16 



MALACOZOA. 



The skin, always soft, and generally sensitive, often 

 forms plaits or folds, which envelope the body in whole 

 or in part. The portion of integument thus raised is 

 named the Mantle. It is often almost entirely free, form- 

 ing two large lamince or lobes, which cover the rest of 

 the animal ; or tlie two laminse unite so as to form a sort 

 of tube ; but sometimes the mantle forms a kind of 

 disk, of which the margins only are free; or surrounds 

 the body in the form of a bag. 



In very many cases the soft skin is protected by a 

 kind of calcareous crust, named the Shell, which is a 

 secretion from the mantle, deposited in successive layers. 

 The secreting glands or folHcles, generally lodged in the 

 edges of the mantle, pour forth a kind of horny sub- 

 stance mixed with carbonate of lime, which soon hardens; 

 and internal laminse are successively formed, so that the 

 shell is gradually increasing in extent and thickness. The 

 outer surface is generally covered by a horny membrane 

 composed of the united margins of the laminse, and 

 named the Ejndermis. Sometimes the whole shell is 

 horny, but most commonly the calcareous part predomi- 

 nates, and the inner surface is more compact. In some 

 cases the shell is internal or lodged in the skin, but 

 generally it is external, and affords complete protection 

 to the animal. Those Malacozoa which have no shell 

 are said to be Naked ; those having a shell are called 

 Testaceous or Conchiferous. The shell varies in form, 

 sometimes resembling a shield which covers the back of 

 the animal, more frequently a conical tube spirally twisted ; 

 or it may be composed of two distinct pieces united by a 

 joint: whence a distinction of these animals into Univalve 

 and Bicalve. The shells arc often ornamented with 



