16 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



respect like those in the periphery of the brain, within the mandibles 

 (Plate 2, Fig. 17) and elsewhere. A delicate nerve fibre from each 

 ganglion cell penetrates between the glandular cells and appears to 

 accompany the filiform processes of the latter as far as the cuticula. 



Mandibles. 



In general form, the chitinous skeleton of either mandible (Plate 2, 

 Fig. 10, md.) is a modified, elongated hollow cone, a cross section of the 

 least modified part being almost circular (Plate 4, Fig. 31, md.). The 

 specialized regions consist of an anterior or dental portion, and a posterior 

 portion, named by Tullberg the fulcrum, for articulation and muscular 

 insertions. The apex of the mandible bears several sharp, incisive teeth 

 on its median side (Plate 2, Figs. 10, 11), invariably five on the right 

 mandible and four on the left in the many cases I have observed. Behind 

 the apex, also on the median side, is an extensive convex molar surface 

 (Figs. 10, 12) composed of minute raised teeth arranged in quincunx. 

 This denticulated molar surface is bounded ventrally by a row of several 

 large rounded conical teeth (de. v.). On the posterior end of the molar 

 face is a single blunt tooth at right angles to the median plane of the 

 head. At the base of the mandible is a conspicuous triangular medio- 

 ventral opening (Fig. 15, of.) through which the large adductor muscles 

 enter. Near the anterior angle of this aperture, on the median dorsal 

 side is a conical projection (Fig. 10, pr J j. con.), serving for the insertion 

 of a rotating muscle. Other muscles are inserted on the mesal face of a 

 dorsal and oblique basal ridge (Fig. 10, crs. ba.). The extreme base of 

 the mandible, a prolongation of its dorsal wall, is formed into a blunt 

 pivot (Fig. 13, cdx.), upon which the mandible turns. This pivot is 

 peculiar in that it does not form part of an ordinary articulation ; it 

 simply rests freely in a chitinous pocket or stirrup (Fig. 13, sta.), from 

 which it is withdrawn when the mandible is protruded. The chitinous 

 stirrup is formed from the cuticular lining of the cavity in which the 

 mandible lies. The end of the stirrup against which the pivot bears 

 when in motion is thickened, thus offering better resistance (Fig. 13, cht.). 

 The pocket of the stirrup is fashioned from one extremity of an elongated, 

 trough-shaped strap, the other end of which passes through the hypo- 

 dermis and is continuous with the external cuticula of the head. I at 

 first thought that the stirrup was free to swing forward and backward, 

 carrying the mandible with it, but am now convinced that it is fixed 

 in place and supports the mandible only when the latter is retracted. 



