BY VERA IBWIN-SMITH. 



427 



sharply-pomted lateral plates (Z.p.). These plates, although firmly united with 

 the median process, in such a way as to allow of no separate movement, can be 

 broken away from it fairly readily, when the pai-ts are dissected out. From 

 their hard, strong ai)iees they extend downward in a deep curve, to unite in 

 the ventral median line in a structure of much softer composition, which forms 

 the ventral Ixjuudury of the oral aperture. Its edge is fringed with several rows 

 of soft processes (Text-fig. 4). 



Immediately below these processes, there is, in the cuticle, a group of 

 what appear, to be the openings of ducts. A pair of these is situated close to 

 the centre line, with two more on each side a little furtlier. back (Text-fig. 6). 



The lateral plates are hollowed out to form sheaths, or sockets, for the 

 two movable mouth jaws (Text-figs. 5, !l). These jaws, although very minute, 

 less than a tenth of a millimetre long, have a very complicated and interesting 

 structure. In form they are somewhat conical, having a broad oval base (Text- 

 fiu'. 10) and a pointed apex. The apex consists of a single, relatively large 

 (0.026 mm. long) tooth of dense chitin, which rests on, and articulates with 

 the chitinous internal framework which strengthens the liack part of tlie jaw 

 (Text-flg. 11). This framework serves, also, as the point of attachment for the 

 muscles which move the tooth. With the exception of these chitin structures, 

 and another ciiitin mass at the base, the entire jaw is composed of several 

 distinctly marked off areas of clear, colourless composition. 



Below the tooth is a soft, rounded, cushion-like swelling, having on its 

 outer face two small, oval patches, which ajipear to be of the nature of sensory 

 papillae. A di'e]i gi'oovc separates this swelling from a phitc wliicli l)pnrs on 



Text-figures 7-13. 

 7. Inner edges of the two jaws, (.x 280). 8. Denticles of jaw. (x 320). 9. Side view of 

 entire jaw. (x 280). 10. Transverse section through lower part of jaws, (x 280). 



11. Apical tooth of jaw, showing attachments of muscles which move it, (x 200). 



12. Oral termiuation of pliarynx, showing the pillars which support the jaws, (x 120). 



13. Attachments of muscles which move the whole jaw. (x 120). 



its outer face from five to seven transverse rows of minute denticles (Text-figs. 

 7, 8, 9). The denticle plate extends to that part of the jaw which is nearest 

 to tiie opposite jaw, but does not continue round its inner face; so that the 

 denticles of the two jaws are hardly opposable. On the back of the jaw, 

 where it moves in the socket of the lateral plate, the cuticle is raised into a 

 series of serrated ridges (Text-fig. 9). 



