274 



PRINCIPLES OF ANIMAL BIOLOGY 



trees or carry disease-producing organisms (mosquitoes, housefiies). 



Some of the insects are parasitic in domestic animals, and the mites may 



attack the skin of man ("jiggers"), poultry, 

 or cattle. Barnacles injure bottoms of ships. 

 Relationship of the arthropods to the 

 annelids has often been suggested, largely 

 because of the segmentation of the body into a 

 longitudinal chain of metameres. In support 

 of this idea is brought the annelidlike Peri- 

 patus (class Onychophora, below). Peripatus 

 has a body superficially like a worm and has 

 segmentally arranged nephridia; but it has 

 tracheae like insects, sinuses in the circulatory 



system, and no coelom. If Peripatus is a primitive form, which may be 



doubted, its value as a connecting link is considerable. 



Fig. 240. — A scorpion 

 (left) and king crab. {From 

 Carolina Biological Supply 



Co.) 



two pairs of 



Class II. Onychophora. Primitive air breathing arthropods with tracheae and 

 nephridia. Peripatus. 



Class III. Myriapoda. Arthropods with tracheae, one pair of antennae, and 

 many similar legs. Centipedes and millipedes. (Fig. 235.) 



Order 1. Pauropoda Order 3. Chilopoda 



Order 2. Diplopoda Order 4. Symphyla 



