396 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



and the crustaceans have attained a high degree of development 

 along this line. The spiders likewise have evolved a two-piece 

 body (fig. 15 B), but Avith this difference from the crustaceans, 

 that the principal viscera are contained in the second body division. 

 The centipedes (C), on the other hand, have progressed but little 

 farther than the acquisition of a six-segment head similar to that 

 of the insects, the rest of the body being composed of a series of 

 segments all pretty much alike. The insects (D) are chai-acterized 

 by liaving a head well separated by a neck from the thorax, by the 



Fig. 15. — I'rincipal types of animals ovolved lu the pbyluin Aitliropoda by difforeiU 

 groupings of the segments 

 A, a crab (Crustacea) with a two-piece body, no distinct bead, the principal 

 viscera in the lirst body division. B, a spider (Arachnida), also with a two-piece 

 body and no distinct head, but witli principal viscera in second part. C, a centipede 

 (Myriapoda), with a bead, and undivided body of many segments. D, an in^^ect 

 (He.xapoda), composed of lu'ad, thorax, and abdomen, with principal viscera in the 

 last. 



specialization of the thorax as the locomotory part, since it carries 

 the legs and the wings, and by the developinent-of the abdomen as 

 a container for the larger viscera. 



THE PRIMITIVE APPENDAGES 



Closely connected with the evolution of the segments, in both 

 insects and other arthropods, is the development of the external 

 organs attached to them. One of the most conspicuous features 

 of nearly all members of the Arthropoda is the number of legs or 

 leg-like structures they have — " crawly " things these creatures are 

 to most people on this account. Insects, it is true, have only three 



