286 GENERAL CHARACTERS OF MYRIAPODA. 



being interposed between each pair of firm rings or plates. This 

 is obviously required by the condensation of the rest of the integu- 

 ment ; since, if it were otherwise, the body would not have the 

 power of bending in any direction. The legs are covered with 

 the same kind of integuments, and are jointed in a similar manner; 

 each terminates in a single claw or hook. Sometimes two pairs 

 of legs seem to be present on each segment (Fig. 507) ; but this 

 results from the fact, that two adjacent segments are soldered (as 



it were) together, so 

 as to appear but one, 

 as is made evident 

 by the existence of a 

 similar doubling of all 



FlG. 507- POLYDESMUS. ,, 



the other organs in 



each division, and by the existence of a deep groove which runs 

 across it, marking the line of union. The first segment or head is 

 furnished with a pair of compound eyes, resembling those of Insects, 

 but less complicated in their structure ; and also with a pair of 

 jointed antennae. The mouth is adapted for mastication ; and 

 possesses a pair of mandibles, followed by a lower lip, and by a 

 pair of appendages somewhat resembling the feet-jaws of Crus- 

 tacea. On the side or under surface of the body may be observed 

 a row of minute pores, a pair usually existing on each segment ; 

 these are the stigmata, or apertures for the admission of air to 

 the respiratory organs, which consist of tracheae or air-tubes 

 resembling those of Insects. These tracheae, however, do not so 

 completely unite into a system, as in that class ( 619) ; for 

 those of the several segments have but little communication with 

 each other. The character of the other organs strongly resem- 

 bles that of the similar parts in Insects, though not so highly 

 developed. The alimentary canal runs straight, or nearly so, from 

 one end of the body to the other ; and possesses a few glandular 

 appendages, in the form of a long tube very imperfectly developed. 

 The circulation is carried on by the aid of a long dorsal vessel, 

 which extends along nearly the whole length of the body. The 

 nervous system presents a multiplication in its ganglionic cen- 

 tres, corresponding with the multiplication in the number, and 



