XI 



PHYLUM ARTHROPODA 



617 



stigmata open. In the Chilopoda each segment bears a pair of 

 jointed legs ; of these the most anterior pair is extended forwards, 

 as already stated, to 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 (see p. 636). The exoskeleton is a thickened 

 chitinous cuticle which is calcified in Diplopoda. Odoriferous 

 glands are present in most Diplopoda on some of the body- 

 segments, 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. There are a pair of salivary 

 glands ; and one or two pairs of Malpighian tubes, having a renal 

 function, open into the beginning of the hind-gut. 



The heart is a greatly elongated tube, divided into a number 

 of chambers. 



The respiratory system resembles that of Insects, which will 

 be fully dealt with later (p. 626, Fig. 497), consisting of air-tubes 

 or trachecc. There is one pail of stigmata in 

 each segment in the Diplopoda, and the branch- 

 ing tracheae do not anastomose. In the Chilo- 

 poda the number of stigmata is in most cases 

 less than the number of segments, and the 

 tracheae anastomose, often forming longitudinal 

 trunks which may extend throughout the body. 

 In Scutigcra, the stigmata are unpaired and 

 dorsal, and lead, not into tracheae, but into air- 

 sacs or lungs. 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. 643). There 

 is a brain, a pair of cesophageal connectives, 

 and a ventral nerve-cord consisting of a series 

 of double nerve- ganglia, one in each segment, 

 with double connectives between them. The 

 double character of the ventral cord is much 

 more distinctly marked in the Chilopoda than in the Diplopoda, 

 the ganglia are more distinct, and the first three are intimately 

 united together into an infra-cesophageal mass. A sympathetic 

 or visceral nervous system is present, at least in the Diplopoda. 



Fio. 489. Paur opus 

 huxleyi. (From 

 Leuckart, after L:it- 

 Mi.) 



