378 PROCEEDINGS OF THE ACADEMY OF [May, 



the dilatation in the case of the first, near the dilatation of the second, 

 and only across the ends of the slits in the last. Hence the deeper 

 the opening the nearer the section passed to the dilatation. Thus 

 the internal cavity of a slit may be represented by the diagram (fig. 4), 

 which shows it to have the form of a flattened funnel with a central 

 enlargement. At least three distinct layers are discernible in the 

 cuticula, but they are not shown in fig. 1. 



The hypodermis (hyp.), usually twice as thick as the cuticula, lies 

 just beneath the latter. Its long cylindrical cells stand at right angles 

 to its outer membrane (o. m.) and to the basement membrane (b. m.). 

 The outer membrane is very thin and never shows any nuclei, while 

 the basement membrane is much thicker and reveals several elongate 

 nuclei (b. m. n.). Directly beneath and on either side near the organ 

 these hypodermal cells appear to be vacuolated at their bases, while 

 at some distance from the organ they are not vacuolated. With Dela- 

 field's hematoxylin and eosin the cell walls and cytoplasm stain very 

 faintly, but the nuclei stand out very conspicuously, and nucleoli 

 are very numerous. There are always just as many sense cells (s. c.) 

 as there are slits in the organ ; if the slits are few in number the sense 

 cells lie at the base of the hypodermis directly beneath the organ. If 

 the slits are many then there is not enough space just beneath the 

 organ, so the sense cells are scattered along the basement membrane 

 of the hypodermis toward the nerve as seen in the drawing. Each 

 sense cell is always as large and sometimes three times as large as any 

 hypodermal cell. It is invariably spindle-shaped with one of its 

 poles running to the nerve of the leg and the other pole connecting 

 with the dilatation (di.) of its respective slit. The nuclei are usually 

 two or three times as large as those of the hypodermal cells and stain 

 slightly darker, although sometimes both kinds of nuclei have the 

 same dimensions and degree of staining capacity when it is difficult 

 to distinguish the two kinds of cells unless one can readily see their 

 walls. Moreover, most sense cells have at least at one end of their 

 nuclei aggregations (ag. cyt.) of dense staining cytoplasm, which does 

 not occur in the hypodermal cells. Also each sense cell has at least 

 one very large nucleolus besides the numerous smaller ones and its 

 cytoplasm stains slightly darker. Therefore, when we consider all 

 these differences, we see that the sense cells are different from ordinary 

 hypodermal cells, but since in many cases these differences are very 

 difficult to distinguish, as with those small sense cells just beneath 

 the organ in the drawing, we must conclude that sense cells are nothing- 

 more than modified hypodermal cells which have taken on a different 



