DEVELOPMENT. 49 



I will not here dwell upon the particular form of egg-capsules 

 produced by each prosobranchiate genus, because I shall illus- 

 trate as many of these as possible in connection with the various 

 monographies which will be hereafter presented. In the present 

 volume I figure only those belonging to the genera which are 

 herein discussed, and for convenience of comparison they are 

 grouped together. How these bodies are developed is still a 

 mystery. (PL 7.) 



We will now rapidly sketch the developmental history of the 

 major portion of the prosobranchiates, illustrated by figures of 

 the common Buccinum undatum and Purpura lapillus. (PI. 8.) 



The transformation of the egg into the embryo is preceded by 

 division of the yolk mass into blastomeres (cells), which begins 

 immediately after fertilization and is speedily completed. The 

 yolk assumes the appearance of a cluster of round nucleated 

 cells, the large ones internal, the smaller external ; forming the 

 embryo except a portion which remains as nutritive material. 

 The yolk first loses its spherical form, elongates somewhat, de- 

 velops upon its entire surface a delicate ciliary covering, and 

 begins its wonderful rotary movement. At the anterior part of 

 the body a circlet or crown of long cilia arises ; the portion of 

 the body supporting this is then elevated into a ridge, then a 

 ring, and finally it develops on each side into a rounded lobe. Both 

 lobes together present somewhat the appearance of the figure 8 : 

 these are the vela which Forskal had already described as the 

 organs of locomotion of the univalve molluscan larva. Immedi- 

 ately below the velum the mouth is developed as an invagination ; 

 at the posterior end the anus is similarly formed ; both open into 

 the intestinal cavity which has been formed by the displacement 

 of the large yolk cells in the centre. An intestinal is now pres- 

 ent, and the large cells which are somewhat heaped up posteriorly, 

 become in large part the liver and intestinal wall. A body- 

 cavity between this large-celled intestinal wall and the small- 

 celled body wall is not yet present, and originates later by a 

 separation of the two walls and the appearance of a fluid between 

 them. 



Below the mouth the foot arises as a blunt ciliated appendage, 

 whilst the ciliated covering of the rest of the body has become 



7 



