CHAPTER 17 



o«>i 



Phylum /Vrthropoda. 



Peripatus, 



Centipedes, 



and Millipedes 



222 



Tor convenience, three classes of arthro- 

 pods are considered together in this chapter. 

 The Onychophora (the name means claw- 

 bearing) are rare; and, although the Chilo- 

 poda (centipedes) and Diplopoda (milli- 

 pedes) are abundant in certain localities, 

 other types of arthropods considered in 

 Chapter 18 reveal the characteristics of this 

 phylum to better advantage. 



The onychophorans resemble more 

 closely than any other animals what is be- 

 lieved to have been the ancestral condition 

 of the arthropods. They possess a thin cuti- 

 cle, a continuous muscular body wall, no 

 joints, one pair of jaws, a tracheal respira- 

 tory system, and a series of nephridial open- 

 ings. 



CLASS ONYCHOPHORA 



Peripatus (Fig. 124), a representative 

 species, is about two or three inches in 

 length, with a cylindrical body, but with- 

 out a distinct head. These animals are 

 especially interesting because they obviously 

 exhibit both arthropod and annelid charac- 

 teristics, as well as peculiarities of their own. 

 Unfortunately, they are gradually disap- 

 pearing and hence becoming more difficult 

 to observe. However, the group furnishes 

 an excellent example of discontinuous dis- 

 tribution. Species have been reported from 

 Central America, Mexico, the West Indies, 

 and the southern hemisphere. Even in the 

 area where a species occurs, specimens are 



Figure 124. Facing page, Peripatus, an onychoph- 

 oran. Top, drawing to show external structure. 

 Bottom, photo of living animal. It is a walking 

 wormlike animal, which is neither an annelid nor a 

 typical arthropod. Because it has both annelid and 

 arthropod characteristics, it is the only living ani- 

 mal that comes near to being a common rela- 

 tive to annelids and arthropods. Therefore it is 

 considered by some as a connecting link between 

 the two phyla. (Photo reproduced by permission 

 from The Biotic World and Man, by L.J. and M.J. 

 Milne, p. 498. Copyright, 1952, by Prentice-Hall, 

 Inc., Englewood Cliffs, N.J.) 



