MOLLUSC A. 303 



to contain a smaller proportionate quantity of fibrine than that 

 of the Vertebrata. There are reasons for believing that their 

 veins fulfil the functions of absorbent vessels. 



Their muscles are attached to various points of their skin, 

 forming tissues there which are more or less complex and 

 dense. Their 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 objects, just as the form of these 

 parts may permit ; but as the limbs are not supported by arti- 

 culated and solid levers, they cannot advance rapidly, or per 

 saltum. 



The irritability of most of them is extremely great, and re- 

 mains 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, Cirrho- 

 poda, and part of the Gasteropoda and Pteropoda, are de- 

 prived 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 resem- 

 blance to a.7nantle; 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 JVaked Mollusca are those in which the mantle is simply 

 membranous or fleshy ; most frequently however one or seve- 

 ral lamince, 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 



