12 INSECTA. 



is the head (PI. I., Fig. 2, A), the enlarged middle 

 region the thorax (Fig. 2, B), and the posterior region 

 the abdomen (Fig. 2, C). The scholars should observe 

 the distinctness of these regions, and the marked de- 

 velopment of the head which especially characterizes 

 insects as a whole and separates them from the Spiders 

 and Crustacea, and they ought to detect in the seg- 

 mented body, especially in the posterior region, one 

 of the structural characteristics common to these three 

 classes and to the Worms. This characteristic of being 

 composed of segments, or successive rings, possessed 

 in common by the Worms, Crustacea, Spiders, Myrio- 

 pods, and Insects, led Cuvier to bring these five types 

 together under the name of the Articulata. The last 

 four classes are now usually grouped together as the 

 Arthropoda, or animals with jointed appendages. 

 These, as one type, are contrasted with the Worms, 

 which either possess simple, unjointed appendages, or 

 none at all, as stated in Guide No. VII., p. 16, and 

 the use of the term " Articulata " has been discon- 

 tinued.^ 



1 While this is a statement of current views, it does not repre- 

 sent the unanimous opinion of investigators. Some naturalists, 

 among them the authors of this Guide, are disposed to uphold a 

 modified form of the Cuvierian classification. The old names 

 Kadiata, Mollusca, and Articulata, like the name Vertebrata, rep- 

 resent obvious relations, and a legitimate grouping of forms. 

 The groups Crustacea, Scorpions and Spiders, Myriopods, and In- 

 sects have a stiff skeleton, and as swimming and walking animals 

 necessarily have jointed appendages. The Worms, being crawl- 

 ers and burrowers or swimmers, do not need a hard skeleton. 

 Their integument being soft, they do not have articulated or 

 jointed legs, but soft paddles and setae. The division of the 

 Arthropoda and Vermes may be used to show such distinctions, 



