316 



PHYLUM ARTHROPODA. 



within the firm external cuticle, and the bands of it pass from 

 point to point across the articulations. The Arthropoda 

 are thus markedly contrasted with the Chordata, in which the 

 skeletal structures are internal and covered by the muscles.* 

 There is no contractile dermomuscular body wall, except in 

 Peripatus. 



On the vastly increased mechanical possibilities afforded by 

 such an exoskeleton, with its plates of varying degrees of 

 rigidity and delicately adjusted systems of levers the active 

 many-sided life of the Arthropoda depends. Associated with 

 the increased powers of locomotion which such a skeleton con- 

 fers we find a high development of the sense organs and 

 nervous system, and, in Insects, the elaborate social instincts 

 which have ever excited the wonder of mankind. 



FIG. 228. Squilla mantis. A', A" first and second antennae ; B', B", B"' the three jiairs .of 

 biramous appendages (6th-8th thoracic) ; Kf, Kj" first and second maxillipeds 

 (after Claus). 



In no Arthropod is the body-cavity coelomic. It consists of 

 blood-spaces, constituting a haemocoele. The development 

 of members of the several groups however reveals the fact that 

 a coelom exists, in a more or less modified form, in all. 



In Peripatus the somites are formed from the mesoblastic 

 bands as paired structures, a pair to each segment at the sides 

 of the elongated blastopore (p. 570). They are hollow from 

 their first appearance and the cavity which each contains is 

 the coelomic cavity. From them are formed (a) the nephridia, 

 a pair to each trunk segment, with their end-sacs ; and (6) the 

 tubular generative organs. The latter arise by the junction of 

 the dorsal portions of the paired coelomic cavities of certain of 

 the posterior segments, and their connexion with a posterior 



* The fibre-cartilaginous endosternum of Limulus and other Arachnids is 

 however comparable, as regards its relation to muscles, with the chordate 

 skeleton. 



