

ARTHROPOD A. CRUSTACEA. 185 



CLASS. CRUSTACEA. 



The members of this class are mainly aquatic and are 

 very abundant as fossils ; they breathe generally by means 

 of gills, but in some cases by the general surface of the 

 body. The chitinous exoskeleton is frequently hardened 

 by a calcareous deposit, hence the name Crustacea. Seg- 

 mentation is usually well-marked, but is absent in the 

 Ostracoda. Three regions may be distinguished in the 

 body : the head, the thorax, and the abdomen. There are 

 five segments in the head, but these are indicated only by 

 the appendages ; the number in the thorax and abdomen 

 is variable in one of the two great divisions, namely the 

 Entomostraca, but is constant in the other, the Mala- 

 costraca. In most forms some or all of the segments of 

 the thorax (and sometimes also the anterior segments of 

 the abdomen) fuse with those of the head to form a 

 cephalothorax, this is covered dorsally by a large, unseg- 

 mented shield or carapace, termed the cephalothoracic 

 shield. The head usually bears fi ve pairs of appendages, viz. 

 two pairs of antennae, one of mandibles, and two of maxillae. 

 The thorax is also provided with appendages, and very often 

 the abdomen too. The mandibles and maxillae serve as 

 jaws, and often also some of the anterior thoracic appen- 

 dages. Eyes are generally present and are placed on the 

 head ; in some cases they are simple, in others compound, 

 consisting of a number of lenses. The sexes are separate 

 except in some cirripeds and isopods. In some cases 

 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 a metamorphosis before 

 reaching the adult stage. The two chief larval forms are 

 known as the Nauplius and the Zocea. In the Nauplius 



