ARTHROPODA. 



143 



Order Isopoda. 



In this order are a number of Crustaceans, of which some inhabit 

 the sea, others are parasitic in their habits, and others are terrestrial. 

 Of the terrestrial forms the common Wood-lice {Oniscus) are familiar 

 examples. Of the aquatic types, the Water-slaters (Asellus) inhabit 

 fresh waters ; but the Rock -slaters {Ligia) and Box-slaters {Idotea) 

 are marine, as are many others. The parasitic forms live upon 

 fishes or other Crustaceans, and have often become much altered 

 by retrograde metamorphosis. 



Fig. 91.— Isopoda. A, Idotea entomon, enlarged. B, Areturtis longicomis, enlarged. C, 

 Serotis Scyfhei. a Antennules ; an Antennse. (After Qerstecker, Spence Bate and 

 Westwood, and Lutken.) 



The Isopod Crustaceans all belong to a section of the class Crus- 

 tacea, in which the eyes are not supported upon movable stalks, and 

 they are therefore said to be " sessile-eyed " {Hedriophthalmata). 

 The head is distinct, and the thorax carries seven pairs of legs ; 

 while the branchiffi are attached to the vinder surface of the ab- 

 domen, the segments of which are often coaleseent. 



Order Amphipoda. 



This order comprises small Crustaceans, in which the breathing- 

 organs are in the form of membranous vesicles attached to the bases 



