No. I.] 772^^ EMBRYOLOGY OF CREPIDULA. 17 



usually very small, and in some cases has almost entirely disap- 

 peared. Quite a complete series of stages in the degeneration 

 of this organ was observed, from the fully formed organ on the 

 one hand to a minute papilla on the other. Sections of such 

 animals show that neither male nor female sexual cells are 

 produced at this time. Although the evidence seems to favor 

 the view that we have in these cases an example of successive 

 hermaphroditism, I am not able to assert that this is really the 

 case, although I have spent considerable time in attempting to 

 decide it. 



3. Types of Development in C. fornicata, C. plana, C. convexa, 

 and C. adunca. 



All the ova produced by one individual are laid at about the 

 same time, and the development proceeds very slowly. In C. 

 plana and C. fornicata it is about four weeks from the time the 

 ova are laid until the fully formed veligers escape from the 

 &%'g capsules, and in C. convexa and C. adunca the period pre- 

 ceding the escape of the young is probably much longer. 

 How long the veligers of the two former species lead a free- 

 swimming life I do not know, since I found it impossible to 

 keep them alive until they were transformed into the spat, or 

 young Crepidulas. From circumstantial evidence, however, 

 I am convinced that in C. fornicata the veligers do not swim 

 about for more than three weeks, probably about two. On 

 July 23, 1890, Mr. Vinal Edwards, collector for the United 

 States Fish Commission, brought me a large number of C. 

 fornicata, dredged from the mouth of the New Bedford river. 

 A large proportion of these were carrying egg capsules, many 

 of which contained fully formed veligers, while most of them 

 were in an advanced stage of development. On August 11, 

 nineteen days later, another lot of Crepidulas were taken at 

 the same place, but no eggs or ^g^ capsules could be found ; 

 the parent shells, however, were covered with the very small 

 spat of this species. During July of the next year (1891) 

 . I kept a lot of veligers of this species in a wooden box, the 

 bottom of which was covered by silk netting. The box was 



