CILIATED PITS OF STENOSTOMA 239 
The epithelium of the growing pit is extended as a region which 
is morphologically different from the general epidermis in that 
its cells are lower and are repeatedly dividing to yield additional 
cells to the formation of the ciliated pit-ganglion; also in that the 
cells which line the fundus of the pit have irregularly placed nu- 
clei, and have lost their cilia, while the cilia on the marginal 
cells have become longer than the cilia on the general body 
epithelium. These characters are shown in figures 2 and 3. 
Up until this stage in their development the cells of the pit- 
epithelium retain their power to elaborate rhabdites, as is illus- 
trated in figures 2 and 3. 
As the pit grows larger no rhabdites are to be found in the 
epithelium of its fundus. But before these rhabdites have to- 
tally disappeared theformation of a peculiar body isstarted, which 
in the mature pit is a highly refractive, homogeneous layer, which 
Ott (92)? has called the ‘homogeneous mass.’ 
The nature of this ‘homogeneous mass’ can best be arrived at 
by observation of the specimens during fixing. As. an animal 
lies in contact with the slide, if it be fixed by dropping the fixing 
fluid upon it, it will adhere to the slide on account of the protec- 
tive discharge thrown out by the cells of the general epithelium. 
To avoid such trouble it was necessary to apply the fixing fluid 
with a dash as the animal swam around in a small drop of water 
on the slide. Thus any adhesions which would injure the fixed 
specimen were avoided. The details of this trouble can be plainly 
observed under the binocular microscope. If an entire speci- 
men be treated with methylen blue, a protective blue sheath 
of mucus with imbedded rhabdites (stained deep blue), will be 
seen to be formed around the animal. If this sheath be removed 
from the specimen and the stain again applied the epidermal 
cells fail to respond and the protective sheath is not formed 
the second time. 
Thus it is evident that in an effort to protect itself the epider- 
mis not only discharges rhabdites but also a mucus which stains 
with methylen blue less deeply than the rhabdites. Now, since 
2 Ott; Jour. Morph., vol. 7, 1892. 
