FOLSOM: MOUTH-PARTS OF ORCHESELLA CINCTA. 13 



plane. The ventral portion of the tentorium consists of a thin frontal 

 plate (Figs. 5 and 7, la./.), to the anterior margin of which are attached 

 the ventral dilators of the pharynx (Figs. 3, 6, and 7), and under which 

 may be seen certain of the muscles which adduct the mandibles (Fig. 7, 

 add. md.). From the frontal plate diverge two anterior arms (Figs. 5 

 and 6, Plate 2, Fig. 10, br. a.), which pass forward and downward, and 

 become united with the paraglossse (Plate 3, Fig. 22, br. a.). The 

 anterior arms bow outwards, and serve for the origin of the protrusors of 

 the mandibles (Plate 2, Fig. 14, prH.). A second, or dorsal, pair of arms 

 (Fig. 5, br. d.) diverge from the tentorium, and extend upwards on 

 either side of the supra-cesophageal ganglion to the skull. Each dorsal 

 arm is differentiated into two parts : a short proximal projection, which 

 is part of the tentorium proper, and a long distal strand, less chitinized 

  than the tentorium proper and distinctly fibrous in nature. As the 

 strand shows no trace of cross striation and is chitinous, it can hardly be 

 regarded as a muscle, but may be called a ligament. The third pair of 

 arras project behind the tentorium (Figs. 5 and 6, br. p.). Each pos- 

 terior arm curves downward, as well as outward,, and consists distally of 

 a ligament such as just described for the dorsal arms. The ligaments 

 not only become continuous with the body of the tentorium, but also 

 are securely attached to the heels (ex.) of the chitinous legs which 

 support the ligula (Fig. 6). 



The responses of the tentorium to stains and to potash prove it to 

 possess three degrees of chitinization, the ligaments being least chitinous, 

 the anterior arms strongly so, and the body of the tentorium inter- 

 mediate in this respect. In preparations rendered transparent with 

 potassic hydrate, whether subsequently stained with safranin or not, no 

 trace of the tentorium is to be seen, except the anterior arms (Plate 2, 

 Fig. 10, br. a.) attached to the paraglossse. When the tentorium is 

 intact, the union of these arms with the rest of the endoskeleton is dis- 

 tinctly indicated by two curving sutures (Fig. 6, sut.). 



The mass of muscles originating in the tentorium is at first bewilder- 

 ing ; it is, nevertheless, possible to trace each muscle to its insertion, or, 

 better, vice versa, and to infer its function. Having done this, I find 

 no muscles which might protrude or retract the tentorium as Meinert 

 ('65) claims for Japyx. This author (Meinert, '67, p. 367) says, 

 "The opposite ends of the flexors of the mandibles, as well as of 

 their tensors, in Japyx are attached to a chitinous plate situated be- 

 tween the mandibles, and steadied by a double set of muscles." The 

 author figures the muscles referred to, and describes them as steadying, 



