ANIMALIA MOLLUSCA. a 
blance to a mantle ; itis often, however, narrowed into a sim- 
ple disk, formed into a pipe, hollowed into a sac, or extended 
and divided in the form of fins. 
The naked mollusca are those in which the mantle is simply 
membranous or fleshy: most frequently, however, it forms 
in its thickness one or several laminz, of a substance more or 
less hard, deposited in layers, and increasing in extent, as 
well as in thickness, because the recent layers always outedge 
the old ones. 
When this substance remains concealed in the thickness of 
the mantle, it is still customary to call the animals naked 
mollusca. Most generally, however, it becomes so much 
developed, that the contracted animal finds shelter beneath it. 
It is then termed a shell, and the animal is said to be testace- 
ous. ‘The epidermis which covers it is thin, and sometimes 
desiccated. 
Until my labours on the subject were published, the testa- 
cea were made 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 be admitted. Besides, 
there are several of the testacea which are not mollusca. 
The variety in the form, colour, surface, substance, and 
brilliancy of shells, is infinite. Most of them are calcareous, 
some are horny ; but they always consist of matters deposited 
in layers, or exuded from the skin under the epidermis, like 
the mucous covering, the nails, horns, scales, and even teeth. 
The tissue of shells differs according to the mode of this dis- 
position, which is either in parallel laminz, or crowded verti- 
cal filaments. 
All the modes of mastication and deglutition are found in 
the mollusca. Their stomachs are sometimes simple, some- 
times multiple, frequently provided with a peculiar armature, 
and their intestines are variously prolonged. They most 
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