170 MANUAL OF ZOOLOGY. 



CHAPTER XXX. 

 AETHEOPODA. 



DIVISION II. AETHEOPODA, OE AETICULATA. The remaining mem- 

 bers of the sub-kingdom Annulosa are distinguished by the 

 possession of jointed appendages, articulated to the body ; and 

 they form the second 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 Arthropoda. 



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 is composed of a series of segments, arranged along 

 a longitudinal axis ; each segment, or ' somite,' occasionally, 

 and some always, being provided with articulated appendages. 

 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 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 traversed anteriorly by the 

 oesophagus. The hasmal system, when differentiated, is placed 

 dor sally, and consists of a contractile cavity, or heart, provided 

 with valvular apertures, and communicating with a perivisceral 

 cavity, containing corpusculated blood. Respiration is effected 

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

 or by tubular involutions of the integument, termed ' trachese.' 

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

 developed. According to Professor Huxley, an additional 

 constant character of the Arthropoda is to be found in the 

 structure of the head, which is typically composed of six seg- 

 ments, and never contains less than four. 



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

 Crustacea, the Arachnida, the Myriapoda, and the Insecta ; 

 which are roughly distinguished as follows : 



1. CEUSTACEA. Respiration by means of gills, or by the general 

 surface of the body. Two pairs of antennce. Locomotive appen- 

 dages more than eight in number, borne by the segments of the 

 thorax, and usually of the abdomen also. 



2. AEACHNIDA. Respiration by pulmonary vesicles, by trachece. 

 or by the general surface of the body. Head and thorax united 



