50 



BRANCH ARTHROPODA. 



BRANCH ARTHROPODA (&r thr6 P -6 da). 



INSECTS AND CRUSTACEANS. 



The AKTHROPODS (ar'thro podz) differ from the 

 jointed, bilaterally symmetrical animals, which we have 

 hitherto studied, in having the appendages jointed,* and 

 the body protected by a hard external shell, f which is 

 generally divided into regions, each of which has special 

 duties to perform. J 



Locomotion is effected by the jointed appendages, 

 FlG . 7(; . which allow the body 



Abdo,,,. Head. to progress without 



the flexions so charac- 

 teristic of the worms. 

 The nervous system 

 consists of a brain, 

 situated in the head, 

 connected with a 

 chain of ventral gan- 

 glia by means of an 

 oesophageal (e so fag'e ai) ring. The alimentary tract is 

 entirely separated from the body cavity, while the circu- 

 latory system is more or less imperfect. The blood, 

 generally colorless, on leaving the dorsally situate heart, 

 passes through lacunce (la ka'ne) (open spaces in the body 

 cavity) before it again enters regular blood-vessels on its 

 return. 



* Compare appendages of Clam-worm. 



t A rigid support is necessary for the attachment of the muscles which serve 

 in flying, jumping, or swimming. 



t The head bears the mouth parts and some of the special sense organs; 

 the eyes, and antennae : the thorax (tho 1 rax) supports the organs of locomotion : 

 the abdomen (ab do' men), generally without appendages, is of use in carrying the 

 eggs. 



An Arthropod Animal. Young Lobster, showing the 

 division of the body into head, thorax, or head-thorax, 

 and abdomen. (Enlarged.) 



