ANNULOSA: MYRIAPODA. 



231 



Fig. 78. Centipede 

 (Scolopendra) reduced. 



and Arachnida. The head is invariably distinct, and there 

 is no marked line of demarcation between the segments 

 of the thorax and those of the abdomen. 

 The body always consists of more than 

 twenty somites, and those which corre- 

 spond to the abdomen in the Arachnida and 

 Insecta are always provided with locomotive 

 limbs. " The head consists of at least five, 

 and probably of six, coalescent and modified 

 somites, and some of the anterior segments 

 of the body are, in many genera, coales- 

 cent, and have their appendages specially 

 modified to subserve prehension." (Hux- 

 ley.) 



The respiratory organs, with one ex- 

 ception, agree with those of the Insecta 

 and of many of the Arachnida in being 

 "tracheae" that is to say, ramified tubes, 

 which open upon the surface of the body 

 by minute apertures, or " stigmata," and 

 the walls of which are strengthened by a 

 spirally coiled filament of chitine. 



The somites, with the exception of the head and the last 

 abdominal segment, are usually undistinguishable from one 

 another, and each bears a single pair of limbs. In some cases, 

 however, each segment appears to be provided with two pairs 

 of appendages (fig. 79). This is really due to the coalescence 

 of the somites in pairs, each apparent segment being in reality 

 composed of two amalgamated somites. This is shown, not 

 only by the bigeminal limbs, but also by the arrangement of the 

 stigmata, which in the normal forms occur on every alternate 

 ring only, whereas in these aberrant forms they are found upon 

 every ring. 



The head always bears a pair of jointed antennae, resembling 

 those of many Insects, and behind the antennae there is gene- 

 rally a variable number of simple sessile eyes. 



The young, in some cases, on escaping from the egg, possess 

 nearly all the characters of the parents, except that the number 

 of somites, and consequently of limbs, is always less, and 

 increases at every change of skin ("moult" or "ecdysis"). 

 In other cases, there is a species of metamorphosis, the embryo 

 being at first either devoid of locomotive appendages, or pos- 

 sessed of no more than three pairs of legs, thus resembling 

 the true hexapod Insects. In these cases the number of legs 

 proper to the adult is not obtained until after several moults, 



