ARTHROPODA 271 



also antennae, the third pair are mandibles, and behind these limbs 

 are one or more pairs of additional jaws (maxillae). In the crustaceans 

 and insects there is commonly a pair of compound eyes of a complex 

 type peculiar to these animals. The trilobites belong to this section, 

 but their appendages behind the first pair are undiff'erentiated. In the 

 arachnid section none of the limbs have the form of antennae or 

 mandibles, the first pair (chelicerae) being usually chelate, the second 

 chelate, palp-like, or leg-like, and the third to sixth pairs leg-like, 

 though often some of the postcheliceral limbs possess biting pro- 

 cesses (gnathobases) on the first joint. The members of this section 

 never possess true compound eyes of the crustacean-insect type. 



The Crustacea differ from the Insecta and Myriapoda in possessing 

 a second pair of antennae, and nearly always in being truly aquatic. 

 The Insecta differ from the Myriapoda in possessing only three pairs 

 of legs, and usually in the possession of wings. 



The series of somites which, with small pre- and postsegmental 

 regions, constitutes the body of an arthropod is marked out, by 

 differences in width, fusions of somites, or features of the limbs, into 

 divisions known as tagmata. In the Onychophora, Crustacea, Insecta, 

 and Myriapoda, the foremost tagma is a short division, known as 

 the head^ which carries the antennae and mouth parts, and the rest 

 of the body, known as the trunk, is often divided into two sections 

 called thorax and abdomen. In the Arachnida, the foremost tagma 

 is the prosoma ("cephalothorax"), and carries legs as well as the 

 limbs used in feeding, while the divisions, if any, of the hinder part 

 of the body (opisthosoma or "abdomen") are known as the mesosoma 

 and metasoma. It is important that the student should recognize that 

 each of these divisions varies in size, and that consequently none of 

 them comprises in all arthropods the same somites, so that, for 

 instance, the thorax of an insect is a quite different entity from that 

 of a crayfish. The most significant variation is that of the head, 

 which, as the organization of its possessor becomes higher, increases 

 in size, taking in behind somites whose appendages become jaws, 

 while, by alteration in the position of the mouth, it adds others, whose 

 limbs become antennae, to its preoral sensory complex. Thus, while 

 the head of the Onychophora comprises only the first three somites, 

 and only the first of these is preoral, in the Crustacea there are 

 in the true head six somites (including the embryonic first somite), 

 of which three are preoral, and thoracic somites, whose limbs 

 (maxillipeds) function as jaws, are often united with the head. 



The paired limbs of arthropods present an enormous variety of 

 form, and attempts have been made to reduce them to a common type. 

 Some of the evidence suggests an archetype with a nine-segmented 

 axis bearing on the median side of the first segment a biting process 



