300 



INVEETEBEATA 



CHAP. 



foot unite with one another in the mid-ventral line so as to form a 

 median protuberance. The shell gland grows greatly in extent and 

 depth so as to occupy the entire dorsal region behind the prototroch ; 

 the cells forming its floor become thin, whilst those constituting its 

 sides remain thick, and subsequently the invagination shallows out 

 and the floor becomes everted as a rounded hump. On this hump a 

 thin thorny membrane is secreted, which is the first rudiment of 

 the shell (Fig. 236). The eversion is due in large measure to the 

 swelling up of the stomach. The thickened edge of the shell- 



FiG. 235. — Ventral views of two stages in the development of the Trochophore 

 larva of Patella coerulea. (After Fatten. ) 



A, stage in wliicll the blastopore extends to tlie posterior pole of tlie embryo. B, stage in whieh the 

 blastopore is closed and the stomodaeum is formed. Letters as in the previous figure. In addition, 

 Up, blastopore ; /, foot ; set, prominences in the pretrochal region bearing stiff setae. 



gland region constitutes the rudiment of the mantle fold ; under it 

 appears a groove, deepest behind, and this deep spot is the rudiment 

 of the mantle cavity. The rounded hump on which the cap-hke 

 shell is secreted is the visceral hump. 



The larva with its projecting foot and cap-like shell is termed a 

 veliger; and its enlarged prototrochal girdle is called the velum. 

 At first the visceral hump and shell project forwards, but in 

 the latest stage observed by Patten they project backwards (Fig. 

 237). How this change was effected Patten did not observe ; 

 but it has been observed by Boutan (1899) in the closely allied 

 genus Acmaea, and in other primitive Gastropoda, like Fissurella 



