3240 



THE JOINTED ANIMALS 



form the function of walking or swimming organs, while the posterior five pairs 

 at least have no exopodites. There are four pairs of jaws, namely, the mandibles, 

 two pairs of maxillae, and one pair of maxillipedes. Although there are many ex- 

 ceptions, it may be said that, as a rule, the body is broad, short, and flattened. 

 Corresponding to the structural variations, the Isopods exhibit great diversity of 

 habits and habitat. Most are marine, occurring in shallow waters or at great 

 depths. Some live in fresh water, others on land, while others have taken to a 

 parasitic life, and have thus to a great extent lost the characteristics of the order. 



Serolis bromleyana. 

 (Natural size.) 



Of the five tribes, the Valvifera have the posterior pair of abdominal append- 

 ages or uropods, transformed into valves or flaps, covering part of the lower sur- 

 face, and constituting a chamber for the gills. The tribe contains two families, 

 the Arcturidce and Idoteidce. The former are distinguished by their slender cylin- 

 drical shape, long lower antennae, and the length of the fourth thoracic segment, 

 which separates the posterior three pairs of thoracic legs from the anterior pairs by 

 a wide space. The anterior thoracic feet are slender and hairy on the inner side, 

 while the posterior feet are strong and prehensile, and enable the animal to fix itself 

 to the branches of corallines. In the Idoteidce the body is longish and narrow, the 



