442 R. I. POCOCK. 



virtue of the development of a poisou gland, accompanied 

 by an increase in tlie thickness of its segments, practically 

 the sole function of prehension was taken over by those of 

 the second pair. 



Sub-class Pleurostigma. 



Chilopods resembling the primitive type in the possession 

 of a system of tracheal tubes, the orifices of which open upon 

 the pleural area of more or fewer of the somites ; and in 

 the presence of a distinct tergum and sternum on each leg- 

 bearing somite, the number of sterna never exceeding that 

 of the terga. Eyes are either preserved or lost; when 

 preserved they are monomeniscous, and variable in number 

 on each side from one to about forty. Correlated with the 

 relative simplicity in the structure of the metameres just 

 mentioned is a mai-ked specialisation of the head and the 

 appendages connected therewith, a specialisation which was 

 initiated apparently by the retroversion of the prasanteunal 

 area of the cephalite resulting in the projection of the 

 antennae from the fore-margin of the head thus formed, and 

 in the backward movement of the mouth with the gnathites 

 of the first and second pairs into more intimate connection 

 with the palpognaths and toxicognaths. As concomitants 

 of this process the last-named appendages shortened and 

 thickened, were axially rotated so as to move and fold in a 

 horizontal plane, one segment of the palpognaths was sup- 

 pressed, and the basal segments of the toxicognaths became 

 firmly fused and incapable of independent movement ; and 

 lastly, the gnathites of the second pair being underlain by 

 the two pairs that succeed them, failed to develop or lost 

 the sense-organ which is found in the other sub-class. 



'&' 



Order 1. — Geophilomorpha. 



Chilopods retaining a large and indefinite number of meta- 

 meres in response to the demands of a subterranean existence 



