ENTOGNATIIY IN APTERYGOTES — TUXEN 4II 



From this it is seen that the general plan of the maxilla in the three 

 groups is very much alike; especially the shape and position of the 

 cardo are identical. The palpus is greatly reduced in most Collembola 

 and without muscle, which, however, is present in Diplura and Pro- 

 tura. Also the galea is much reduced in Collembola. In all three 

 groups the lacinia carries an arm or platelike process to which one 

 or more muscles are attached, running to the stipes, to the hind wall 

 of the head, or to both. And in all three groups powerful muscles 

 connect the stipes and the distal part of the cardo, as it seems, to the 

 fulcrum. In Diplura and Collembola some dorsal muscles from the 

 stipes join the corresponding ones from the opposite side below the 

 oesophagus ; this is not the case in Protura, the maxillae of which are 

 extensively modified into piercing organs. 



6. CONCLUSIONS 



It is my hope that this comparison between selected types of the 

 three groups of entognathous apterygotes will have shown how great 

 is the likeness in their head structures. Where the embryology is 

 known, the entognathy comes about through the formation of two 

 plicae orales uniting at or below the sides of the labium. At the same 

 time a change in the relative position of the mandibles and maxillae 

 may take place, indicating an ingrowth of these mouth parts into the 

 head, in which the mandibles may reach even to the hindmost part. 

 From these changes a prognathous head results from a hypognathous 

 one in so far as the functional mouth — the opening of the atrium — 

 is directed forward, though the real mouth, the opening of the 

 pharynx, is still directed downward. In connection with this prog- 

 nathy stands the fact that the prosternum more and more overgrows 

 the underside of the head, mostly so in Protura, where nearly half 

 of the head is covered. This again, in Protura, involves a displace- 

 ment of the forelegs, which take over the function of the antennae, 

 as the latter become reduced to small organs with unknown function, 

 if any — the pseudoculi. 



A cavity on each side results from the coalescence of the plicae 

 orales with the labium, which latter again, being pushed forward, 

 coalesces more or less with the underside of the head, leaving only 

 the hypopharynx (lingua and superlinguae) free, or the place where 

 the hypopharynx should be if it is lacking (Protura). A fold from 

 the plicae orales may grow in between the mandibles and the maxillae, 

 resulting in two more or less definite cavities on each side, the gnathal 

 pouches. Probably as stififenings in the gnathal pouches, a skeleton is 



