254 ZOOLOGY 
I. BRANCHIATA. 
Ciass CRUSTACEA. 
CHARACTERISTICS.— Aquatic Arthropods, which breathe either 
through the general surface of their skin 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 abdomen may 
or may not bear appendages. Some of the limbs are modified 
to form jaws. The gills ave usually extensions of the basal 
joint of some of the appendages. The whole group, both in 
its internal and external features, 1s, 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 inelude 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 Nawplius stage. 
The ENTOMOSTRACA consists of four orders : 
1. PHYLLOPODA. 
2. OSTRACODA. 
3. COPEPODA. 
4, CIRRHIPEDIA. 
Orper 1. PHYLLOPODA. 
CHARACTERISTICS.—Crustacea, with usually elongated and well- 
seymented body, partially covered by a shield-like carapace, 
which may be laterally prolonged to form a bwalved shell. 
The number of segments and appendages varies greatly, but 
there are never less than four leaf-like lobed swimmang-feet. 
