46 LAMELLIBRANCHIA. 



sion into right and left palps being more apparent than real. This 

 condition appears to be brought about by the stronger growth of the 

 lateral parts of these structures.] 



This origin of the labial palps partly confirms the assumption made by 

 LOVEN that the velum of the larva may pass over directly into the adult 

 palps. In the double nature of this organ LOVEN finds agreement with the 

 double velum of the Gastropoda, a resemblance which is strengthened by 

 ZIEGLER'S observation of the median division of the ciliated area of Cyclas. 

 The small section of the ciliated area lying above the mouth might then 

 be regarded as the last vestige of the former ciliated ring of the Trocho- 

 phore. The ciliated area of Cyclas, however, as we have just shown, seems 

 rather to correspond to the ad-oral ciliation of the larva (p. 40). And since 

 this serves for feeding more than for locomotion we see that a part of the larval 

 body passes over into a similarly functioning organ of the adult animal. 

 The significance of the labial palps lies principally in their relation to the 

 capture of food, in which they assist through their position and their ciliation 

 (THIELE, No. 55). An exact knowledge of the fate of the entire velum, i.e., 

 of the pre-oral ciliated portion of the body in a marine Lamellibranch, would 

 be of great value. 



The metamorphosis of the Lamellibranch larva into the adult is 

 characterised chiefly by the complete degeneration of the pre-oral 

 part of the body which was so large in the former. In the larva, the 

 highly developed velum spreads out between the mouth and the 

 shell (Figs. 15-18, pp. 31-36 and Fig. 19, p. 40), but, as develop- 

 ment proceeds, this area becomes contracted (Fig. 21 A), and finally 

 almost entirely disappears, in keeping with the condition of the 

 adult, in which the cephalic region is almost completely lost. 



While the external changes of form just described have been taking- 

 place in the embryo, a marked advance has also taken place in the 

 inner organisation, but this will be entered into later on. The young 

 of Cyclas and Pisidium leave the mother only when they possess, on 

 the whole, the same organisation as the adult. 



Divergencies in the Metamorphosis accompanying the Monomyarian 



Condition. 



Among those forms classed by the older malacologists as the 

 Monomyaria, the transition from the larva to the adult has only been 

 well investigated in Ostrea, and their development, judging from 

 this form, seems up to a certain point to agree closely with that of 

 other marine Lamellibranchia. We have already seen how close the 

 agreement is in the early stages of development (p. 28 Fig. 14 

 and Fig. 16, p. 33). The TrochopJwre larva already possesses an 



