320 ANIMALS 



surface of the shell there is " mother-of-pearl," which consists of alter- 

 nate, thin layers of limestone and horny material. The shell is made 

 (secreted) by the mantle. Some mollusks form pearls (which are 

 mother-of-pearl in round masses), if some irritating substance comes 

 between the mantle and the shell. 



The shell of the mussel springs apart when the animal is dead. 

 While alive, the creature holds the halves together by means of two 

 powerful muscles (anterior and posterior "adductors" of Fig. 261). 

 One end of each muscle is attached to the right valve, and the other 

 end of each to the left valve. When the muscles contract, the valves 

 come together with great force. The mussel, like the earthworm, has 

 a nervous system, with a brain at the anterior end of the body, over 

 the mouth. 



Mussels, clams, and oysters are called bivalves, because of the two 

 halves of their shells. Snails are called univalves, because the shell is 

 in one piece. Slugs are like snails, but they form no shell. To the 

 mollusks belongs, also, the devil fish, or octopus, the " nightmare of 

 the sea." - - 



338. Crustaceans. Crustaceans are so called from 

 their hard body covering. The class includes crayfishes, 

 lobsters, crabs, barnacles, etc. (Fig. 262). 



The crayfish, like the mussel and the earthworm, has 

 its parts arranged symmetrically. Like the earthworm 

 it has a number of joints, or segments, each of which 

 makes up a part of the length of the body. In the cray- 

 fish many of these joints have very well developed parts, 

 or appendages, popularly called "legs," "claws," "feel- 

 ers," etc. The hard covering of the body has already 

 been spoken of. This holds and protects the soft organs 

 within. At the posterior end (abdomen) the body cover- 

 ing is in several pieces; the remainder of the body, that 

 is, the head and thorax (chest), are inside one piece of 

 covering. The so-called ' ' legs ' ' and ' ' claws ' ' are attached 



