FOLSOM: MOUTH-PARTS OF ORCHESELLA CIXCTA. 15 



teeth of the epipharynx are directed towards those of the paraglossae, 



ia conjunction with which they appear to hold the food (Plate 4, 

 Fig. 30). 



The only muscles within the labrum are dilators of the pharynx 

 (already described) and depressors of the labrum. The latter consist 

 of a pair of muscles (Plate 1, Fig. 3, dep.), which originate on the lower 

 margin of the clypeus in paramedian positions and converge downward 

 toward the place of insertion, which is a chitinous ledge or shelf project- 

 ing: inward from the anterior wall of the labrum. The contraction of 

 these muscles doubtless closes the upper lip. I find no muscles which 

 could conceivably elevate the labrum in opening the mouth. This being 

 the case, the most satisfactory alternative which suggests itself is to 

 assume that the external cuticula, which is bent like the letter S at the 

 clypeo-labral suture (Plate 1, Fig. 3), possesses an elasticity sufficient to 

 raise the labrum when the depressors are relaxed. 



The labrum is supplied by a pair of short nerves, which originate from 

 the oesophageal commissures where the latter merge into the supra- 

 cesophageal ganglion. The nerves soon ramify and become distributed 

 between the hypodermal cells of the epipharynx and other regions. The 

 labrum is lined with a single layer of deeply pigmented hypodermis 

 cells with moderately large round nuclei (Plate 1, Fig. 8, h'drm.). Near 

 the base of the labrum and surrounding its central lumen, or body cavity, 

 are grouped large oval nuclei (Fig. 8) ; each nucleus occupies the base 

 of a filiform cell, which may often be traced directly to the base of one 

 of the stout seta? (set. sns.) which cover the exterior of the upper lip. 

 Sommer ('85, p. 703) regarded these in Tomocerus as sensory bristles, 

 and the large oval nuclei as belonging to ganglion cells, although he 

 gave little attention to the subject : " Was die Sinnesorgane betrifft, so 

 muss ich mich darauf beschrauken, dass sich eigenthumlich gestaltete 

 Borsten, welche ich als Sinnesborsten bezeichne, an den Beinen, den 

 Palpen, so wie der Ober- und Unterlippe vorfinden . . . sie stehen, wie 

 ich das an denjenigen der Oberlippe direkt nachweisen konnte, mit Ner- 

 venfiiden in Verbindung, welche aus einen Haufen von Nervenzellen 

 hervortreten." 



The so called " neiwe-cells " have no direct connection with the central 

 nervous system, however ; they do adjoin a network of connective tissue 

 (Fig. 8). I am disposed to consider the filiform cells as glandular in 

 function, since they probably serve to produce the chitin of the sensory 

 bristles. Deeper than the zone of large oval nuclei may be seen true 

 ganglion cells, the nuclei of which are small and round, and in every 



