CHAPTER XL VII 



ARTHROPODS IN GENERAL 



From what has already been said in regard to the different groups 

 included in this phylum the general characteristics have become evident 

 and need only to be briefly reviewed. 



333. Characteristics and Advances. — The animals belonging to 

 Arthropoda are metameric (Fig. 213), and all exhibit more or less of a 

 tendency for the different metameres to be grouped into three regions, 

 namely, head, thorax, and abdomen. In Onychophora and Myriapoda 

 the latter two are not evident ; in the spiders and in many Crustacea the 

 head and thorax are combined into a cephalothorax; in insects, however, 

 the three regions are distinct. Another characteristic of the group is 

 that typically each metamere bears a pair of jointed appendages, though 



Antenna ^Supraesophageal 



Bloodvessel Alimenfviry canal 



Moufh 



Suhesophagec^l 

 ganglion 



Nerve cord 

 Fig. 213. — Diagrammatic representation of the structure of an arthropod. {From Schmeil, 

 " Text-book of Zoology," by the courtesy of A. and C. Black, and Quelle and Meyer.) 



in many groups a greater or less number of these appendages are absent. 

 The cuticula often becomes highly chitinized and in the Crustacea is 

 also hardened by the addition of lime salts. The nervous system resem- 

 bles that of the annelids in having metamerically arranged ganglia, but 

 in the higher forms it shows a pronounced tendency toward fusion of the 

 ganglia and to cephalization, or the localization of control in the head 

 region. The increase in the number and variety of appendages, which 

 makes for variety in action, as well as the higher development of both 

 central nervous system and of sense organs, is the chief advance shown 

 by the phylum. There is also a high degree of specialization in the ali- 

 mentary canal, which is divided into distinct regions and modified in 

 accordance with the character of the food. 



334. Classification. — The classes of this phylum have been discussed 

 but may be systematically presented as follows: 



310 



