TYPE TRACHEATA. 



483 



head vary in number in different genera from eleven {Glo- 

 meris) to over one hundred, and the number of appendages is 

 much greater still, since the majority of the segments bear 

 two pairs of limbs and in reality represent each two meta- 

 meres. The four or five anterior trunk-segments are, however, 

 single, bearing but a single pair of limbs (Fig- 222), and one 

 of them — in some cases the 

 first, in others the second, but 

 more usually the third (Itdus) — 

 is entirely destitute of appen- 

 dages. The last few segments 

 also carry but a single pair of 

 appendages, as does also the 

 seventh segment in the males, 

 the appendages of which are 

 usually modified to serve as 

 copulatory organs. 



Stigmata occur on each of 

 the trunk-segments, the double 

 segments bearing two pairs, 

 situated on the ventral surface 

 near the coxal joints of the 

 limbs. Each one has in connec- 

 tion with it a bunch of un- 

 branched tracheae, a condition 

 recalling somewhat that of Peri- 

 patus, although the location of 

 the stigmata is much more regular and definite. Upon the 

 dorsal surface of the body there is in most species a row of 

 pores which have been mistaken for stigmata, but are really 

 the openings of glands {gl. repugnatoria) secreting an oily 

 evil-smelling fluid which serves as a means of defence. In 

 the genus Polydesmus the secretion contains hydrocyanic 

 acid. Crural glands do not as a rule occur, but protrusible 

 warts occurring on the coxal joints of a number of legs in 

 some genera {Zysiopetalum) have been regarded as homolo- 

 gous structures. 



The nervous system has the characteristic Myriapodan 

 arrangement, each of the double segments possessing two 



Fig. 232.— Antbeiob, Portion of a 



DiPLOPOD. 



at = antenna. 

 cop = copulatory appendages. 

 mx = maxilla. 



ul = upper lip. [segments. 



1, 2, 3, 4 = the four anterior trunk- 



