436 



PHYLUM MOLLUSCA 



First Type of Mollusca. The Snail (Helix), one of the 

 terrestrial (pulmonate) Gasteropods 



Habits. — The common garden snail (H. aspersa), or the 

 larger edible snail [H. pomatia), which is rare in England 

 but abundant on the Continent, serves as a convenient type 

 of this large genus of land-snails. They are thoroughly 

 terrestrial animals, breathing air directly through a pulmon- 

 ary chamber, and drowning (slowly) when immersed in 

 water. Their food consists of leaves and other parts of 

 plants, but they sometimes indulge in strange vagaries of 

 appetite. They are hermaphrodite, but there is always 



Fig. 249. — Roman snail {Helix pomatia). 



Note shell covering visceral hump ; p.ap., pulmonary aperture 

 (hicluding anus and opening of ureter) ; /., the foot ; g.ap., 

 genital aperture ; m., mouth ; e., eye on long horn ; s.Ji., one of 

 short horns. 



cross-fertilisation. The breeding time is spring, and the 

 eggs are laid in the ground. In winter snails bury them- 

 selves, usually in companies, cement the mouths of their 

 shells with hardened mucus and a little lime, and fall into a 

 state of " latent Ufe," in which the heart beats feebly. They 

 have been known to remain dormant for years. 



General appearance. — A snail actively creeping shows 

 a well-developed head, with two pairs of retractile horns or 

 tentacles, of which the longer and posterior bear eyes. The 

 foot, by the muscular contraction of which the animal 

 creeps, is very large ; it leaves behind it a trail of mucus. 

 The viscera protrude, as if ruptured, in a dorsal hump, 

 which is spirally coiled and protected by the spiral shell. 



