xi PHYLUM ARTHROPODA 601 



The alimentary canal is straight, and is much simpler in 

 character than that of the Insecta. There is 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, consisting of air-tubes or trachea. There 

 is one pair of stigmata in each true segment in the Diplopoda (two 

 pairs in each apparent segment). Each stigma leads into an air- 

 chamber from which a large number of tracheae are given off. In 

 some Diplopoda the tracheae are branched, in others unbranched : 

 the tracheae of one group do not anastomose with those of other 

 groups. In the Chilopoda 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 Scutigera the stigmata are unpaired and dorsal, 

 and each leads into a large air-chamber which gives off on either 

 side a large number of radially arranged short air-tubes the whole 

 forming a sort of lung. 



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 p. 628). 

 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. 



The sexes are always separate. There is usually an unpaired 

 gonad with paired ducts. In the Chilopoda the single genital 

 aperture is situated at the posterior end of the body : in the 

 Diplopoda and Pauropoda the two apertures are placed far forwards 

 towards the anterior end. 



The ovum, as in most Arthropods, contains a large quantity of 

 food-yolk. The centrally-placed segmentation-nucleus divides so 

 as to give rise to a number of nuclei, this division being accom- 

 panied by a partial division of the yolk into a number of masses. 

 The nuclei then, for the most part, migrate to the surface, some 

 being left behind in the yolk. Those that reach the surface, sur- 

 rounded each by its little clump of protoplasm, become arranged 

 into a continuous superficial layer of cells the blastoderm. On 

 the surface of this appears a thickening the ventral plate and 

 along the thickening is formed a groove which may perhaps repre- 



