MOLLUSCA. 233 



prolongations of their different parts, or a relaxation of the same, 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 advance rapidly, 

 or by leaping. 



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 Acalepha, Brachiopoda, 

 Cirrhopoda, and part of the Gasteropoda and Pteropoda, are deprived 

 of eyes ; the Cephalopoda on the contrary have them at least as com- 

 plicated 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 inclosed 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 

 tins. 



The Naked Mollusea are those in which the mantle is simply mem- 

 branous or fleshy ; most frequently, however, one or several laminae, 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 skia 

 under the epidermis, like the mucus covering nails, hairs, horns, scales, 

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

 this deposition, which is either in parallel laminae or in crowded vertical 

 filaments. 



All the modes of mastication and deglutition are visible in the 



