402 



ARTHROrODA 



longitudinal trunks i^tig. 45-\ Ih). so that spiracles occur in oiily a part of 

 the segments, these supplying the whole l.ioily. Although the trachea^ 

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

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

 ing the spiracles. Then aquatic lar\';e often have tracheal gills, thin- 

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

 penetrated by numerous tracheal twigs (tig. 453). 



The alimentarv tract always has excretory organs, the Malpigliian 

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

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

 tract. They dilTer from the physiologicallv similar tulies of the Arachnida 

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

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

 wiiile the ducts (at least in some cases modi lied nephridia) open \'entrally 

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



In the subdivision of the ' trachea te' arthropods a group of ^Ivriapoda is 

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

 types are in reality very diti'erent. The centipedes (Chilopoda) show in al! 

 structural features close relationships to the Hexapoda. while the Diplopoda 

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

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

 blances and ditferences. 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 anteiiiix and usually 



Fig. 43Q. — Diagram of transverse section of a centipede (orig.V J, digestive tract; 

 S, gonad; h, ncr\c cord; .v. spiracle and trachea'. 



numerous ocelli, which only in Sciitigrra show a tendencv to become com- 

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

 of maxilkc, both uiiited iir the median line, the first pair forming a 'giiallio- 

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

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



