304 ELEMENTARY BIOLOGY. [CHAP. IV. 



displacement, upon the development of the foot. 

 An integumental pit known as the shell-gland, 

 appears at the hind end of the body during this 

 stage. 



e. The Veliger larva. A head-segment is now be- 

 coming differentiated, and the rotatory movements 

 of the animal are less marked. Bilateral symmetry 

 is becoming disturbed. Note especially — 



«.. The velum; a ciliated pre-oral fold, occupying 

 the position of the original trochal-ridge. 



ft. The foot ; now greatly increased in size and 

 utilized for purposes of locomotion. 



y. The eye-spots; two black masses at the sides of 

 the head segment. 



/ The advanced larva. Tentacles are now appearing, 

 as paired outgrowths of the cephalic wall (the eyes 

 are not carried up by them as in Helix). The 

 original bilateral symmetry no longer obtains. 

 Note the beat of the heart ; the presence of the 

 visceral sac and of its related shell — at this stage 

 a transparent chitinous cap. The velum is under- 

 going a marked reduction. 



If any difficulty is experienced in examining the 

 embryos in the living state, they should be liberated 

 from the surrounding albuminous investment by 

 means of a couple of needles, and submitted, for 20 

 minutes, to the action of \ p.c. Osmic acid solution. 

 They may then be transferred to alcohol of in- 

 creasing strength, and preserved for future ex- 

 amination. 



