104 HITCHCOCK'S ANATOMY. 



by some eminent writers, have a skin more or less hard, and 

 sometimes spinous. 



Fig. 117. 



Microscopic structure of the Spine of Sea Hedge-Hog. 



226. Skeleton of the Mollusca. — In all the three 

 great classes of Molluscs, the Acephala, the Cephalophora, . 

 and the Cephalopoda, the animal possesses a mantle by which 

 it is able to secrete a solid deposit, mainly composed of car- 

 bonate of lime, which it spreads over nearly all the body, and 

 is fastened to it by muscles. In the Acephala this skele- 

 ton is in two pieces, called valves ; in the Cephalophora, it is 

 in one piece, except that a calcareous plate is attached to the 

 foot to close the orifice. The organic base of these shells 

 sometimes, but rarely, predominates over the calcareous part. 

 In the Cephalopoda there are cartilages for the attachment of 

 muscles similar to the same substance in vertebrate animals. 

 In the Argonauta and Nautilina of this class, the mantle 

 secretes an external shell ; but in the Loligina, or Cuttle Fish 

 family, it is internal. 



The structure of the shells of Molluscs is often complicated 

 and beautiful. 



227. Skeletons of the Radiata. — Among the Polypi 

 some are entirely soft ; others have a solid skeleton, which 



226. Describe the skeleton of Molluscs. 22T. WTiat are the skeletons of Radiates ? 

 What is their chemical composition ? 



