PHYLUM ARTHROPODA 263 



the insects ; but the insects also have simple eyes 

 (ocelli). 



Class Prototracheata 



The arthropods, with their jointed legs and usually 

 hard chitinous covering, seem isolated in the animal 

 kingdom. The various soft-bodied and legless mem- 

 bers of the group, such as the female scale insect, are 

 obviously not primitive, but highly specialized. Look- 

 ing for some real relative outside of the arthropod 

 phylum, we can turn only to the higher worms. These 

 are adapted for life in the water or in moist earth, 

 whereas the majority of the arthropods live on the sur- 

 face of the earth or on plants. The Crustacea do indeed 

 inhabit the waters in great numbers, but they show 

 little resemblance to worms. There is, however, one 

 group of animals which, although terrestrial, is soft- 

 bodied, without chitinous body rings, and doubtless 

 primitively so. Superficially, at least, it seems to com- 

 bine the features of a worm with those of a centipede. 

 The Peripatus, first discovered in the Island of St. Vin- 

 cent, West Indies, was taken by its discoverer for some 

 strange kind of slug. This was in 1826, and since that 

 time many related species have been found, widely 

 scattered over the earth, so that today over 70 differ- 

 ent forms can be enumerated. It is found that these Thetracheal 

 animals breathe by means of trachea, - - that is to say, 

 minute tubes connected with small openings on the sur- 

 face of the body. Hence the group has been called 

 Prototracheata, or first (in the sense of primitive) 

 trachea-breathing animals. 1 They resemble the terres- 

 trial arthropods in this feature, but the tracheal open- 

 ings are scattered over the surface of the body, instead 



1 It is also called Protracheata and (more generally) Onychophora. 



