362 PHYLUM ARTHROPODA. 
mostly generalised types, prophetic of rather than referable 
to the modern orders. 
As to the pedigree of insects, the wingless Collembola 
and Thysanura are doubtless primitive. In /votopteron, 
for instance, there are appendages on the first four seg- 
ments of the abdomen, and the genital apertures are paired. 
Similarly, Aceventomon is a little blind creature, without 
antenne, without cerci, without stigmata, with suctorial 
mouth-parts, with eleven abdominal segments, with a 
peculiar anal segment, with an unpaired genital aperture 
Fic. 190.—Acerentomon, a very primitive insect. 
H, Head; TH.1, TH.?, TH.%, terga of thoracic segments ; 7, 2, 7, the 
thoracic legs; 4.1, 4.2, 4.3, A.4, abdominal appendages; P.A., 
eighth abdominal tergum ; G., genital aperture ; AP.P., post-anal 
appendix, 
on the eleventh urosternite. For Acerentomon, Acerentulus, 
and Zosentomon (with stigmata) the special order Protura 
has been proposed. These and similar primitive forms 
lead us back to some of the less specialised Myriopods 
(e.g. Scolopendrella), back further to the level represented 
by Leripatus, which helps to link the Tracheate to the 
Annelid series. 
But though the primitive wingless insects, the simple 
types of Myriopods, and /erpatus, represent ascending 
steps in evolution, what the actual path has been we do 
not know. 
