

 change in position. 



I should not attempt to apply such an explanation to the move- 

 ments of the oral plate and ventral plate, but would rather explain 

 them as caused by the necessities of precocious segregation which 

 must often isolate organs which later are intimately related. 



Two factors are responsible for the redistribution of the 

 sensory hairs in the glochidium, vis., the backward motion of 

 the thread-gland, and the invagination of the larval mantle. I 

 have just considered the movement of the thread-gland; now as 

 to the larval mantle. I call this the larval mantle because all 

 authors who have described its later history state that it does 

 not form (or does in part only) the mantle of the adult, but 

 degenerates, giving us the so-called "fungus-like bodies" of 

 Braun. The larval mantle is established certainly in a very 

 curious way, and yet I think that a little consideration will 

 convince us that at bottom it is not much different from 

 the mantle of other forms. Its borders are formed by 

 the evaginated edges of the shell-gland to which the shell 

 remains attached. Practically the same thing is true for the 

 embryonic mantle of all Molluscan forms. The difference 

 is simply that the cavity is so enormous in this larva ; and 

 instead of being a groove-like cavity above the foot, deeper at 

 the (primitive) posterior end, here the cavity is so great that 

 the embryo lies within it at one end. Schmidt has attributed 

 this concentration of the embryonic area to the immense de- 

 velopment of the adductor muscle. It seems to me, however, 

 an unnecessary assumption to make, for the parts in question 

 no doubt occupy all the space they require. We should expect 

 the embryonic material to assume a compact form, and its 

 position at the posterior end is the natural one. The pos- 

 terior end is always the growing zone of the embryo. 



