GENERAL CHARACTERS OF CEPHALOPODA. 



339 



cipal part of the body is contained within a. shell. Animals of 

 this kind were formerly much more abundant. At present the 

 naked species, as they are termed in which the shell is rudi- 

 mentary, and is inclosed between folds of the mantle instead of 

 being external to the body are the chief inhabitants of our 

 seas. In some of these, the shell still retains considerable size 

 and density, as the common pounce-bone ; but in the long slender 

 species, which swim by the movements of the whole body, it is 

 necessarily flexible ; and here we find it very narrow, and com- 

 posed of a light horny substance, so as to bear some resemblance 

 to a feather. 



878. The trunk of these animals is inclosed in the mantle ; 

 which is shaped like a bag, sometimes nearly spherical, some- 

 times more or less 

 elongated ; inclosing 

 all the viscera, and 

 being only open in 

 front (Fig. 541, o). 

 The head, projecting 

 from this opening, is 

 round, and generally 

 provided with two 

 large eyes, of a struc- 

 ture very similar to 

 those of Vertebrated 

 animals. The mouth 

 is situated in the 

 centre of the circle of 

 arms ; and is armed 

 with a pair of horny 



mandibles, which have very much the form of the Parrot's beak. 

 The arms are sometimes all of equal length, as in the Octopus 

 (Fig. 545) ; and sometimes two are much longer than the rest, 

 as in the Calamary. Cephalopods are essentially aquatic, and 

 breathe therefore by gills. These organs are concealed beneath 

 the mantle in a particular cavity (Fig. 541), the walls of which 

 dilate and contract alternately, and which communicates with 



FIG. 541. GILLS OK POULP. 



