ARTHROPODA. 275 



307. General Survey. The symmetry of the Arthropods 

 is very pronounced, except in the case of fixed, parasitic, or 

 otherwise degenerate forms (as barnacles, Sacculina, etc.). 

 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 

 successful 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. 



308. 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 (cray-fish), may 

 fuse to form the thorax. These two regions, head and 

 thorax, are often fused into one piece the cephalothorax. 

 The abdominal segments are usually unfused. 



309. 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 



