ENTOGNATHY IN APTERYGOTES — TUXEN 399 



tion of its own (fig. 4). In Protura also two "dorsal arms" are given 

 off from the coalesced middle part ; from their position they seem to 

 support the two large basal muscles of the maxillae, though I have 

 not been able to follow this in detail. They were hitherto found only 

 in Acerentomon, 



As already pointed out, the fulcrum in Diplura and Protura de- 

 velops later than the mouth parts and, in Diplura, the hypopharynx ; in 

 Collembola, however, it would seem from Folsom (1900, fig. 25) that 

 they have already attained their connection with the hypopharynx in 

 stage 7, i.e., before hatching. 



A true tentorium is not found in any of the three groups. In Col- 

 lembola Denis (1928) described a very elaborate structure which he 

 calls tentorium, but which, being mesodermal, cannot be compared 

 with the tentorium of Thysanura and Pterygota. Snodgrass (1951, 

 p. 87) points out that "it has the appearance of being a tissue com- 

 parable to the endosternum of Arachnida." It has been termed "archi- 

 tentorium" by Hansen (1930), and Paclt (1956b) follows him, but 

 this term also is unhappy, the structure being no forerunner of a 

 tentorium. I propose to call it endosternum. A similar endosternum is 

 present in Campodca (see Nassonow, 1887), though I have not been 

 able to trace it in my slides. The ending of some muscles, however, 

 shows its presence (fig. 15). And also in Protura I suppose an endo- 

 sternum to be present, though I have never seen it. 



4. THE MANDIBLES 



The mandibles in Diplura are hollow at the inner and upper side 

 and for nearly four-fifths of their length apart from the teeth. One- 

 fifth of the distance from the proximal end a sclerotized rod bridges 

 this cavity. In Campodea a separate prostheca is found, already 

 seen by Meinert (1865), to whose beautiful drawings the student 

 should still be referred ; a long ligamentous cord connects this pros- 

 theca with two muscles at the hind part of the head (fig. 15, &). A 

 prostheca is absent in Japyx. The proximal end of the mandible lies 

 free in the gnathal pouch, no supporting rods being found. 



Among the muscles the following may be pointed out (fig. 15) : 

 The three muscles c and d run from the roof of the head to the dorsal 

 side of the mandibles retracting them. Two muscles, /, from the dor- 

 sal side of the mandible to the endosternum are placed dorsal to 

 the ligament of the prostheca. An oblique muscle, g, runs from the 

 underside of the bridging rod to the endosternum, below the said liga- 

 ment. Proximal to / the two muscles e and i run from the dorsal 



