IV.] THE COMMON SNAIL. 303 



Examine from the side in optical section. 



)8. The archenteroji ; a sac-Hke pit opening externally 

 by the blastopore, its walls are formed of the 

 invaginate yolk-bearing endodervi cells. 



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

 product of the more transparent rapidly dividing 

 ones referred to above. 



S. The cleavage-cavity or blastocoele; a spacious cavity, 

 enclosed between the investing and invaginate 

 layers. 



d. 'J'he Trochosphere larva; recognizable by its egg- 

 shaped contour and rotatory movements. 



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

 enlarged end. 



ft. 'Y\\& trochal ridge ; a saddle-like band encircling 

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

 cilia. 



Examine in optical section and note — 



y. T\\^foot (it first appears at this stage); a median 

 ventral outgrowth of the body wall just behind 

 the mouth. 



8. The stomatodceiwi ; a blind sac-like involution of 

 the integument, its aperture giving rise to the 

 mouth. 



£. The archciiteroii ; 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 



