No. I.] SENSE-ORGANS OF LUMBRICUS AGRICOLA. 21 3 



Distribution of the Sense-Organs. 



In a surface view of the cuticula the two layers of fibres 

 which form it, the comparatively large openings of the gland- 

 cells, the nephridial openings, the cuticular sacs of the setae, 

 and the intersegmental grooves may be readily identified {PL 

 XIII, Fig. 4, and PI. XIV). Attention is usually first directed 

 to the cuticular elevations over the sense-organs by the absence 

 of the gland-pores over a little area around each elevation. 

 These cuticular elevations themselves appear as irregularly 

 rounded raised spots, which contain numerous openings smaller 

 than the pores of the gland-cells (PI. XIII, Fig. 4). Each one of 

 these openings is the outer end of one of the pore-canals seen in 

 sections, and, normally, a sense-hair protrudes through each. 

 Each opening lies at the intersection of two short straight lines, 

 each of which is a line of contact of two adjacent cuticular 

 fibres. The existence of these two intersecting lines shows 

 that both layers of the cuticular fibres are present over the 

 sense-organs. The pore-canals through which the sense-hairs 

 pass are thus spaces left between the fibres of the cuticular 

 layers at their intersection. That is, the pore-canals are such 

 openings as might be made by taking a layer of fibres, laying 

 a second layer over and at right angles to the first, and then 

 forcing a blunt instrument through both layers between their 

 threads. Whether these openings always exist between the 

 cuticular fibres, or are made by an outward growth of the 

 sense-hairs, has not been determined. Even if they always 

 exist, the presence of the cross-like marking at each opening 

 shows that the sense-hair pushes the fibres still farther apart. 

 When the cuticula is removed, the sense-hairs are usually 

 pulled out of these canals. Occasionally the hairs are torn 

 from their cells and remain projecting above the cuticula. A 

 study of the distribution of the cuticular spots over the sense- 

 organs forms the most ready means of determining the distri- 

 bution of the latter. 



The cuticular spots are found on every metamere of the body, 

 and appear on the clitellum metameres after the clitellum has 

 disappeared, and their size and distribution shows the following 

 facts : 



