304 



MOLLUSCA. 



Mollusca. Most generally, however, it becomes so much de- 

 veloped, that the contracted animal finds shelter beneath it; 

 it is then termed a shell, and the animal is said to be testaceous; 

 the epidermis which covers it is thin, and sometimes desic- 

 cated(l). 



^ The variety in the form, colour, surface, substance and bril- 

 liancy of shells, is infinite ; most of them are calcareous ; some 

 are horny, but they always consist of matters deposited in lay- 

 ers, or exuded from the skin under the epidermis like the 

 mucous covering, nails, hairs, horns, scales, and even teeth. 

 The tissue of shells differs according to the mode of this de- 

 position, which is either in parallel laminse or in crowded ver- 

 tical filaments. 



All the modes of mastication and deglutition are visible in 

 the Mollusca ; here the stomachs are simple, there multiple, 

 and frequently provided with a peculiar armature ; their in- 

 testines are variously prolonged. They most generally have 

 salivary glands, and always a large liver, but neither pancreas 

 nor mesentery : several have secretions which are peculiar to 

 them. 



They also present examples of all the modes of generation. 

 Several of them possess the faculty of self-impregnation; 

 others, although hermaphrodites, require a reciprocal coitus, 

 while in many the sexes are separated. The first are vivipa- 

 rous, and the others oviparous ; the eggs of the latter are 

 sometimes enveloped with a harder or softer shell, and some- 

 times with a simple viscosity. 



These varieties of the digestive and generative processes are 

 found in the same order, and sometimes in the same family. 



The Mollusca in general appear to be animals that are but 

 shghtly developed, possessed of but little industry, and which 

 are only preserved by their fecundity and vital tenacity(l). 



(1) Until my labours on the subject were made public, the Tes/ocea constituted 

 a particular order; but there are so many insensible transitions from the naked 

 Mollusca to the Testacea, and their natural divisions form such groups with each 

 other, that this distinction can no longer exist. Besides this, there are several of 

 the Testacea which are not Mollusca. 



