THE SNAIL. 339 



consists of a. ventricle and two auricles. Respiratory organs 

 are most typically represented by a pair of vascular processes 

 of the body-wall (ctenidia or gills), but one or both of these may 

 be absent. At the base of the gills there is generally an 

 olfactory organ or osphradium. The sexes are separate or 

 united. There are two common larval stages, — the Trocho- 

 sphere, which resembles the same stage in some Annelids, and 

 the more characteristic Veliger (Fig. 149); but the develop- 

 ment is often direct. The Mollusca form a very large group, 

 exhibiting much diversity of habit. 



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. pomaiia), 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 

 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 life," 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. 

 On slight provocation the animal retracts itself within its 

 shell, a process which drives air from the mantle cavity, and 

 thus helps indirectly in 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 



