ARTHEOPODA. 



83 



base of the legs. Thorax with six or seven free segments. Abdo- 

 men of seven segments. Tail natatory or saltatorial. 



In this order the four anterior legs) including the gnathopoda, 

 are directed forward, the three posterior backward ; the five pos- 

 terior legs [pereiopoda] are the true walking-legs. Behind the 

 legs are three pairs of appendages [pleopoda], plumosely fringed ; 

 these are the swimming-organs. The terminal segment of the 

 body is the telson, varying much in structure. The eyes are 

 sometimes so slightly differentiated as to disappear after death. 

 The largest species, from Lake Baikal, is five inches long. 



Dana includes this and Isopoda in one order Choristopoda. 

 It comprises Crevettina \ and Hyperina of Glaus. 



Dulichiidcs. 



Gammaridce. 



Vibiliida. 



Order III. ISOPODA. 



POLYGONATA. 



No branchial vesicles ; the respiratory organs placed beneath 

 the abdomen, and modified in various ways. Body depressed. 

 Tail well developed. 



The head is almost always distinct from the thorax ; the latter 

 consists of seven segments bearing seven pairs of legs, all, the two 

 anterior excepted, more or less conformable. The epirnera of the 

 dorsal portion of the segments are small or wanting. The young 



