PHYLUM ARTHROPOD A 233 



insects belonging to the order Thysanura. The pauropods are 

 minute forms usually under one millimeter in length (Fig. Ill) 

 bearing nine pairs of functional legs. 



Class 7. Diplopoda 



The thousand-legs, or millipeds, are terrestrial - arthropods 

 with the body consisting of two regions, head and trunk. The 

 cylindrical body is composed of numerous segments each of 

 which seems to bear two pairs of short legs inserted near the 

 median line of the body. The apparent doubling in number of 

 appendages on each somite, which is so uncommon in arthropods, 

 seems to be explained on the grounds that each seeming segment 

 is, in reality, a double somite, for on the ventral surface the 

 double nature of the skeletal plates is evident. Some few of 

 the segments, especially near the anterior extremity, remain 

 simple and bear but a single pair of legs each. The distinct head 

 bears a pair of short antennae, usually lateral groups of ocelli, and 

 the mouth parts. The latter comprise an unpaired upper lip, a 

 pair of mandibles, and one or two additional pairs of jaws. 



Most millipeds are harmless and of little importance, for they 

 feed largely upon decaying vegetable matter. Julus, Glomeris, 

 Polyxenus, and Spirobolus are commonly encountered genera. 



Class 8. Chilopoda 



The centipeds are terrestrial tracheate arthropods with only 

 two body regions in which each of the flattened trunk somites 

 bears but a single pair of legs. On the first trunk somite these 

 are modified as poison jaws bearing ducts of poison glands. The 

 legs are lateral in position and are not borne near the median 

 ventral line as in the diplopods. 



The antennae are long and many jointed. The head also bears 

 a pair of mandibles and two pairs of maxillae. The common 

 house centiped, Scutigera forceps, is a representative of this class. 

 Lithobius, Geophilus, and Scolopendra are other genera. 



Outline of Classification 



Phylum Arthropoda, continued. 



II. Class Acerata. — No antennae; body usually cephalothorax and 

 abdomen; usually six pairs cephalothoracic appendages; abdomen 

 almost always lacking appendages in adult; respiration by abdominal 

 gills, lung books or trachcao. 



a. Subclass Gigantostraca. — Marino; gills on abdominal appendages 

 two to six; a pair compovuid eyes and one pair median ocelli. 



