. Animal biology. Zoology; Biology. CHAPTER XLVII 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. 331. Characteristics and Advances.—The animals belonging to Arthropoda are metameric (Fig. 200), 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 thor
. Animal biology. Zoology; Biology. CHAPTER XLVII 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. 331. Characteristics and Advances.—The animals belonging to Arthropoda are metameric (Fig. 200), 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 Blood i'essel Alimeniviry canal. Moufh So be so phages I ganglion Nerye cord Fig. 200.—Diagrammatio 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. 332. Classification.—The classes of this phylum have been discussed but may be systematic
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