METAZOA SYMPHYLA. 387 



head near the insertion of the antennae, and the tracheae 

 (colored blue in fig. 2) are simple air tubes without spiral 

 threads, which extend only a short distance back into 

 the trunk. 



Besides these organs there are nine or ten pairs of 

 ventral sacs or "blood gills" (colored red in fig. 2), 

 which are situated within the body near the vestigial feet 

 but which can be turned outward as seen in fig. 2. 



Unfortunately, the embryology of Scolopendrella is un- 

 known (Packard, 1898), but it is most probable that the 

 larva has six legs like the Diplopods, and like them de- 

 velops more legs with growth. 



The Symphyla introduces us naturally to the next and 

 last group of invertebrates, the Insecta. 



