340 



COMPARATIVE ZOOLOGY 



eight in the thorax, and seven in the abdomen. In the 

 adult, however, the rings of the head and thorax are 

 often soldered together into one shield, called cephalo- 

 thorax ; and in the horseshoe crab the abdominal rings 

 are also united. The shell of crustaceans is periodically 



cast off, for the 

 animals continue 

 to grow even after 

 they have reached 

 their mature form. 

 This molting is a 

 very remarkable 

 operation. How 

 the lobster can 

 draw its legs 

 from their cases 

 without unjoint- 

 ing or splitting 

 them was long a 

 puzzle. The flesh 

 becomes soft, and 

 is drawn through 



FIG. 295. Diagram of an Insect: A, head bearing the 



eyes and antennae; B, prothorax, carrying the first pair the joints, th.6 

 of legs, G; C, mesothorax, carrying the second pair of , . 



legs, H, and first pair of wings, K; D, metathorax, carry- WOUndS tilUS 

 ing the third pair of legs, I, and second pair of wings, L; caused QUicklv 

 E, abdomen, with ovipositor, F; i, coxa, or hip; 2, tro- J 



chanter; 3, femur, or thigh; 4, tibia, or shank; 5, tarsus, healing. The 



or foot; 6, claw. 



cast-off skeleton 



is a perfect copy of the animal, retaining in their places 

 the delicate coverings of the eyes and antennae, and even 

 the lining membrane of the stomach with its teeth. 



The horny crust of insects differs from that of crus- 

 taceans in consisting mainly of a horny substance called 

 chitin and in containing no lime. The head, thorax, and 

 abdomen are distinct, and usually consist of fourteen 

 visible segments one for the head, three for the 



