IV.] THE COMMON SNAIL. 303 



Examine from the side in optical section. 

 {3. The archenteron ; a sac-like pit opening externally 



by the blastopore, its walls are formed of the 



invaginate yolk-bearing endoderm cells. 

 y. The ectoderm ; a single layer of investing cells, the 



product of the more transparent rapidly dividing 



ones referred to above. 

 8. The cleavage-cavity or blastoccele ; a spacious cavity, 



enclosed between the investing and invaginate 



layers. 



d. The Trochosphere larva; recognizable by its egg- 

 shaped contour and rotatory movements, 

 a. The mouth; a small median orifice situated at the 



enlarged end. 



/?. The trochal ridge; a saddle-like band encircling 

 the dorso-lateral area pre-orally. Look for its 

 cilia. 



Examine in optical section and note 



y. TheyW (it first appears at this stage); a median 

 ventral outgrowth of the body wall just behind 

 the mouth. 



S. The stomodtzum ; a blind sac-like involution of 

 the integument, its aperture giving rise to the 

 mouth. 



e. The archenteron ; now partially surrounded by a 

 conspicuous large-celled granular mass the 

 digestive gland, arising as an outgrowth of its 

 wall. 



Note the bilateral symmetry of the larva at this 

 period. The blastopore appears shifted back, 

 as the result of elongation of the embryo and 



