GASTEROPODA. 261 



GASTEROPODA. 



The Gasteropoda (belly-creeping animals) are characterised by their 

 being provided with a fleshy disk, serving as a foot upon which to 

 creep. The back is covered with a cloak, in or upon which the shell 

 is secreted, and may consist of one or more pieces. All the shells are 

 remarkable for the small amount of the animal as compared with that 

 of the earthy matter, so that they are extremely brittle, and their frac- 

 tured surfaces have a crystalline appearance. In some the shell is of 

 a horny texture, for example, in the Aplysia, Sea-hare (Plate I. No. 

 10) ; it is also transparent and flexible. 



The majority of the Gasteropoda are furnished with a shell, which 

 has been denominated spirivalve. The cause of this spiral arrange- 

 ment is said to be owing to the shape of the body of the animal inha- 

 biting the shell, which, as it grows, principally enlarges its shell in one 

 direction ; thus, of course, making it form a spine, modified in shape 

 according 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 imparted to the early stage of the embryo ; 

 the first deposit of calcareous matter forming the axis, the tube con- 

 tinues to rotate upon its axial pillar or columella, as it is called j and 

 by reason of some other peculiar vital tendency, the shell is gradually 

 deposited in a series of cells ; thus enlarging its conical form, and 

 winding obliquely from right to left. 



Every turn around the axis is termed a wJwrl ; and when the 

 columella is hollow, it is said to be umbilicated. 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 more vascular portion 

 placed around its aperture. 



The mouth of such of the Gasteropoda as devour vegetable matter 

 consists of a strong muscular cavity, with a single crescent-shaped 

 horny tooth, furnished along its upper edge with sharp points, sepa- 

 rated by semicircular cutting spaces, admirably adapted for the divi- 

 sion of vegetable food, and furnishing beautiful objects for the micro- 

 scope especially under polarised light. Several kinds of molluscous 

 animals are to be found in shallow water, in brooks, and ditches. One 

 of the most frequent of these is commonly known as the horny coil- 

 shell, or Planorbis corneus. The shell of this creature at first sight 

 looks like that of one of those little flat snails which are sometimes 



