Crustaceans 183 



ARTHROPODS 



We come now to that great assemblage of animals 

 which bear a chitinous armor on the outside of the 

 body, and, as the name implies, are possessed of jointed 

 feet. This group is numerically dominant in the world 

 today on sea and land. It is roughly divisible into 

 three main parts; crustaceans, spiders and insects. 

 The crustaceans are the most primitive and the most 

 wide-spread in the water-world; so with them we will 

 begin. 



The Crustaceans include a host of minute forms, such 

 as the water fleas and their allies, collectively known 

 as Entomostraca, and a number of groups of larger 

 forms, such as scuds, shrimps, prawns and crabs, col- 

 lectively known as the higher Crustacea or Malacos- 

 traca. A few of the latter (crabs, sow-bugs, etc.) live 

 in part on land, but all the groups are predominately 

 aquatic, and the Entomostraca are almost wholly so. 



The Entomostraca are among the most important 

 animals in all fresh waters. They are perhaps the chief 

 means of turning the minute plant life of the waters into 

 food for the higher animals. They are themselves the 

 chief food of nearly all young fishes. 



There are three groups of Entomostraca, so common 

 and so important in fresh water, that even in this brief 

 discussion we must distinguish them. They are: 

 Branchiopods, Ostracods and Copepods. 



The Branchiopods, or gill-footed crustaceans, have 

 some portion of the thoracic feet expanded and lamelli- 

 form, and adapted to respiratory use. The feet are 

 moved with a rapid shuttle-like vibration which draws 

 the water along and renews the supply of oxygen. The 

 largest of the entomostraca are members of this group ; 

 they are very diverse in form. 



