850 TKAXSACTIOXS LIVEKPOOL BIOLOGICAL SOCIETY. 



lono'itudiiial ridges. One very curious organ must also be 

 mentioned. Protruding slightly from the pallial cavity 

 is a vesicle (fig. 64, Pul.), which is apparently an 

 evagiuation from the floor of the cavity. This vesicle 

 contracts rhythmically. It has been noticed in other 

 larval gastropods and termed the " larval heart." It is 

 not, however, to be regarded as an early stage in the 

 development of the true heart, which is situated much 

 further back. 



Further development results in the absor]ition of the 

 velum, increase in size, and spiral coiling of the now 

 asymmetrical shell. Of the thousand eggs that might 

 have been in the capsule, perhaps ten may eventually 

 reach the stage, of which the shell is figured on PI. VIII. 

 This reduction is due in the first place to the small 

 number of eggs that can develop into young embryos, and 

 secondly to the caunil):ilism exhibited hy fhose same 

 embryos practically as soon as the first rudiments of an 

 nlimentary canal are developed. 



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