TYPE TRACHEATA. 481 



just as many chambers and pairs of alar muscles as there are 

 trunk-segments. The number of stigmata vary, in some 

 forms only a single pair occurring, while in others there is a 

 pair on each segment of the trunk; and the form of the 

 tracheae varies also, as they are sometimes branched and 

 sometimes arranged in bunches composed of a number of un- 

 brauched tubes. 



The digestive tract is almost always a straight tube, ex- 

 tending through the body to the terminal anus. The mouth 

 is guarded in front by a well-developed upper lip or labrum, 

 while the fusion of the maxillae behind it in many forms pro- 

 duces a lower lip. It leads into an ectodermal fore-gut, and 

 this into an endodermal mid-gut, which is usually provided 

 with a number of unbranclied diverticula termed hepatic caeca. 

 One or two pairs of Malpighian tubules open into the ante- 

 rior end of the ectodermal rectum, and serve as excretory 

 organs. 



The nervous system except in the head region shows but 

 little trace of concentration, there being as a rule in each 

 segment of the trunk a pair of ganglia. The antennal ganglia 

 are fused with the supracesophageal gaugliouic mass which 

 sends off branches to the ocelli ; these may be quite numer- 

 ous, though compound eyes do not as a rule occur. A sympa- 

 thetic system is present as in other Tracheates. 



There are no nephridia so far as known in the group, the 

 excretion being performed by the Malpighiau tubules. The 

 reproductive organs are paired, and open to the exterior in 

 some cases by paired orifices, but more usually by a single 

 opening, which may be situated either far forwards, or else 

 in other cases near the posterior extremity of the body. 



1. Order Pauropoda. 



The order Pauropoda contains a few small forms in which 

 the trunk possesses but ten metameres and nine pairs of ap- 

 pendages, which, with the exception of the first pair, are six- 

 jointed and terminate in a claw. In some species, when viewed 

 from the dorsal surface, the segments appear to be less nu- 

 merous than the appendages, a condition which results from 



