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AUTHROPODA. 



143 



Order Isopoda. 



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

 the sea, otliers are parasitic in their habits, and others are terrestriah 

 Of the terrestrial foi'ms tlie common Wooil-lice (Oniscus) are familiar 

 examples. Of the aquatic types, the Water-slaters (Ase/liis) inhabit 

 fresh waters ; but the Eock-slaters (Ligia) and Box-slaters (Idotea) 

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

 fishes or other C'rustaceans, and have often become much altered 

 by retrograde metamorphosis. 



Fig. 01. — Isoxiodn. A, hhjtca eiil<nn<:ii, (;\]\i\y'^ed. U, ^lidnnts longicvrnis, ii-ti]nv'^ei\. C, 

 Si'-ToHs ^cytlicL a Antennules ; n/;. Aiitcniiii'. (AftiT Gersta^cker, Spent'e Bate and 

 AVestwood, and Lutken.) 



The Isopod C'rustaceans all belong to a section of the cla.ss Crus- 

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

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

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

 wliile the branchi;e are attached to the under surface of the ab- 

 domen, the segments of which are often coalescent. 



Order Amphipoda. 



This order compiises small Crustaceans, in whicli tlie breathing- 

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



