254 ZOOLOGY 



I. BRANCHIATA. 



Class CRUSTACEA. 



Chaeacteeistics. — AqvMtic Arthropods, which breathe either 

 through the general surface of their shin or through 

 specialised extensions of the same, the branchiae or gills. 

 Two pairs of antennae are found, and the appendages are 

 as a rule biramous. A limb-bearing thorax is either free or 

 united with the head. The usually segmented aidomen Truly 

 or may not bear appendages. Some of the limbs are modified 

 to form jaws. The gills are usually extensions of the basal 

 joint of some of the appendages. The whole group, both in 

 its internal and external features, is, with few exceptions, 

 rigidly bilaterally symmetrical. 

 The Crustacea are divisible into two series : (A) the 



Entomostraca and (B) the Malacostraca. 



A. ENTOMOSTRACA. 



A. The Entomostraca include many comparatively small 

 and simply-organised Crustacea, the number of whose segments 

 varies within wide limits. A large carapace, which may enclose 

 the whole body, is often present. The demarcation between 

 thorax and abdomen is often shown by the opening of the 

 generative organs. Paired compound eyes and an unpaired 

 simple eye often coexist. There is no masticating stomach. The 

 developement almost always includes a Nauplius stage. 



The Entomostraca consists of four orders : 



1. Phyllopoda. 



2. osteacoda. 



3. copepoda. 



■ 4. ClEEHIPEDIA. 



Order 1. PHYLLOPODA. 



Chaeacteeistics. — Crustacea, with usually elongated and well- 

 segmented body, partially covered by a shield -like carapace, 

 which may be laterally prolonged to form a bivalved shell. 

 The number of segments and appendages varies greatly, but 

 there are never less than four leaf-like lohed swimming-feet. 



