ARTHROPODA. CRUSTACEA 291 



telson) of the abdomen. Eyes are generally present, 

 commonly a pair of compound eyes, and sometimes a 

 median simple eye. The sexes are separate except in 

 most of the cirripedes and in some parasitic isopods. In 

 the Malacostraca the genital apertures are on the sixth 

 thoracic segment in the male, and on the eighth in the 

 female ; in the Entomostraca the position of the apertures 

 is variable. 



In some Crustacea development is direct, that is to 

 say, the young individual has the same form as the adult; 

 but generally this is not the case, the young undergoing 

 metamorphosis before reaching the adult stage. The two 

 chief larval forms are known as the nauplius and the 

 zocea. In the nauplius the body is unsegmented, and 

 possesses three pairs of appendages representing the two 

 pairs of antennae and the mandibles. In the zoaea stage 

 some of the thoracic appendages are present also, and the 

 abdomen is segmented but possesses no appendages. 



The Crustacea are divided into six sub-classes: — 

 (1) Trilobita, (2) Branchiopoda, (3) Ostracoda, (4) Cope- 

 poda, (5) Cirripedia, (6) Malacostraca. The Copepods are 

 not definitely known as fossils. 



The first five sub-classes are usually grouped together 

 as the Entomostraca, but they differ considerably from 

 one another and are not united by the possession of 

 important features common to all. In comparison with 

 the Malacostraca they are generally of simple organisation, 

 usually with the number of segments in the thorax and 

 abdomen varying widely, and (with the exception of the 

 Trilobita) they are generally of small size, with the 

 abdomen usually ending in a caudal fork, and often 

 without a clear differentiation of the trunk into thorax 

 and abdomen. A median unpaired eye is usually present. 



19—2 



