12 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



immediately above the depressors of the labrum. The more dorsal pair 

 pass directly backward, and are inserted side by side on the anterior 

 wall of the pharynx (Plate 4, Fig. 29, dil. phy.). The other pair are 

 similar, but longer, and are inserted considerably below the preceding. 

 These four muscles evidently pull upon the toothed epipharyns, and 

 serve to withdraw it from the teeth of the paraglossae. On the dorsal 

 side of the anterior part of the oesophagus are seven pairs of long 

 slender muscles which originate on the front, and are affixed to the 

 chitinous intima of the oesophagus (Fig. 3, dil. ce.). The members of 

 each pair are widely separated in origin, but converge as they approach 

 the cesoj)hagus, penetrate between the circular muscles and epithelial 

 cells, and are inserted dorso-laterally on the intima of the oesophagus 

 by means of short spreading tendons. The posterior three muscles of 

 either side unite to form a single head. Opposed to these dorsal muscles 

 are four pairs on the ventral side of the oesophagus, which have a 

 common tendinous origin on the anterior margin of the tentorium, and 

 run forward under the oesophagus, to which they are affixed ventro- 

 laterally in the same manner as the dorsal muscles. The function of 

 these dorsal and ventral muscles is manifestly to enlarge the gullet ; 

 thus they are antagonistic to the circular muscles previously de- 

 scribed. 



Before describing the mouth-parts, it is best to consider an endo- 

 skeletal structure which is intimately concerned with them. 



Tentorium. 



The tentorium of Collembola has never been described ; to dissect it 

 out is extremely difficult ; in potash preparations it is partially de- 

 stroyed, and it is not easy, owing to its form, to make serial sections of 

 it which will permit of accurate reconstruction. The failure to recog- 

 nize the tentorium of Collembola as being the place of origin of the 

 principal cephalic muscles, and homologous with the same structure in 

 Orthoptera and other mandibulate insects, has led students to assign an 

 altogether undue importance to the " Stiitzapparat " of the ligula, which 

 has erroneously been regarded as a sort of substitute for a tentorium. 

 Partly as a result of this error, systematists have acquired an exagger- 

 ated opinion of the differences which separate Collembola and Thysanura 

 from insects of other orders. 



The tentorium (Figs. 5 and 6) is a chitinized structure in the middle 

 of the head, underlying the oesophagus, extending upward on either side 

 of it, and held in place by three pairs of arms diverging from the median 



