MOLLUSCA. 17 
of the pallial membrane; the former we have found 
to be more or less covered by glands appointed for 
the purpose, situated in the circumference of the 
mantle, but as these glands do not exist elsewhere, 
no colouring matter 1s ever mixed with the layers 
that increase the thickness of the shell, so that the 
latter always remain of a delicate white hue, and 
form the well-known iridescent material usually 
distinguished by the name of nacre, or mother of 
pearl,” * 
This lucid description of the process specifically 
applies to the Conchifera, or Bivalves; but the 
formation of the shell in the Gasteropoda is not 
marked by any important point of difference. 
No species of this great Division of animals is 
furnished with limbs, properly so called: unless 
we may consider as such the long flexible ten- 
tacular arms of the Poulpes and Cuttles (Cepha- 
lopoda) which are used as instruments of an un- 
gainly sort of crawling, as well as for seizing prey 
and dragging it to the mouth: yet various modes 
of locomotion are by turns practised among the 
Mouuusca. In one extensive Class, the Gaster- 
opoda, of which the Limpet and the Snail are 
examples, an even gliding movement is that which 
is most characteristic ; a broad expanded muscular 
disk, called the foot, being applied to the surface 
over which the animal crawls. Many of the 
aquatic members of the Class are able to float at 
the surface by the aid of the same organ. They 
erawl to the top of the water up the stem of a 
plant, or the side of a rock, and stretching out the 
bottom of the foot along the surface, the back being 
downward, it presently dries by contact with the 
* Jones’s Animal Kingdom, p. 385. 
CG 
