PHYLUM ARTHROPODA 



3°9 



four orders adopted in this book are ranked as phyla by some 

 zoologists. The chief distinguishing characteristics of the group 

 are: (i) a distinct head with one pair of tentacles and one pair of 

 mandibles, (2) numerous body segments bear- 

 ing similar leglike appendages, (3) tracheae 

 with segmentally arranged apertures, and 

 (4) excretory organs (malpighian tubules) 

 opening into the intestine. 



Order 1. Pauropoda (Fig. 231). — These 

 are small myriopods less than 2 mm. in 

 length which prey on microscopic animals 

 or eat decaying animal and vegetable 

 matter. They are without eyes, heart, and 

 special respiratory organs, and evidently 

 breathe through the general surface of the 

 body, as in the earthworm. The head is 

 distinct, and the body contains twelve seg- 

 ments and bears nine pairs of legs. The 

 Pauropoda are apparently primitive myrio- 

 pods related to the millipedes (Diplopoda). 

 Pauropus and Eurypauropus are North American genera. 



Order 2. Diplopoda. — The Diplopoda are called millipedes 

 (Fig. 232). The body is subcylindrical, and consists of from 

 about twenty-five to more than one hundred segments, accord- 



Fig. 231. — Order 

 Pauropoda. Pauropus 

 huxleyi. (From Sedg- 

 wick's Zoology, after 

 Latzel.) 



Fig. 232. — A millipede. (From Shipley and MacBride, after Koch.) 



ing to the species. Almost every segment bears two pairs of 

 appendages (Fig. 232, 3), and has probably arisen by the fusion 

 of two segments. The mouth parts are a pair of mandibles and 



