70 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 8l 



fibers, he adds, '' pass under the tentorium and become continuous 

 with similar fibers from the opposite mandible." Folsom, it will be 

 noticed, says the adductor fibers connecting the mandibles pass be- 

 neath the tentorial arms. In Japyx the tendon of the dumb-bell muscle 

 distinctly lies dorsal to the hypopharyngeal apodemes. In Machilis 

 the apodemes are so loosely connected with the base of the hypo- 

 pharynx and so strongly united with the lateral inflections of the head 

 wall, that in dissections their hypopharyngeal connections are easily 

 lost, and the impression is given that the tendon of the dumb-bell 



lAnt 



J-~. 



iMxp 



A 



Fig. 28. — Head of Gammarus Jocusta (amphipod crustacean). 



A, lateral view of head, showing tergal abductors (/) and adductors 

 (/) of left mandible, and base of ventral adductor (KL). B, postero-ventral 

 view of back of head, showing origin of ventral adductors {KL) on posterior 

 tentorial bar (PT). 



I Ant, first antenna ; 2Ant, second antenna ; /, abductor of mandible ; /, dorsal 

 adductor of mandible ; KL, ventral adductor of mandible ; Lm, labrum ; Md, 

 mandible; MdPIp, mandibular palpus; lALr, first maxilla; pMx, second maxilla; 

 iMxp, first maxilliped ; pt, posterior tentorial pit ; PT, transverse posterior 

 tentorial bar. 



adductor lies ventral to the apodemes. It does lie ventral to the sus- 

 pensorial plates uniting the apodemes with the lateral walls of the 

 head, but it passes anterior, i. e., dorsal, to the ends of the apodemes 

 themselves. Folsom's statement, therefore, should be verified, for 

 a discrepancy in the relations of the parts in question seems hardly 

 permissible if we are dealing with homologous structures. 



The mandibles of the Protura, as described by Berlese (1909), are 

 provided each with retractors and protractors that have their origins 

 on the head wall, and with a protractor arising on the tentorial 

 apodeme. Berlese, however, does not mention a muscle continuous 

 between the two mandibles. The muscles present clearly represent 

 the usual tergal muscles, and the hypopharyngeal adductor. 



