DEVELOPMENT OF THE PECTINIBRANCH1ATA. 351 



12. After an interval of at least eight weeks, we see the 

 young leave their capsule ; the shell is a little more elongated 

 (about 2 millim. long), hard, frangible and semitransparent. 

 The lobes have disappeared, and the young animal creeps like 

 the adult ; it is still distinguishable by the number of the turns 

 of its spire (there are only one or two). We should observe, 

 that we have hitherto found no traces of the generative organs 

 in these young. 



13. Finally, the grouped ova are sufficiently numerous to fill 

 the posterior part of the shell. 



Purpura lapillus. 



1. The ova, dispersed through an extraordinarily thick and 

 viscous humour, fill the ovigerous capsules of the animal, which 

 are flask-shaped. 



2. The size of the ovum is about 0*194 millim. This ovum 

 is composed of a delicate chorion, of an albumen, of a vitelline 

 membrane and yelk. 



3. The vitellus undergoes a very irregular cleavage. The 

 cleavage masses have no nuclei. 



4. After a certain progress of the cleavage, the ova begin to 

 be grouped together. 



5. On the twelfth and thirteenth days the ova have become, 

 so to say, a compact mass, subdivided into many masses of ova, 

 disposed in bunches. 



6. On the sixteenth day the separate groups were more 

 sharply circumscribed, and projected from the remainder of the 

 mass. These projecting groups soon took a cylindrical or pyri- 

 form shape, and were fixed to the rest by means of a peduncle. 

 The microscope showed them to be composed of a delicate 

 ciliated membrane, and that they enclosed a mass of ova; a 

 transparent substance exuded from the two sides of the peduncle, 

 upon which fine cilia appeared (the foot) ; and at the base of 

 this same peduncle the first traces of the lobes were distinguish- 

 able. Finally, many of these pyriform bodies became detached 

 from the mass and rotated upon themselves ; these were the 

 embryos. 



7. It is impossible to determine the exact number of the ova 

 which go to form one embryo, as it varies greatly. In each capsule 



