130 IXVEIITEIil.'ATK ANIMALS. 



CHArTEi; XIII. 

 AKTHllOPdDA. 



1. GeNEHAI, CiIARACTER.S of TIIK AkTHROPODA. 2. CriARACTERS 

 AXD IllVlSIiiKS oh' THE t'RUSTACEA. 



The remaiiiinij; nioiilierw of the xvib-kingdom Aniuildsa are dis- 

 tinguished liy the ];()Ssessi(ii[ of juintrd ajipeivlages, articulated to 

 till' liiidf/ ; aiid they folia ii great jirimavy di\'isioii — often called by 

 the iiaiue An iriilntii . As this name, however, has been em])loyed 

 in a wider sense than is understood by it here, it is perhaps best to 

 adopt the iiiine modern term Artlimjioila. 



The members iif this divisinn, comprising the CniMacea (Lobsters, 

 t'rabs, kv.), the Anu-linida (Sjiiders and Scorpions), the Mi/riajmhi 

 (Centipedes), and the I/isriin, are distinguished as fnUcjws : — 



The liuily (tig. TlJ is Composed of a series of segments, arranged 

 ailing a hmgitudinal a.xis ; each segment or "somite," occasionallv, 

 and Slime almost aKvay.s, being iirovided with articulated a]ipen- 

 dages. Iloth tlie segmented body and the articulated limbs are 

 more or less comjiletely jiroteeted by a chitinous exoskeleton, 

 formed by a, hardening of the cuticle. The appendages are hollow, 

 and the iiiiiseles are jjrolongeil into their interior. The nervous 

 sjstem in all, at any rate in the embryonic condition, consists of a 

 double rhain of ganglia, ]>laeed along the ventral surface of the 

 body, iiiiiled by longitudinal coniniissures, and traversed anteriorly 

 liy the osnphagiis. Tlie blood-system, when ditl'erentiated, is)ilactd 

 doi'sally, and consists of a. contractile cavity, or heart, provided 

 with vahiilar apertures, .-11111 eomiminicating with a ]ieri\-isceral 

 cavity, eoiifainiiig corpuseiilated blood. Eespiration is effected 

 by the general surface of the body, by gills, by j.iilmoii.ary sacs, 

 or by tubular in\'olutioiis of (he iuteguiiient, termed " tracheie." 

 Jo uo iiiemlirr of tlie ili\isioii are vibratile cili.i known to be 



llrv,l,,|„.,|. 



t hi- Ai//i,-i,/i,,i/ii ;li-l- divided into four great classes— viz. the 



