CHAPTER VIII. 



THE SNAIL {HELIX POMATIA, HELIX HORTEA'SIS, ETC.). 



The best snail for dissection is (on account of its large size) the 

 so-called " Roman snail." But if this cannot be conveniently 

 obtained, the common Garden Snail, Helix hortensis, will do. 

 The snail belongs to the same great group as the Swan mussel ; 

 but whereas the latter is bivalve, symmetrical, unisexual, and 

 aquatic, the snail is univalve, symmetrical, terrestrial, and 

 hermaphrodite. It has, moreover, a distinct head, from which 

 the tentacles bearing the eyes protrude, and is on this account 

 placed in a separate division of the Mollusca, the Cephalous 

 Mollusca. To prepare the snail for study, it is best to kill by 

 drowning. A number of snails should be placed in a glass 

 vessel, upon the top of which a glass plate can be placed so as 

 to shut out the air. If the glass vessel be then completely 

 filled with water, the snails immersed and the cover placed 

 upon it, they will be found, after twelve or fifteen hours, dead, 

 and in a fully extended condition. 



Before dissecting the animal there are a number of external 

 characters that should be noted. When the body is fully 

 extended, it is seen to be divisible into two regions : the 

 nmscalax foot, with the head at the anterior end, and the visceral 

 hump, which lies within, and has the shape of, the shell. Iii a 

 living snail, just beneath the shell, may be seen a round orifice 

 which alternately shuts and opens. This is the pulmonary 

 orifice, and leads into the mantle cavity, a cavity formed by a 

 fold of the integument, as is the mantle of the Anodon. The 

 eyes are borne upon a pair of long and retractile stalks. Below 

 these are a second and smaller pair of tentacles. The mouth 



