CHAPTER V. 



CLASS I. CRUSTACEA." 



Aquatic Arthropoda usually breathing by means of gills, with 

 two pairs of antennae and numerous pairs of frequently bi-ramous 

 legs on the thorax and on the abdomen. 



The crustacean body is usually more or less distinctly divisible 

 into three main regions the head, which contains a constant 

 number of segments, and the thorax and abdomen in which the 

 number varies considerably in the several groups. 



In recent Crustacea the head usually differs from the suc- 

 ceeding parts of the body in the absence of external separation 

 between the segments, although in the fossil Trilobites the 

 more or less complete grooves which traverse it appear to bear 

 witness to its original segmentation (Fig. 243). It differs also 

 in the character of its limbs, and in the absence from them of 

 branchial lobes. 



The thorax usually bears the main locomotory appendages. Its 

 segments are usually larger than those of the rest of the body 

 and though often distinct they are generally less movable on 

 one another than those which follow. The abdomen usually 

 consists of narrower and more mobile segments and in most 

 groups carries appendages, which differ in character from those 

 of the thorax. In many Entomostraca however (Apus, Cope- 

 poda, etc.) the distinction between thorax and abdomen is little 

 marked. The orifices of the generative organs are frequently 

 situated near the junction of thorax and abdomen. 



The groove separating the head from the thorax is well marked 

 in the Trilobites, the Phyllopods, Nebalia, as well as in the 

 larval forms of some Copepods, and of several of the Malacostraca, 



* In addition to the literature cited below the student is referred, for 

 further details, to the work of A. Gerstaecker, completed by A. E. Ort- 

 mann, on Crustacea in Bro tin's Thierreich, Leipzig, 1866-1901, and to the 

 section on Crustacea by K. Heider, inKorschelt and Heider's Text-book of 

 Embryology. 



342 



