THE SNAIL {Helix aspersa) 



External Features. 



1. Make a drawing of the Snail, viewed from the right side, to show : 



{a) The univalve spirally coiled shell (a right-handed conical spiral) within 



which is the twisted visceral hump containing the chief internal organs. 



Round the mouth of the shell is the collar, the thickened edge of the 



mantle. 

 {b) The head, with two pairs of tentacles and the mouth. The mouth is 



situated ventrally at the anterior end of the head. The tentacles are 



retractile ; the upper and larger pair have eyes at their tips, the lower 



and smaller pair arise in front of the larger ones, 

 (c) The large muscular foot, constituting the organ of locomotion, on which 



the animal glides. It is the flattened ventral part of the body-wall. 



Fig. 35. — The Snail (Helix aspersa). x IJ. 



{d) The Opening of the slime or pedal gland, below the mouth. 



(e) The genital aperture, situated at the end of a groove along the right side, 



just below and behind the right large tentacle. 

 (/) The pulmonary or respiratory aperture, situated on the right side in the 



deep notch of the collar. The anus opens beside the pulmonary 



aperture. 



2. Carefully break and detach the shell. Insert a seeker through the pul- 

 monary aperture and determine the extent of the pulmonary chamber. 



Fix the Snail down in water by pinning it through the edge of the foot. 

 Make a dorsal-view sketch showing : 



(a) The mantle, which forms the roof of the pulmonary, chamber and on 

 the underside of which the pulmonary veins are spread out. The 

 mantle secretes the shell. 



(6) The collar, which is formed by the thickening of the edge of the mantle. 



T14 



