506 
by the silver are present in a single cross section 30 uw in thickness. 
At the posterior end, the groups are especially well marked (Fig. 5, 
Nos. 1 and 2), containing three or four cells lying close together, yet 
retaining the characteristics of the grouped condition, as described. 
C. The sense organs are composed of from four to seven sense 
cells in close contact, with no intervening supporting cells (Fig. 5, 
No. 4). The organ, as a whole, is a broadly conical structure, at the 
base 8 «u to 16 « in diameter, and tapering to one-half or one-third 
this size at the upper end. The inner cells of the organ are flask 
shaped; with a round or oval body 2 u to 4 u in thickness, lying at 
the base of the epidermis, and a thick slightly tapering neck, 6 «u to 
8 w in length, which reaches the cuticle. The outline of the body is 
continuous with that of the neck, there being no abrupt shoulder between 
them. The nucleus nearly fills the cell body; and, like the body, 
may show by its flattened or kidney shape the effect of pressure from 
the surrounding cells. The nerve fibre, as far as can be made out 
in the vom Ratu material, issues as a slender process from the base 
of the cell. The outer cells of the organ are thicker than the inner 
cells, and are more or less curved about them. The body of these 
cells is larger than that of the inner cells, and the nucleus is usually 
at a slightly higher level. A flattening of the nucleus, if present, is 
now in a direction at right angles to the surface of the epidermis. 
The nerve fibre from these cells may arise from a flattenned base, 
as in the inner cells, or may be a prolongation of the tapering base; 
the cell in this latter case having the shape of a slightly curved 
spindle. The sense organs are not bounded by specially modified 
supporting cells of the epidermis. 
An elevation of the cuticle forming a large concavity in its lower 
side, into which project the necks of the sense cells, is characteristic 
of the sense organ. In the center of this elevation, within an area 
of 2 u to 3 w in diameter, the sense hairs penetrate the cuticle. 
Over this area in many of the vom RATH preparations, is a shallow 
depression, from the bottom of which issue the hairs. The tactile 
hairs from the cells of a sense organ are slightly more slender, and 
about one-fourth longer than those of either the isolated, or grouped 
sense cells. As far as observed, each cell of an organ bears a 
single hair. 
I have not been able to determine the arrangement upon the body, 
of the organs, the groups of sense cells, or the isolated sense cells; 
and, therefore, cannot say whether there is, in this species of Tubifex, 
a grouping of these into bands or girdles about each segment, as 
