CHAPTER X 



THE PHYLUM ARTHROPODA 



Bilaterally symmetrical, segmented Metazoa; with, on some or all of 

 the somites, paired limbs, of which at least one pair function as jaws ; 

 a chitinous cuticle, which usually is stout but at intervals upon the 

 trunk and limbs flexible so as to provide joints; a nervous system 

 upon the same plan as that of the Annelida ; the coelom in the adult 

 much reduced and replaced as a perivisceral space by enlargement of 

 the haemocoele; without true nephridia, but often with one or more 

 pairs of coelomoducts as excretory organs ; and (except in Peripatus) 

 without cilia in any part of the body. 



The Arthropoda have much in common with the Annelida, and 

 must be regarded as derived from the same stock as the Polychaeta 

 in that phylum. The key to most of their peculiar features is an in- 

 crease in the thickness of the cuticle. This entrains the necessity for 

 joints; and the stout, jointed limbs can become jaws. In order to ' 

 move the complex of hard pieces constituted by the jointed cuticle, 

 the continuous muscular layer of the body wall of an annelid has 

 become converted into a system of separate muscles; with this, and 

 with the fact that turgescence of the body wall is no longer a factor in 

 locomotion, is perhaps connected the replacement of the perivisceral 

 coelom by a haemocoelic space. The loss of the nephridia which in 

 annelids lie in the coelom is probably due to the reduction of that 

 cavity. An interesting feature of difference between the Arthropoda 

 and Annelida is the absence from the former phylum of the chetae, v^ 

 imbedded in and secreted by pits of the skin, which characterize the 

 annelids ; though bristles, formed as hollow outgrowths of the cuticle, 

 are common on arthropods. 



One small group of the Arthropoda stands apart from all the rest. 

 The Onychophora have a thin cuticle, without joints; a continuous 

 muscular body wall; eyes (p. 274) 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, 

 embryonic structure without external representation in the adult. 



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 



