CHAPTEE V 



CKUSTACEA {CONTINUED) : MALAC03TEACA : LEPTOSTEACA ■ 



PHYLLOCARIDA : EUMALACOSTEACA : SYNCAEIDA ANAS- 



PIDACBA : PEEACAEIDA MYSIDACEA CUMACEA ISOPODA 



AMPHIPODA : HOPLOCAEIDA STOMATOPODA 



SUB-CLASS II.— MALACOSTRACA. 



The Malacostraca are generally large Crustacea, and they are 

 oharaoterised by the presence of a definite and constant number 

 of segments composing the body. In addition to the paired eyes 

 we can distinguish two pairs of antennae, a mandibular segment, 

 and two maxillary segments composing the head-region proper; 

 there then follow eight thoracic segments, the limbs belonging 

 to the anterior thoracic segments being often turned forwards 

 towards the mouth, and modified in structure to act as maxilli- 

 pedes, while at any rate the last four are used in locomotion and 

 are termed " pereiopods." ^ The abdomen is composed of six 

 segments, which typically carry as many pairs of biraihous 

 " pleopods,'' and the body terminates in a telson. Not counting 

 the paired eyes or the telson, there are present nineteen segments. 

 The excretory organs in the adult open at the bases of the second 

 antennae, and are known as "green glands," but in the larva 

 maxillary glands may be present homologous to those which per- 

 sist in the adult Entomostraca. This is the typical arrange- 

 ment, but sometimes the maxillary glands persist in adult 

 Malacostraca, e.g. Nebalia, Anas2ndes, and some Isopods. 



The hepato-pancreatic diverticula are directed posteriorly, and 

 not anteriorly as in most Entomostraca, and the stomach is often 

 furnished with chitinous teeth and ridges forming an elaborate 

 gastric mill, especially in the larger Decapods. 



' The term pereiopod is applied to those thoracic limbs which are used in 

 locomotion, and are not specially differentiated for any other purpose. 



