INTERNAL ^N ATOMY. 313 



of the median line, and they are all of nearly equal size, narro\ving from a broad 

 origin to about 0.7 mm. in diameter. 



From the dorsal surface of the bulb arises a strong pair of muscles, the 

 superior protractor muscles which diverge forward and outward to their inser- 

 tion in the antero-dorsal body-wall, above the mouth. 



From the postero-lateral face of the bulb arise the long slender dorso- 

 lateral protractor muscles. These are formed on either side by the union of a 

 broad system of superficial muscles from the postero-lateral region of the pharyn- 

 geal bulb into a narrow band, which passes upward and forward to an insertion 

 in the body-wall at the side of the mouth. 



From a deep origin slightly in front of the middle of the bulb arise the 

 very strong paired ventro-lateral protractor muscles. They pass upward, 

 forward, and outward to the antero-lateral body-wall. 



Plate 8, fig. 5, illustrates the arrangement of the internal structures of the 

 pharyngeal bulb as seen in a dorsal view. The bulb has been slit longitudinally 

 in the median line and the sides reflected. Guarding the opening at the ante- 

 rior end is the pair of lateral mandibles, m, behind these in the median line is 

 the large radula, and on the roof of the cavity, above and behind the radula 

 are the spinous palatal folds, I. f. The structure of each of these will be taken 

 up in detail. 



Mandibles. — The amber colored mandibles are in the form of two strong 

 triangular plates on the sides of the mouth-opening. They measure 7.5 mm. 

 in height and 4.0 mm. in greatest width. The broader basal portions are wide 

 apart, being separated by fully 7.6 mm. while the apices nearly meet in the 

 median line above. Plate 8, fig. 4, exhibits the relations of the mandibles in 

 D. hasseltii as seen in a transverse section of the pharyngeal bulb. The plane 

 of the section passes somewhat behind the tip of the mandibles, in about the 

 region indicated by the dotted line m, (Plate 8, fig. 5). This causes the tips 

 of the mandibles to seem much more separated than they really are. The 

 anterior edge of each mandible is frayed and broken through use, wliile the 

 posterior border disappears in a deep sulcus of the wall, in which the mandible 

 is developed. This is shown at g Plate 8, fig. 4, and in detail Plate 7, fig. 1. 

 The cuticle (Plate 8, fig. 4) is everjTvhere much thicker than the underlying 

 epithelium which produced it, as shown by the narrow shaded basal band, 

 representing the latter, in comparison with the clearer strip above it. In the 

 region of the mandible the cuticle becomes much increased in thickness and is 

 differentiated into rodlets. These are closely packed structures of a sUghtly 



