334 



THE CRAYFISH AND THE ARTHROPODA 



and a small number, like the sow-bugs and the land crabs of the 

 tropics, have become adapted for terrestrial existence. A few 

 are parasitic upon larger animals. Since the number of somites 

 and the division of the body into three principal regions are con- 

 stant for all Malacostraca, a schematic representation of a mala- 

 costracan shows an animal essentially like a crayfish. The body is 



Brood-chamber 



Diyesfive tract 

 Abdomfmtl processes **, 



Abdominal setae 



Abdominal clav/s' i 

 I 

 I 

 I 



Post' abdomen 



Frotltttf oryon 



^tledian c/e 



'^,'lst antenna 



""T^/red eyes 



^id antenna 

 ^Caudal sfy/ets 



Fig. 162. — Representative entomostraca. 



.\bove, a water-flea, Daphnia pulex, one of the Cladocera. Below, a fairy shrimp, 

 Branchinecla packirdii, one of the Phyllopoda; left, anterior view of male showing second 

 antennae modified as clasping organs; right, lateral view of female with eggs in uterus and 

 showing appendages. (After Dodds, University of Colorado Studies, Vol. XI.) 



divided into head, thorax, and abdomen, which are composed, 

 respectively, of five, eight, and six somites, each bearing a pair of 

 appendages. The anterior part of the head bears the eyes, which 

 are probably not homologous with the appendages; while pos- 

 teriorly the abdomen terminates in a telson. There are thus 

 nineteen somites with paired appendages, in addition to the most 



