202 LANGDON. [Vol. XL 



of these walls. The dorsal wall curves evenly ventrad into the 

 buccal cavity, while the ventral wall is much convoluted longitu- 

 dinally. The sense-organs in the first part of the buccal cavity 

 are shorter and of greater average width as compared with 

 their height than those of the epidermis, about 'jGii high and 

 from 50 to 6o;U' wide ; toward the caudal end of this cavity they 

 diminish in size, and become only 20\x high and \o\x wide. 

 The sense-hairs in the first half of the buccal cavity are from 

 4 to 6/x long, over twice as long as those of the epidermal 

 sense-organs. The cuticula over a sense-organ in the buccal 

 cavity is no thinner than that covering the surrounding cells, 

 and the summit of a sense-organ does not project above these 

 cells. Sometimes the cuticula passes evenly over the sense- 

 organ from the surrounding cells; more often, however, it is 

 indented around the sense-organ so that the summit of the 

 latter really projects into an elevated area of cuticula, but this 

 area is not elevated above the general cuticular level. The 

 base of the sense-organ is often rounded and then presses the 

 basement-membrane, which is very thin under the epithelium, 

 out among the muscle-fibres. The sense-cells of the organs in 

 the cephalic part of the buccal cavity do not differ from those 

 of the sense-organs in the epidermis. In the caudal part, they 

 are comparatively wider and have a more rounded nucleus. 



Even in the cephalic part, the sense-cells and also the epithe- 

 lium cells seem less crowded, and the former have larger spaces 

 between them than in the epidermis. This may be due to the 

 absence of the gland-cells from the epithelium. In these clear 

 spaces between the sense-cells one often sees delicate fibres. 

 These are sometimes cut walls of cells, but usually prove to be 

 the very slender bases of some sense-cells. It was suggested' 

 in the description of the sense-organs of the epidermis, that 

 the varying height of the nuclei of the sense-cells was merely 

 due to the necessity of these cells adapting themselves to the 

 space they were in. This receives confirmation from the posi- 

 tion of the nuclei of the sense-cells in the buccal cavity. In 

 this region, where the sense-cells have more room, all nuclei 

 are generally found below the middle height of the sense- 

 organs (PI. XIII, Fig. 2). 



