97 



cut of! in a similar manner, and then by division of the 

 first four small cells a total number of twelve is reached. 

 By the segmentation of the small cells themselves, and 

 by the addition of others a layer of cells (ectoderm) is 

 formed which gradually grows round the egg until a 

 space is left at the opposite pole. Here the four large 

 cells remain visible and form part of the wall of what 

 may be termed a blastula. The gastrula is formed by the 

 depression or invagination (very slight) of the large cells, 

 and the growth of the ectodern at the blastopore. The 

 further details of this development do not actually apply 

 to Buccinum. This, however, gives one a hint of the 

 processes by which the early carnivorous stage is formed. 

 As soon as stomach and oesophagus are developed, the 

 embryo starts devouring the eggs in the capsule. 



The ectodern is ciliated, and outlines of head and 

 foot are soon to be observed. The otocysts are visible 

 very early, and are closely followed by the eyes. At this 

 stage the embryo is quite symmetrical. The stage shown 

 in fig. 64 on PI. VIII, represents a fully developed 

 embryo some time before the shell and body attains the 

 size which may be termed the " young whelk " stage, at 

 which it leaves the capsule. At this period the head is 

 well developed, and eyes and tentacles are obvious 

 structures. The velum (fig. 64), in the form of a bilobed 

 structure fringed with particularly large cilia, has 

 reached its maximum development, and by that means 

 the larva moves actively round in the capsule. The foot 

 (fig. 64) is well developed — the pedal groove [Fed. gl.) 

 being relatively large, and the otocysts can still be seen, 

 though they have sunk away from the epithelium and lie 

 quite deep in the body. The larval shell is also 

 conspicuous. It is a very delicate chitin-like structure, 

 not yet coiled in a spiral, and marked by delicate 



