244 MOLLUSCA. 



motions consist of various contractions which produce inflexions 

 and prolongations of their different parts, or a relaxation of the same, 

 by means of which they creep, swim, and seize upon various ob- 

 jects, just as the form of these parts may permit; but as the limbs 

 are not supported by articulated and solid levers, they cannot ad- 

 vance rapidly, or per saltum. 



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 Aca- 

 lepha, Brachiopoda, Cirrhopoda, and part of the Gasteropoda and 

 Pteropoda, 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 within a particular 

 cartilaginous box. 



Nearly all the Mollusca 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, 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, one or several Ja- 

 minas, 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 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 testaceous; the epidermis which covers 

 it is thin, and sometimes desiccated. 



The variety in the form, colour, 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, nails, hairs, horns, 

 scales, and even teeth. The tissue of shells differs according to the 



