136 INVERTEBRATE ANIMALS. 



CHAPTEE XIIL 



ARTHROPOD A.. 



1. General Characters op the Arthropod a. 2. Characters 

 AND Divisions of the Crustacea. 



The remaining members of the sub-kingdom Annulosa are dis- 

 tinguished by the possession of jointed appendages, articulaied to 

 the body ; and tliey form a great primary division — often called by 

 the name Articulata. As this name, however, has been employed 

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

 adopt the more modern term Artlirnpoda. 



The members of this division, comprising the Crustacea (Lobsters, 

 Crabs, &c.), the Arachnida (Spiders and Scorpions), the Myriapoda 

 (Centipedes), and the Insecta, are distinguished as follows : — 



The body (fig. 71) is composed of a series of segments, arranged 

 along a longitudinal axis ; each segment or " somite," occasionally, 

 and some almost always, being provided with articulated appen- 

 dages. Both the segmented body and the articulated limbs are 

 more or less completely protected by a chitinous exoskeleton, 

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

 and the muscles are prolonged into their interior. The nervous 

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

 double chain of ganglia, placed along the ventral surface of the 

 body, united by longitudinal commissures, and travereed anteriorly 

 by the cesophagus. The blood-system, when differentiated, is placed 

 dorsally, and consists of a contractile cavity, or heai-t, provided 

 with valvular apertures, and communicating with a, perivisceral 

 cavity, containing corjnisculated blood. Respiration is effected 

 by the general surface of the body, by gills, by pulmonary sacs, 

 or by tubular involutions of the integument, termed " tracheae." 

 In no member of the division are vibratile cilia known to be 

 developed. 



The Arthropoda are divided into four great classes — viz., the 



