20 CONKLIN. [Vol. XIII. 



side, the dorsum, and the left side of the embryo. In the end 

 the course of the mesenteron is like a figure 8 open at the top. 



In well-advanced embryos the head vesicle and the velar 

 folds become separated by a deep constriction from the poste- 

 rior part of the embryo. The latter contains all the yolk, and 

 it alone becomes asymmetrical ; the head vesicle, velar lobes, 

 and foot, all of which lie anterior to this constriction, retain 

 their bilateral symmetry. 



At the point of constriction there is a large spherical promi- 

 nence on each side, just dorsal to the foot ; this is the primi- 

 tive excretory organ ("urniere"). 



On the right side of the embryo, just posterior to this con- 

 striction, a depression appears in the ectoderm which becomes 

 the branchial cavity. 



The formation of the gills, permanent kidney, pericardium, 

 and heart does not occur until a later period than is shown in 

 the figures. 



In later stages the head vesicle decreases rapidly in size, the 

 velum is largely, if not entirely, absorbed, the foot becomes rela- 

 tively very large, and the shell, which during the veliger stage was 

 of the spiral type, takes on the form characteristic of the adult. 



In this condition the young or spat resemble the adult forms 

 in all essential respects, and the embryology may be considered 

 as finished. 



5 . A bnormalities. 



Under entirely normal conditions all the eggs of C. fornicata 

 and C. plana develop into perfect embryos and veligers (I have 

 not studied C. convexa and C. adunca with reference to this 

 point) ; still it is not uncommon to find one or two small, ab- 

 normal embryos in each ^gg capsule, even though taken from 

 an individual living in what seems to be a normal environment. 

 But when the adult Crepidulas are removed to the laboratory, 

 and kept in the best possible conditions, the percentage of 

 these abnormalities increases, and when the egg capsules are 

 removed from the mantle cavity of the mother, and kept in 

 dishes of sea- water, the monstrosities increase to such an ex- 

 tent that after a few days not a single normally developing egg 

 or embryo can be found. 



