MOLLUSCA. 



of their different parts, by means of which they creep, swim, and 

 seize upon various objects, just as the form of these parts may permit; 

 but as the limbs are not supported by articulated and solid levers, 

 they cannot perform very rapid advances in progression. 



The irritability of most of them is extremely great, and remains 

 for a long time after they are divided. Their skin is naked, very 

 sensible, and usually covered with a humour that oozes from its 

 pores ; no particular organ of smell has ever been detected in them, 

 although they enjoy that sense ; it may possibly reside in the entire 

 skin, for it greatly resembles a pituitary membrane. All the Cephala, 

 Brachiopoda, Cirrhopoda, and part of the Gasteropoda and Ptero- 

 poda, are deprived of eyes; the Cephalopoda on the contrary have 

 them at least as complicated as those of the warm-blooded animals. 

 They are the only ones in which the organ of hearing has been 

 discovered, and whose brain is enclosed with a particular cartila- 

 ginous box. 



Nearly all the MoUusca have a development of the skin which 

 covers their body, and which bears more or less resemblance to a 

 mantle; it is often however narrowed into a simple disk, or is formed 

 into a pipe, or billowed into a sac, or lastly is extended and divided 

 in the form of fins. 



The Naked MoUusca are those in which the mantle is simply 

 membranous or fleshy ; most frequently however one or several 

 laminee, of a substance more or less hard, is formed in its 

 thickness, deposited in layers, and increasing in extent as well 

 as in thickness, because the recent layers always overlap the old 

 ones. 



When this substance remains concealed in the thickness of the 

 mantle, it is still customary to style the animals Naked MoUusca. 

 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 testaceous ; the epidermis which 

 covers it is thin, and sometimes desiccated;* it is called i/rapmar- 

 in{a). 



The variety in the form, colour, surface, substance and brilliancy 



* Until my labours on the subject were made public, the Testacea constituted a 

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

 MoUusca 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 MoUusca. 



(j:^ (a) This name is given to a woolly texture which covers the outside of 

 several univalve shells. Eng. Ed. 



