1007.] NATURAL SCIENCES OF PHILADELPHIA, 349 



oriian in this animal. I have not observed the destiny of these excre- 

 tory cells in Fiilyur, l>ut in Crcpidula, where they are relatively larger, 

 they are ultimately pinched off and set free with their load of nitrogen- 

 ous waste. In Fasciolaria, as Glaser (1905) has shown, the external 

 kidneys become colossal organs, which are filled with nitrogenous 

 waste substance. Here the great size of these organs is probably 

 associated with high metabolism of nitrogen, the result in this case of 

 the cannibalism of the embryo. The small size of these organs in 

 Fulgur probably indicates low nitrogenous metabolism in the embryo. 

 The definitive kidney has no structural connections with the larval 

 ones. It appears as a single structure near the gastral end of the 

 intestine (figs. 32 et seq.), and a duct develops from it which runs along 

 the right side (left in ventral view) of the intestine (figs. 34, 35). As 

 a result of the invagination which forms the mantle chamber, both the 

 kidney and the intestine come to lie in the roof of that chamber, and 

 since the kidney lies near the edge of the mantle it undergoes little 

 movement during this invagination ; the intestine, which lies farther 

 from the mantle edge, moves under the kidney during this invagination, 

 and thus the relative positions of these two organs are interchanged, the 

 kidney coming to lie on the left side of the intestine (fig. 36). 



10. GILL AND HEART. 



At its earliest appearance the gill lies on the anterior side of the 

 kidney and in close contact with it (figs. 33, 34). Before the formation 

 of the mantle chamber it is merely an aggregation of cells and shows no 

 characteristic structure. After the formation of the mantle chamber 

 it lies on the left side of the kidney and its anterior border becomes 

 crenated, each of these lobules giving rise later to a tentacle-like 

 process. 



The larval heart (figs. 35, 36, L.H.) appears as a blister under the 

 blastoderm at the right of the larval kidney (fig. 34). The walls of this 

 vesicle contain smooth nuiscle cells and are pulsatile. I have observed 

 no definite vessels leading to or from this heart, and it probably serves 

 merely to keep lymph moving through irregular channels. In later 

 stages the larval heart is carried up on to the dorsal side of the embryo 

 behind the head vesicle ami velum (fig. 36) ; this movement is a part of 

 the general twisting of all the organs, which were originally posterior 

 to the foot. 



I have not observed the manner of origin of the definitive heart and 

 have not been able to recognize its anlage, unless it may be the dark 

 body between the reference letters M and Pa in fig. 36. 



