46 HOWARD AYERS. 
test (figs. 12, 15, 20, 26.) In the living condition it is impos- 
sible to detect the cell boundaries or nuclei of the epithelial 
cells, much less those of the more internal nerve-cells, and the 
presence of cilia is known only by the currents caused by their 
motion. By adding under the cover-glass a drop of chromic 
acetic acid solution! to the seawater containing the spheeridia 
the cell-bounding nuclei are rendered distinctly visible and the 
cilia killed. The latter are long (measuring 14—2 mm. in 
length), slender, perfectly homogeneous filaments. They 
should be studied immediately on the addition of the reagent 
as they curl close up to the cell wall soon after death (fig. 27). 
The reagent causes a slight separation of the cells due to the 
swelling of the cell protoplasm, but otherwise the cells remain 
in excellent histological condition. The cilia do not regularly 
occur distributed over the entire surface of the spheerid, as 
one would infer from Lovén’s figure and description; but are 
usually confined to different sized patches situated on the sides 
of the neck and globule (fig. 20). The current caused by the 
cilia varies with the position of the patch. In Spherechinus 
the patches on the head set in motion a current toward the 
base, while those on the neck cause a current towards the head. 
The epithelium consists of a single layer of relatively large 
cells resting directly on the calcareous body, underlaid, however, 
in the neck region by the muscle and fibrous elements of the 
joint. At the joint it is thickened in the form of a ring 
(sometimes two, one above and one below the joint), (figs. 1, 
15, 20). On isolating the epithelial cells by macerating in 
osmo-acetic acid solution they are seen to be irregular in shape, 
having a rounded outer end, a branching inner part, and not 
unfrequently provided with branches or processes at the sides 
(figs. 22, 25, a.6.) The nuclei are situated in the outer halves 
of the cells and are frequently surrounded with green chroma- 
tophores. In the region of the joint, large, brown or purple 
chromatophores are of frequent occurrence, they nearly fill the 
cell in which they are formed. 
In studying the nervous tissues of the spheridia the best 
1 See Fol, ‘ Lehrbuch d. Vergl. Mikr. Anat., &c.,’ pp. 90—100. 
