ENTOGNATIIY IN Al'TEKYGOTES — TUXEN 



397 



From this it would seem evident that entognathy in the entognath- 

 ous apterygotes is a rather complex business. It results first and 

 foremost from the building on either side of the head between labrum 

 and labium of a plica oralis which coalesces with the labium, forming 

 one or two pouches on each side enclosing the mandibles and maxillae. 

 At the inner edges of these pouches some stiff enings arise, forming the 

 fulcrum, which also carries the hypopharynx if such a structure is 

 present. But besides this the following happens: i, The sternal part 

 of the head coalesces in the middle line with the labium. 2, The man- 

 dibles (in Diplura and Collembola) and the maxillae (in all three 



Fig. 14.— Acercntoinon dodcroi Silv. Head in side view. Mandible closely 

 dotted, maxilla and fulcrum openly dotted. 



groups) become hollow at the side, turning against the interior of the 

 head, and some of their muscles take their origin in these cavities. 



3, The head, which is originally hypognathous, becomes more or less 

 prognathous, the mandibles and maxillae turning from a vertical to a 

 horizontal direction and simultaneously changing more or less their 

 relative positions, the mandibles extending in the Diplura and Pro- 

 tura farther back than the maxillae, reaching even the hindmost part 

 of the head. Whether this change in relative position has any con- 

 nection with a rotation noticed during embryogeny is not known. 



4, The plicae orales, approaching the under side of the head, push the 

 labium, or at least its anterior part, before it to the apex of the head, 

 meeting in the middle line or leaving a smaller or broader remnant 

 of submentum between them. 



The entognathy therefore seems to me to arise not only by way of 



