Phylum 6. Arthropoda. 



The body is divided into a number of segments demarcated 

 externally by constrictions^ and provided witb jointed limbs, 

 wMch constitute efficient locomotor organs : it resembles, therefore, 

 the Annelid body in the former respect, but differs in the latter. 

 Moreover, there is a greater dissimilarity in the formation of the 

 body segments than in the Annelids; among the Arthropoda, the 

 body (exclusive of the head), is usually divided into two or more 

 regions, which are distinguished by a special modification of the 

 constituent segments, and the individual segments of each region 

 often differ considerably from one another. This dissimilarity is 

 manifest both externally and internally. Furthermore, the limits 

 between certain of the somites are often obliterated so that they 

 come to be more or less intimately united to form a compound 

 structure, the origin of which can only be made out from a comparison 

 with other forms, or from a study of the development. The most 

 anterior region of the body, the head, is always composed of several 

 fused segments ; some of the appendages thus brought together serve 

 for feeding, and are called mouth-parts ; there are usually, also, one 

 or two pairs of feelers or antennae. 



In tte majority of Ai-thropods, three pairs of mouth-parts are present ; the 

 fii-st are the mandibles, usually strong hard-hiting organs ; the second and 

 third are known respectively as the first and second m a x i 11 se ; they are 

 almost always more feebly developed than the mandibles. These three pau-s may 

 be augumented by others called maxillipeds, when more segments are 

 included in the head. 



As in the Annelids again, the body, with its appendages, is 

 invested by a cuticle, secreted by the epidermis. It differs in an 

 apparently trifling, but in its results very important, respect from that 

 of the Annelids ; for it is usually of a much greater thickness 

 and hardness than in these, forming as it were an armour for 

 the body, an exoskeleton. Only at the constrictions between 

 the segments, both of the body proper and of the limbs, does it retain 



