2 I 8 MANUAL OF ZOOLOGY. 



the Myriapoda (Centipedes), and the Insecta, are distinguished 

 as follows : 



The body (fig. 75) 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 k anteriorly by the oeso- 

 phagus. The hsemal system, when differentiated, is placed 

 dorsally, and consists of a contractile cavity, or heart, provided 

 with valvular apertures, and communicating with a perivis- 

 ceral 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 " tracheae." In no member of the division are vibra- 

 tile cilia known to be developed. According to Professor 

 Huxley, an additional constant character of the Ai'thropoda 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 : 



1. 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. 



2. ARACHNIDA. Respiration by pulmonary vesicles, by trachea, 

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

 into a cephalothorax. Antenna (as such) absent. Legs eight. 

 Abdomen without articulated appendages. 



3. MYRIAPODA. Respiration by trachea ; head distinct ; re- 

 mainder of the body composed of nearly similar somites. One 

 pair of antenna. Legs numerous. 



4. INSECTA. Respiration by trachea. Head, thorax, and 

 abdomen distinct. One pair of antenna. Three pairs of legs 

 borne on the thorax. Abdomen destitute of limbs. Generally 

 two pairs of wings on the thorax. 



