XXVI 
INTEODUCTION. 
eye, supposing these to he reversed ; the convexity being 
inwards. Each is an oval depression, with a transverse 
slit across the middle. When closed, this slit may some- 
times be discerned merely as a dark line (Plate XI. fig. 5, m), 
the optical expression of the contact of the two edges ; but, 
when slightly opened (Z), a brilliant line of light allows the 
passage of the rays from the lamp to the beholder. From 
this condition the lids may separate in various degrees, 
until they are retracted to the margin of the oval pit, and 
the whole orifice is open (k). 
The dimensions of the cinclides vary not only with the 
species, and probably also with the size of the individual, 
but with the state of the muscular contraction of the integu- 
ments, and, as I think, with the pleasure of the animal. 
In a small specimen of S. dianthiis, I found the width of 
a cinclis, measured transversely, ^th of an inch ; but that 
of another, in the same animal, was more than twice as 
great, viz. yg^th of an inch. This was on the thickened 
marginal ring, or parapet, Avhich in this species surrounds 
the tentacles, where the cinclides are larger than elsewhere. 
Watching a specimen of S. nivea under the microscope, 
I saw a cinch’s begin to open, and gradually expand till it 
was almost circular in outline, and absth of an inch in 
diameter. I slightly touched the animal, and it in an 
instant enlarged the aperture to y^th of an inch. In a 
specimen of S. bellis, less than half grown, I found the 
cinclides numerous, and sufficiently easy of detection, but 
rather less defined than in dianthusov nivea. They occurred 
at about every fourth intersept, three intersepts being blind 
for each perforate one, and about three or four in linear 
series, but not quite regularly, in either of these respects. 
In this case they were about shth of an inch in transverse 
diameter, a large size, — and I measured one which was 
even -ghtli of an inch. By bringing the animal before the 
window, I could discern the light through the tiny orifices 
with my naked eye. 
From several good observations, and especially from 
one on a cinclis, widely opened, that happened to be close 
to the edge of the parapet of a diantJms, I perceived that 
the passage is not absolutely open, at least in ordinary, but 
that an excessively thin film lies across it. By delicate 
focusing, I have detected repeatedly, in different degrees 
of expansion, and even at the widest, the granulations of a 
