THEIR KIN: 



Fig. 15.1. Subph\ium Trilobitomorpha. .4, 

 dorsal view, and /?, ventral view of a gen- 

 eralized trilobite. The appendages are all 

 relatively unspecialized; the fringed lateral 

 processes are interpreted as epipodites, which 

 probably served for gaseous exchange. (Re- 

 drawn from R. E. Snodgrass, Textbook of 

 Arthropod Anatomy, copyright 1952 by Cornell 

 Universitv Press, printed by permission.) 



and the first pair of ventral appendages are modified into pincer-Hke struc- 

 tures termed chelicerae. The Cheficerata include four classes: the class 

 Xiphosurida, containing 'iiorseshoe crabs" of the genus Umulus; the class 

 Eurypterida, an extinct group represented by the "sea scorpions" such as 

 Eiirypteriis : the class Pycnogonida, peculiar, long-legged animals known as 

 "sea spiders"; and the class Arachnida, which contains the familiar spiders 

 and scorpions. 



The subphylum Mandibulata, as the name implies, includes those arthro- 

 pods in which a pair of appendages flanking the mouth are adapted as mandi- 

 bles, or jaws. Of the several classes grouped in this subphylum, the most 

 important are the class Crustacea, mostly aquatic forms such as the lobster, 

 the crayfish, and the crabs; the class Chilopoda, or centipedes, with long, 

 many-segmented bodies and reproductive ducts which open posteriorly; the 

 class Diplopoda, in which the segments are fused in pairs and the openings of 

 the reproductive ducts are anterior; and the class Insecta, which are mostly 

 terrestrial, wing-bearing arthropods with three pairs of walking legs. Famil- 

 iar insects include flies, beetles, and butterflies. 



The habitats of arthropods are more diverse than those of any other 

 phylum. Arthropods are found abundantly in the ocean, in fresh water, and 

 on land, and most of the in.sects are adapted for flight. In correlation with 

 this diversity of habitat the species present a great variety of habits and 

 structural modifications, although the arthropod type of organization is well 



423 



