THE ARTHROPODS 



127 



119. Peripatus (class Onychophora). It is generally be- 

 lieved that the Crustacea, insects, and spiders, together 

 with their numerous relatives, trace their ancestry back to 

 animals that bore a certain resemblance to the segmented 

 worms. Most of these ancient types have 



long been extinct, but here and there 

 throughout the earth we occasionally meet 

 with them. 



Among the most interesting of these 

 are a few widely distributed species belong- 

 ing to the genus Peripatus (Fig. 72), but as 

 they are comparatively rare we shall dis- 

 miss them with a very brief description. 

 They usually dwell in warm countries, un- 

 der rocks and decaying wood, emerging at 

 night to feed on insects, which they ensnare 

 in the slime thrown out from the under 

 surface of the head. Their external form, 

 their excretory system, and various other 

 organs are worm-like. On the other hand, 

 the appendages are jointed, and one pair 

 has been modified into jaws. The peculiar 

 breathing organs characteristic of the in- 

 sects are also present. Peripatus therefore 

 gives us an interesting link between the 

 worms and insects, and also affords an idea 

 of the primitive insects from which the 

 modern forms have descended. 



120. The centipeds and millipeds (class 

 Myriapoda). Many of the myriapods that 

 is, the centipeds and thousand-legged worms 

 are familiar objects under logs and stones 

 throughout the United States. The first of these (Fig. 73) 

 are active, savage creatures, devouring numbers of small 

 animals, which they sting by means of poison-spines on the 

 tips of the first pair of legs. The bite of the larger tropical 



FIG. 72. Peripatua 



(Peripatus eiseni). 

 Twice the natural 

 size. 



