PHYLUM ARTHROPODA 



259 



minal division. No doubt the early Crustacea, with 

 branched appendages, were essentially swimmers, and 



Photograph by W. P. Hay 



FIG. 72. A prawn (Palcemonetes), which occurs in great numbers along the 

 south Atlantic coast and is extensively used as food. 



the unbranched leg has evolved for walking, whether in Evolution of 

 the water or on the land. Adaptation for walking in the w g egs 

 sea, and on the rocks and seaweed at low tide, made pos- 

 sible the eventual population of the land by arthropods. 

 The Entomostraca are mostly small Crustacea, especially 

 abundant in fresh water, and including some very an- 

 cient types. The "higher" and mostly larger Crustacea 

 are called Malacostraca. Some, such as the crab and 

 crayfish, possess a carapace or shield covering the thorax ; 

 others, including the terrestrial wood lice, are without 

 this structure. 



Class Arachnoidea 



A great series of arthropods in which the antennae 

 are wholly absent and the head and thorax are usually 

 fused into a cephalothorax. 



