402 ARTHROPODA 



longitudinal trunks (fig. 452, Ib), so that spiracles occur in only a part of 

 the segments, these supplying the whole body. Although the trachea? 

 are for aerial respiration, there are aquatic insects, but these also breathe 

 air, which they carry about with them entangled among the hairs surround- 

 ing the spiracles. Then aquatic larva? often have tracheal gills, thin- 

 walled processes of the integument which project into the water and are 

 penetrated by numerous tracheal twigs (fig. 453). 



The alimentary tract always has excretory organs, the Malpighian 

 tubules, connected wdth it. These vary in number between wide limits, 

 but are always placed at the junction of the rectum with the rest of the 

 tract. They differ from the physiologically similar tubes of the Arachnida 

 in being of ectodermal origin, so that no homology can be traced between 

 them. The gonads are always paired and placed dorsal to the intestine, 

 while the ducts (at least in some cases modified nephridia) open ventrally 

 at the hinder end of the body. The spermatozoa are motile. 



Tn the subdivision of the 'tracheate' arthropods a group of Myriapoda is 

 usually recognized, containing the centipedes and 'galley worms.' These two 

 types are in reality very different. The centipedes (Chilopoda) show in all 

 structural features close relationships to the Hexapoda, while the Diplopoda 

 differ in almost every respect, except the presence of numerous walking legs, 

 from the Chilopoda. Hence, since the object of classification is to show resem- 

 blances and differences, the Myriapoda has been dismembered, the Chilopoda 

 being considered here, the Diplopoda as a distinct class at the end of the group 

 of Arthropoda. 



Sub Class I. Chilopoda. 



The most striking characteristic of the chilopods is their long, flattened 

 bodies, each of the numerous similar somites bearing a pair of six- or 

 seven-jointed limbs. The head bears a pair of long antennae and usually 



FIG. 439. Diagram of transverse section of a centipede (orig.). d, digestive tract; 

 g, gonad; n, nerve cord; s, spiracle and tracheae. 



numerous ocelli, which only mScutigcra show a tendency to become com- 

 pound. The mouth parts (fig. 440) are a pair of mandibles and two pairs 

 of maxilke, both united in the median line, the first pair forming a ' gnatlio- 

 cliilarium,' the second the lower lip. Besides, the first pair of legs (5), 

 with their fused bases, extend forward beneath the head and form the 



