NOTES ON SOME AQUATIC OLIGOCHATA. 191 
state; then narrows, as Claparéde stated, before dilating to 
form the “head.” 
This “head” is flattened from in front backwards in the 
natural position of the cheta as seen in a living worm, and 
expanded laterally; it is not cup-shaped: this appearance is 
an optical illusion. When seen from in front this head is seen 
to be formed by seven or eight blunt teeth ; and each is slightly 
curved forwards, so that the free edge of each *‘ tooth” is marked 
by athicker line. The “ teeth ” appear at first sight to be sepa- 
rated from one another by very fine spaces, but I believe these 
spaces are in reality occupied by an extremely thin trans- 
parent membrane, as in Psammoryctes and Tubifex. 
I was at first inclined to regard the “ teeth” as separate 
from one another; but, as Professor Lankester remarked to 
me, if this were so, the lines between the “‘ teeth ” would ap- 
pear much more pronounced, owing to refraction along the 
edges, than is the case. 
And further observation served to assure me that there is 
such a membrane; it can be distinguished by the use of Zeiss’s 
homogeneous immersion, with compensating eye-piece 4. More- 
over if the chetz, during the movements of the bundles to 
and fro, be looked at from above (with a Zeiss’ EK, and 
No. 2 eye-piece), so that the free edge only of the structure 
be seen in focus, we get a curved beaded appearance (fig. 7) ; 
the thicker parts are the “ teeth,” the thinner are the inter- 
vening membranes. This interpretation is further confirmed 
by a very lucky find of an abnormal variety of the cheta, in 
which the ‘‘ teeth” are scarcely differentiated, and we have a 
continuous membrane with extremely faint lines across it 
(fig. 9). This is not a young cheta, but occurred in middle of 
a bundle of palmate chetz, and was of same size as these. 
The whole of the ‘‘ head ” is curved at its sides, as shown in 
fig. 7, and at the same time is bent forwards, as in fig. 6. 
Hence the whole ‘“‘ head” can never be in focus at one and the 
same time under a high power. This leads to the appearance 
represented in fig. 5, where the two most laterally placed or 
outer teeth or ridges are in focus, whilst the remainder are 
