3o8 THE INVERTEBRATA 



only one, the Crustacea, remain predominantly of that habit. No other 

 invertebrate phylum has so large a proportion of terrestrial members. 



A more detailed survey of the organization which we have now out- 

 lined, necessitates a brief exposition of the principal groups into which 

 the phylum falls. One small section stands apart from the rest. The 

 Onychophora have a thin cuticle, without joints; a continuous mus- 

 cular body wall; eyes (p. 310) of annelid type; only one pair of jaws, 

 which moreover are constructed on a different principle from those 

 of other arthropods, biting with the tip and not with the base of the 

 limb ; and a long series of coelomoducts, of which the pair that are 

 the oviducts are ciliated. Only in this group, too, does the first somite 

 bear a pair of limbs : in all others that somite is an evanescent, em- 

 bryonic structure without external representation in the adult. In 

 all these respects the Onychophora show a lower degree of develop- 

 ment of the peculiar features of arthropods than the rest of the 

 phylum. 



The remaining groups of the phylum fall into two sharply different 

 sections, the crustacean-insect-myriapod section and the arachnid 

 section. In the first of these sections, the first pair of limbs (those of 

 the second somite) are antennae, the succeeding pair, if present, are 

 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 undifferentiated. 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 processes 

 (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, In- 

 secta, 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 



