GENERAL CLASSIFICATION 253 



type, of which the majority live in fresh water, though 

 some, such as the curious Apus, are found in brine- 

 pools ; (2) the Cladocera, or water-fleas, most of which 

 occur in fresh water and are partly enclosed in a bi- 

 valve shell of their own; (3) the marine and fresh- 

 water Ostracoda, in which the entire head and body 

 are concealed by a bivalve shell ; (4) the Copepoda, 

 most of which are marine, and many parasitic on 

 fishes ; and (5) the Cirripedia, or barnacles, with 

 which every one is familiar. Only a small number of 

 Entomostraca are comprised in the true abyssal fauna. 



The sub-class Malacostraca contains all the large 

 and highly-organised Crustacea, whose body is as a 

 rule built up of twenty-one segments, each of which 

 usually carries a pair of multipartite appendages. In 

 this sub-class are grouped (i) the marine and lacus- 

 trine Amphipoda, of which the common ''sand-hopper" 

 is a familiar example ; (2) the Isopoda — exemplified by 

 the common wood-louse " and ''slater" — which are 

 found in the sea and in fresh water as well as on 

 land ; (3) the marine Stomapoda or locust-shrimps, 

 whose eyes are borne on stalks ; (4) the sessile-eyed 

 Cumacea, which resemble the larvae of prawns, and live 

 in the sea ; (5) the marine stalk-eyed Schizopoda, 

 or opossum-shrimps ; and (6) the Decapoda, a great 

 Order of stalk-eyed forms, widely distributed through 

 all seas and fresh waters, as well as on land, and 

 comprising all the crustaceans that we use as food. 



