348 MOLLUSC A. 



be acquired by free swimming forms, e.g., Heteropods. The 

 foot usually contains a mucus gland, and tends to be divided 

 into three regions the pro-, me so-, and meta-podium. There 

 is a single reproductive organ and genital duct. 



A type of GASTEROPODA The snail (Helix). 



Mode of Life. 



The common garden snail (H. aspersd) and its larger 

 neighbour species (H. pematia), rare in England but 

 abundant on the Continent, are so like one another, except 

 in size, that the same description will serve for both. They 

 are thoroughly terrestrial animals, breathing air directly 

 through a pulmonary chamber. They drown (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 their 

 sexual relations are by no means simple. The breeding 

 time is spring, and the eggs are laid in the ground. In 

 winter snails bury themselves, usually in companies, cement 

 the mouths of their shells with hardened mucus and a little 

 lime, and fall into a state of "latent life" in which the 

 heart beats feebly. In such a state they have been known 

 to survive 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. On slight provocation the 

 the animal retracts itself within its shell, a process which 

 drives air from the mantle cavity, and thus promotes 

 respiration. Around the mouth of the shell is a very thick 

 mantle margin or collar, by which the continued growth of 

 the shell is secured. On the right side of the expanded 

 animal, close to the anterior edge of the shell, there is a large 

 aperture through which air passes into and out of the 

 mantle cavity. Within the same aperture is the terminal 



