ARTHROPODA 279 



The group presents great diversities expressive of a high de- 

 gree of adaptation to almost every conceivable mode of life. 

 They may be parasitic — internal or external, symbiotic, social, 

 or independent; they may be aquatic, terrestrial, burrowing or 

 aerial; they use all sorts of food; they bore, crawl, swim, jump, 

 fly, or may be fixed. In geographical distribution they are 

 practically cosmopolitan. The group is one of the most suc- 

 cessful in the animal series. None of the living species, however, 

 attains a very great size. The king-crab and the lobster 

 are among the largest. Many are microscopic. 



315. The Segments. — There is a great deal of diversity 

 among the segments of the body as to size, shape, the form 

 and use of their appendages, as well as in their contained 

 structures. In the more primitive forms (Peripatus and the 

 centipedes) and in the larval condition, the somites are well 

 marked externally, but in the majority of forms there is more 

 or less fusion of contiguous somites in certain body regions. 

 A variable number of segments at the anterior end, which 

 bear the mouth parts and sense organs, form the head. Be- 

 hind these a group of three (insects), or more (crayfish), may 

 fuse to form the thorax. The head and thorax are often fused 

 into one piece — the cephalothorax. The abdominal segments 

 are usually unfused. 



316. The Appendages also differ much in form in the 

 various representatives and on different segments of the same 

 individual. This diversity of structure is closely connected 

 with the variety of work to be done, and is an excellent illus- 

 tration of the differentiation which accompanies ' ' division of 

 labor." They are unquestionably serially homologous organs 

 as is shown by their similarity of origin and by the funda- 

 mental likeness of structure, — clearly to be seen in the primi- 

 tive forms. They may be said to consist typically of a basal 

 portion with one or more segments, supporting two jointed 

 branches, — a median and an external. Appendages may be 

 entirely wanting (as in the abdominal segments of almost all 

 adult insects) ; and yet these may appear in a rudimentary form 

 in the early stages of the embryo, only to disappear later. 



