GASTEROPODA. 421 



a cloak mantle, in or upon which the shell is secreted ; 

 this may consist of one or more pieces. All the shells are 

 remarkable for the small amount of the animal as com- 

 pared with that of the earthy matter, so that they are 

 extremely brittle ; the fractured surfaces have a crystalline 

 appearance. 



In the Chitonidoe, coat-of-mail shells, the shell consists 

 of eight transverse plates, imbedded in the mantle ; in the 

 Limpets, the ordinary form is that of a cone. 



The majority of the Gasteropoda are furnished with a 

 shell, denominated spirivalve. The cause of this spiral 

 arrangement is said to be owing to the shape of the body 

 of the animal inhabiting the shell, which, as it grows, 

 principally enlarges its shell in one direction; thus, of 

 course, making it form a spine, modified in shape accord- 

 ing to the degree in which each successive turn surpasses 

 in bulk that which preceded it. It would rather appear 

 that this is principally owing to the ciliary motion im- 

 parted to the early stage of the embryo; the first deposit 

 of calcareous matter forming the axis, the tube continues 

 to rotate upon its axial pillar or columella, as it is called; 

 and by reason of some other peculiar vital tendency, the 

 shell is gradually deposited in a series of cells ; thus en- 

 larging its conical form, and winding obliquely from right 

 to left. Every turn around the axis is termed a whorl; 

 and when the columella is hollow, it is said to be umbili- 

 cated. In the spirivalve-shelled Gasteropoda, we find a 

 difference in structure between that part of the mantle 

 which envelopes the viscera, and which is always concealed 

 within the cavity of the shell, and the portion placed 

 around its aperture. 



The mouths of most Gasteropoda consist of a strong 

 muscular cavity, and a single crescent-shaped horny tooth, 

 armed along its upper edge with sharp points, and sepa- 

 rated by semicircular cutting spaces, admirably adapted for 

 the division of the vegetable food upon which they feed ; 

 furnishing beautiful objects for the microscope, and depo- 

 larising light, as in Fig. 210. Several kinds of Snails are 

 found in our brooks and ditches. One of the most frequent 

 of these is known as the Planorbis corneus, horny coil- 



