2/4 MANUAL OF ZOOLOGY. 



CHAPTER XXIX. 

 ARTHROPODA. 



DIVISION II. ARTHROPODA, OR ARTICULATA. The remaining 

 members 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 (fig. 112) 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 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. Xhe nervous system in all, at any rate in the em- 

 bryonic 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 

 haemal system, when differentiated, is placed dorsally, and con- 

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

 apertures, and communicating with a peri visceral 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 de- 

 veloped. According to Professor Huxley, an additional con- 

 stant character of the Arthropoda is to be found in the structure 

 of the head, which is typically composed of six segments, 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 

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

 the thorax and, usually, of the abdomen also. 



