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 with one or more pairs 

 of coelomoducts as gonoducts and often 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 brings with it the necessity 

 for joints ; and the stout, jointed limbs can now be adapted for various 

 purposes to which those of polychaetes were not convertible. Always 

 at least one pair of them become jaws ; with this is usually associated 

 the specialization for sensory functions of one or two pairs which have 

 come to stand in front of the mouth, and thus the process of cephaliza- 

 tion, begun in the polychaetes, proceeds further here. Other limbs 

 commonly become legs. 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 prob- 

 ably 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, 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. This 

 difference, too, may be connected with the difference in the stoutness 

 of the cuticle. Lastly, it is perhaps that thick covering, hindering the 

 loss of water by evaporation from the surface of the body and pro- 

 viding the skeleton which the lack of support from the medium 

 necessitates, which has enabled arthropods very successfully to invade 

 the dry land. Like those of all other phyla, their earliest known 

 members, the trilobites, were aquatic. Of their surviving groups. 



