No. I.] SENSE-OIiGAA'S OF LUMBRICUS AGRICOLA. IQ-] 



become indistinct. Not only are the supporting cells more 

 nearly black, but their walls have an uneven appearance, and 

 a cross section shows that the silver is deposited on them ir- 

 regularly and rather thickly. The sense-cells present a smooth 

 appearance, and look as if their walls were really stained. But 

 cross sections of these cells show a delicate unstained wall, 

 which has the same clear, glistening appearance noticed in the 

 nerve-fibre, and, on the outside, a very thin, evenly applied de- 

 posit of the silver. 



Comparatively few of the sense-cells in a given organ are 

 stained. But a study of a thin section under the oil immersion 

 shows plainly the outline and nuclei of the unstained cells (PI. 

 XIII, Figs. 11-13). All the sense-cells, both stained and un- 

 stained, present a plump, rounded form, and taper to both ends 

 from an enlarged part in which the nucleus lies. In short, they 

 are of the same form as the sense-cells described in the sense- 

 organs stained by haematoxylin, and are clearly the same thing. 

 The summits of the sense-cells cannot be traced through the 

 cuticula; the slender, converging peripheral ends of the cells 

 blend into a dark mass, which is continuous with the blackened 

 cuticula. It is impossible to trace a single cell through this 

 mass, but a careful study of the summits of the sense-organs 

 almost always reveals a group of short black or colorless hairs 

 projecting from its surface. The summits of the sense-organs 

 in which the cells were stained are not so elevated as in any 

 other preparations; they appear as if there had been an un- 

 usual contraction of the sense-cells. Other facts support this 

 interpretation. The sense-hairs do not project above the cuti- 

 cula as great a distance as in the unstained organs, and the 

 cells themselves appear more thickened. 



The silver nitrate stain permits the character of the bases of 

 the sense-cells to be clearly seen. The cells which have a nu- 

 cleus near the cuticula extend toward the base of the sense- 

 organ into a slender fibre as in the haematoxylin prepara- 

 tions, and this fibre is a nerve-fibre (PI. XIII, Fig. 15, a and b). 

 Those cells which have their nucleus at the center or near the 

 base of the sense-organs sometimes end abruptly at the base 

 (PI. XIII, Fig. 15, g). In this case a nerve-fibre is attached 



