366 SUBDIVISIONS OF GASTEROPODA. 



mouth of the shell ; but it is sometimes made to fit it, at all 

 stages of growth, with the most beautiful accuracy. Some of the 

 land species also possess an operculum ; but in general they are 

 destitute of it, and they form during hybernation a temporary 

 closure to the mouth of the shell, by a viscid secretion, which 

 hardens into a thin plate, and includes within it a bubble of air. 

 Behind this, a second and even a third similar partition is occa- 

 sionally found, as in the common Snail. 



980. The sub-division of this extensive class into Orders is 

 effected by arranging the different tribes according to the cha- 

 racter and position of the respiratory organs. The class may be 

 divided in the first place into the Gasteropoda proper, in which 

 the foot forms a muscular disc, adapted to enable the animal to 

 walk or creep ; and the ffeteropoda, already referred to ( 947), 

 in which the foot is small and usually compressed, and the ani- 

 mals generally swim. The true Gasteropoda form three 

 Orders. 



I. PULMONIFERA. These are for the most part terrestrial 

 species, and all of them are adapted to breathe the air by means 

 of a pulmonary sac, or air cavity, the orifice of which they can 

 open or close at will. They are usually furnished with a large 

 spiral shell ; but in some species the shell is small and internal, 

 and in others entirely wanting. The other two Orders are bran- 

 chiferous. 



II. PROSOBRANCHIATA. In which the branchiae are situated 

 in an arched chamber formed by the mantle above the neck, and 

 in front of the heart. All these animals possess a shell, which is 

 usually large enough to conceal its possessor when retracted. 



III. OPISTHOBRANCHIATA. In these the branchiae are more 

 or less exposed on the back of the animal, usually towards the 

 hinder part of the body, and consequently behind the heart. 

 Most of the species are destitute of a shell ; a few have an in- 

 ternal shell. 



IV. HETEROPODA. The second, or aberrant section of the 

 Gasteropoda, forms only a single Order. The foot is not adapt- 

 ed for creeping, and the animals swim about freely in the 

 open sea. The branchiae, when present, are usually placed on 



