SKELETONS OF INVEllTEBRATES 



255 



nective tissue of a flexible character, so that, like a chain- 

 mail shirt, the parts of the body may bend and still be 

 protected and supported. 



The moUusks (oysters, clams, snails, etc.) have a cer- 

 tain portion of the body, called the mantle, whose function 

 it is to secrete hard material, and this material collects to 

 form a structure called a shell. These shells are external 

 and completely in- 

 close the soft body. ^ 

 Those mollusks 

 that require an 

 opening for the en- 

 trance of food and 

 air, and that, un- 

 der ordinary condi- 

 tions, are station- 

 ary, secure this re- 

 sult by having the 

 shell hinged and 

 provided with mus- 

 cles which can open 

 and close it when 



necessary. Others, such as the snails, have the shell of one' 

 piece, but so mounted that the animal can carry it on his 

 back when moving about, and when disturbed can with- 

 draw the body into it until the disturbing factor has 

 disappeared. Still other forms of mollusks have the shell 

 inside the body, where it serves to give the body a cer- 

 tain stiffness, as in the cuttlefish and squid. These 

 internal shells are of no use in protecting the soft parts 

 of the body. 



In the arthropods (insects and lobsters, etc.) we have 



Fig. 105 — Left sliell of clam ; h, liinge ligament ; t, c, 

 t', liinge teeth ; a, a', points of attacliment of tlie 

 muscles, by means of which the clam closes its shell. 



