ARTHROPODA 



Chapter 7 



The group or phylum Arthro^oda consists of those invertebrates that have 

 jointed appendages, and includes an immense number of diverse forms. Over 

 three-fourths of a million species have been named and described, and about 

 eighty-five per cent of all the known animals belong in this group. They are 

 to be found wherever life is possible and consequently show remarkable adapta' 

 tions. Practically all of them, however, possess jointed appendages, at least in 

 the adult stages, and the body also is made up of a series of segments, usually 

 from eighteen to twenty-three. Partial fusion often makes it difficult to dis- 

 tinguish all, however, especially in the head region. Typically each body seg- 

 ment bears a pair of appendages, but in most cases many of these appendages 

 serve some other purpose than that of locomotion. Those on the head region 

 have been especially modified and may even serve as jaws, so that the novice is 

 surprised to find that the jaws of the crayfish or grasshopper work inward from 

 each side instead of up and down like those of the vertebrates. The skin is 

 hardened by secretions of chitin, an advantage in most respects but a decided 

 disadvantage when growth occurs. Periodically the arthropod becomes too 

 large for its skin, which must then be shed and replaced by one of a larger size. 

 These cast-off skins are frequently encountered and are often perfectly com- 

 plete, only a slit in the back indicating how the former occupant escaped. 



The scientist divides the Arthropoda into several groups. One of these, 

 the Onychophora, is a resident of the subtropics, but is of much theoretical 

 interest because its members combine several typically annelid structures with 

 other equally typical arthropod structures and thereby form a connecting link 

 between tw'o major groups. The other groups are generally distributed and are 

 abundant in most parts of the United States. 



Practically every stream and pond contain representatives of the Crw 

 stacea. This division of the Arthropoda is characterized by the presence of 

 gills, usually concealed under an outgrowth of the exoskeleton, and by two 

 pairs of antennae, in typical forms. The crayfish is our largest fresh-water 

 example, but it is outnumbered by hosts of almost microscopic forms. Most of 

 the Crustacea are essentially aquatic, while the great majority of the rest of 

 the arthropods are land-dwelling animals or only temporary dwellers in the 

 water. 



Another large group of the Arthropoda, the Arachyiida, is characterized 

 by the lack of antennae. The majority of them have the head and thorax 



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