THE PHYLUM ARTHROPODA 339 



accustomed to think of the chordates as the dominant forms of 

 Ufe upon the land surface at the present day, but they are far 

 exceeded in numbers of species and individuals by the Arthropoda. 

 In view of these facts, we may ask what are these Arthropoda, 

 aside from the Crustacea, that so swarm upon the land surface as 

 literally to contend with the terrestrial chordates for possession 

 of the earth. 



There are five classes within the Arthropoda: the Crustacea, 

 as described in the preceding pages; the Onychophora, a small class 

 which is especially important because of its primitive organiza- 

 tion; the Myriopoda, or thousand-legged and hundred-legged 

 animals; the Insecta or Hexapoda, which are the great terrestrial 

 class of the phylum; and the Arachnida, or spiders. 



Fig. 165. — Peripatus entangling a cockroach in sticky threads discharged from 



two papillae beneath the head. 



(From Pearse, "General Zoology," copyright, 1917, by Henry Holt and Co., reprinted 

 by permission.) 



Class Onychophora. — The members of the Class Omjchophora 

 are of interest because they appear to represent a type somewhat 

 similar to the probable ancestors of the Class Insecta. In other 

 respects they link the whole arthropod phylum with the Annulata. 

 They are represented only by the genus Peripatus (Fig. 165) and 

 several closely related genera. These are worm-like animals living 

 in tropical regions, in moist places under bark or objects on the 

 ground, much as do the larvae of many beetles. The general 

 resemblance to an annulate worm, with its terminal mouth and 

 anus and a pair of appendages on each segment of the body is 

 apparent. 



Class Myriopoda. — In the Class Myriopoda are included the 

 centipedes, or "hundred-legged worms" (Fig. 166), and the milli- 



