204 ARTHROPODA [CH. 



The Crustacea are usually divided into two groups, the En- 

 tomostraca (Gr. CVTO/X.OS, cut in pieces ; oo-rpaKov, a shell) and the 

 Malacostraca (Gr. //.aAaKo'?, supple) ; and each of these again is 

 divided into four and three Orders respectively. 



Sub-class A. ENTOMOSTRACA. 



This group may be regarded as a lumber-room for all Crustacea 

 which are not included in the well-defined division Malacostraca, 

 and the only character which can be attributed to all the members 

 is that of not possessing the marks of Malacostraca. 



For the most part they are small Crustacea of simple structure. 

 The number of their segments varies within wide limits ; some 

 Ostracoda having only seven pairs of limbs, whilst in Apus there 

 are sixty-eight pairs. The dorsal part of their head has, in many 

 cases, grown backwards and downwards like a mantle to form a large 

 hood or shell, termed the carapace, which may cover a large part 

 of the body, and in some cases this becomes divided into two lateral 

 halves hinged together like a mussel's shell ; but unlike the carapace 

 of the Malacostraca it does not become fused with the terga of 

 the thorax. In many descriptions of Entomostraca the words 

 " thorax " and " abdomen " are used to describe regions of the 

 body. Such terms are in strictness applicable only to the higher 

 Crustacea, where the trunk is sharply differentiated into two regions 

 distinguished by the character of their appendages. Amongst the 

 Entomostraca however the appendages of the trunk form a uniform 

 series : often it is true the last segments are devoid of appendages, 

 and to these the term abdomen (16, Fig. 77) is usually applied, but 

 to us- this seems an unjustifiable and misleading use of a term 

 which has an exact significance only amongst Malacostraca. 



Entomostraca have no internal teeth in their stomach. As a 

 rule the young are not like their parents but are larvae of a special 

 kind called Nauplii ; these after a number of ecdyses, during which 

 the number of segments increases, grow up into adults. 



The Nauplius possesses an oval, unsegmented body, a median 

 simple eye, three pairs of appendages and a large upper lip. The 

 first pair of limbs representing the first antennae of the adult are 

 simple and unjointed, the other two pairs have a basal piece and 

 two branches. The inner branch of one or both pairs has a hook 

 for masticatory purposes. These two pairs of appendages become 



