SUBDIVISIONS OP GASTEROPODA. 369 



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. 



911. The subdivision of this extensive Class into Orders, 

 may be best effected by arranging the different tribes according 

 to the character and position of the respiratory organs. The 

 following are those adopted by Cuvier : 



I. PULMONEA. These are for the most part terrestrial species, 

 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. Many 

 have no shell. 



II. NUDIBRANCHIATA. These, as well as all the succeeding 

 Orders, are aquatic, being adapted to respire water by gills, like 

 other Mollusca. The animals of this Order have no shell ; and 

 they carry their branchiae, which present various forms, on some 

 part of the back. 



III. INFEROBRANCHIATA. These are similar in many respects 

 to the preceding, but the branchiae are situated under the margins 

 of the mantle. 



IV. TECTIBRANCHIATA. In the greater part of the animals 

 contained in this Order, the branchiae are situated upon the back 

 or on the side, and are covered in by a fold of the mantle, and this 

 fold usually includes a shell more or less developed. 



V. PECTINIBRANCHIATA. The animals of this Order, to which 

 belong all the spiral-shells, except those of the Pulmonea, are so 

 named from the comb-like form of the gills, which are usually 

 situated in a cavity behind the head ; corresponding with the 

 respiratory sac of the Pulmonea. This is by far the most 

 numerous Order of the whole. 



VI. TUBULIBBANCHIATA. These have many affinities with 

 the last Order, but the shell is spiral only at its apex, where it 

 is commonly fixed to (or rather inclosed by) other bodies, and is 

 prolonged in the shape of a tube more or less regular. 



