406 



ARTHROPODA. 



rigid surfaces, which are obtained partly by the development of 

 internal chitinous tendons and plates, and partly by the hardening 

 of the integument and the fusion of several segments to form 

 larger armoured regions. It is only when the movements are 

 simpler and resemble those of Annelids, that all the segments 

 remain independent and bear similar appendages along the whole 



length of the body 

 (larva?, Myriapoda). 

 In general, three 

 regions of the body 

 can be distinguished, 

 the head, the thorax, 

 and the abdomen, the 

 appendages of which 

 possess respectively a 

 different structure 

 andfunction(fig.326). 

 The head constitutes the short and compact anterior region of the 

 body, is covered by a hard integument, encloses the brain and bears 

 the sense organs and mouth-parts (jaws). The appendages of this 

 region are modified to form the antennce and jairs. The' head of 

 Arthropods, as compared with that of Annelids, contains, besides the 

 frontal (pra?oral) or antenna! segment and the oral segment, in 



FIG. 326. Head, thorax and abdomen of an Acridium, seen 

 from the side. St., Stigmata ; T, tympanum. 



FIG. 327. Squilla nxnit>. A', A" Antenna?; Kf, Kf" the anterior maxillipeds on the 

 cephalo-thorax ; B', B", S'", the three pairs of biramous feet. 



addition at least one jaw segment, the appendages of which may, in 

 larval life (Nauplius), still function as legs. Usually, however, 

 several of the succeeding segments whose appendages function as 

 jaws form part of the head. 



The middle portion of the body, or thorax, is likewise distinguished 

 by a relatively intimate fusion of some or all of its segments, as well 

 as by the hardness of its integument. It is sometimes sharply 

 marked off from the head, sometimes fused with the head to form a 



