:: 



PHYLUM ARTHROPODA 



569 



form a pair of poison jaws (maxillipedes), at the extremity of 

 the pointed terminal joint of which opens the duct of a poison 

 gland. In the Diplopoda each segment behind the fourth or fifth 

 bears two pairs of legs, the four or five most anterior having only 

 one pair each. In most of the Diplopoda the appendages of the 

 seventh segment are modified in the male to form copulatory 

 organs. 



The integument and body wall do not differ widely from 

 those of Insects. Odoriferous glands are present on some of the 

 body segments in the Diplopoda, and open on the dorsal surface. 

 Scolopendrella possesses spinning glands. 



The alimentary canal is straight, and is much simpler in 

 character than that of the Insecta. Connected with the mesen- 

 teron are a number of short hepatic caeca, and one or two pairs of 

 Malpigliian tubes, having a renal function, open into the beginning 

 of the hind gut. 



The heart is greatly elongated, and consists of as many 

 chambers as there are segments in the body. 



The respiratory system resembles that of Insects to be fully 

 dealt with later (p. 578, Fig. 459), consisting of air- tubes or 

 trachece. There is one pair of stigmata in each 

 segment in the Diplopoda, and the tracheae are 

 unbranched. In the Chilopoda the trachea? 

 are branched, and the number of stigmata is 

 in most cases less than the number of seg- 

 ments. In Scutigera, one of the stigmata is 

 unpaired and dorsal. In the Symphyla there 

 are only two stigmata, and these are situated 

 on the head. 



The nervous system is, in accordance with 

 the form of the body, much less concentrated 

 than in the Insecta (see below, p. 594). In the 

 Chilopoda there is a double ganglion in each 

 segment of the trunk ; in the Diplopoda there 

 are two. The longitudinal commissures be- 

 tween the ganglia are usually distinct. The 

 ganglia of the first two segments unite to 

 form an infra- cesophageal ganglion from which 

 the nerves to the jaws are given off. A 

 sympathetic or visceral nervous system is 

 present as in Insects. 



The sexes are always separate. The reproductive organs 

 ovaries or testes are usually paired, with paired ducts, having a 

 common terminal portion ; but in the Diplopoda there is an 

 unpaired ovary or testis with paired ducts, which jjemain distinct 

 throughout, and open by separate apertures. lofthe Chilopoda 

 the external aperture is situated on the penultimate segment ; in 



FIG. 451. Paur opus 

 huxleyi. (From 

 Leuckart, after Lat- 

 zel.) 





