1 9O MANUAL OF ZOOLOGY. 



CHAPTER XXX. 

 ARTHROPODA. 



DIVISION II. ARTHROPODA, OR ARTICULATA. The remaining 

 members of the sub-kingdom Annulosa are distinguished by 

 the possession ot 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 (fig. 52) 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 

 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 longi- 

 tudinal commissures, and traversed anteriorly by the oesophagus. 

 The haemal system, when differentiated, is placed dorsally, and 

 consists of a contractile cavity, or heart, provided with valvular 

 apertures, and communicating with a perivisceral cavity, con- 

 taining 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 " tracheae." 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 : 



i. CRUSTACEA. Respiration by means of gills, or by the general 

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

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

 thorax, and usually of the abdomen also. 



