508 
to add some details to our previous knowledge. For the sake of 
clearness of description, I have considered the sensory cells as existing 
in three conditions, isolated, loosely grouped and gathered into sense 
organs. All intermediate conditions appear to exist between the 
isolated cell and the sense organ, so that my recognition of three 
conditions with respect to the relation of the sense cells, is largely 
formal. If my descriptions are correct, we have, in this form, stages 
connecting the isolated sense cell and the sense organ of the usual 
type. These may be genetic stages, or, on the other hand, the cell 
groups may owe their origin to modifications (degeneration?) of sense 
organs, arising from the tubiculous habit of the animal. The existence 
of isolated sense cells and of loose groups of sense cells over the 
entire body, and the restriction of the sense organs to the anterior 
end, probably indicates a difference in function between the sense 
cells found isolated, and those found in the organs. This difference 
might be experimently determined. 
Physiologically the epidermis, regarded as a sensory structure, 
presents three well marked regions: the middle part of the body, 
constantly within the tube; the posterior region, generally above the 
mud in the water and light, but with-drawn, presumably into the 
tube, at the slightest alarm; and the head, protruded from the opposite 
end of the tube into the mud, in the absence of light. As before 
suggested, experimental study is neccessary to determine whether 
there is a difference in reaction to stimuli from these three portions 
of the body. In addition to organs of extreme sensitiveness to mechanical 
stimuli, we should expect to find in the epidermis of the head region 
those of a gustatory sense. Presumably the sense organs described 
in this paper are gustatory, but the question should be investigated 
experimentally. 
I have not detected any arrangement of the sensory structures of 
Tubifex, either in longitudinal rows, as have been described by 
WHITMAN, and others, for the leeches; or into girdles about the body, 
as found by Miss Lanapon in Lumbricus; and BRODE, in Dero. 
Such an arrangment may, nevertheless, exist, and have remained 
undetected by the methods I have found it thus far possible to use. 
Summary. 
1) The epidermis consists of a single layer of cells, except at 
the caudal end, where a scattering basal layer is present. 
2) All the cells of the epidermis appear to arise in the growing 
zone at the caudal end. 
