Physiology of some Zoophytes. 389 
stalk of a Pedicellina echinata, and press it aside. The tooth-like 
process (fig. 7 c) is hollow, has an aperture in its upper edge, 
and in several specimens I have observed it filled with a fibrous 
contractile substance which expands and rises upwards through 
the aperture, and after remaining stationary for a time it re-enters 
the process. It rises only a short distance above the aperture, 
and when expanded presents the appearance of the upper and 
outer angle of the containing process with the curve turned in 
the opposite direction. When expanding it moves from without 
inwards, gradually rising above the edge of the aperture, and it 
re-enters the process by a sudden jerk in the opposite direction. 
These movements of expansion and contraction commonly occur 
after long intervals, and it is in general only by watching a portion 
of the polypidom for a considerable time under the microscope that 
they can be detected. More rarely these movements occur in ra- 
pid succession. I can form no conjecture regarding the function 
of this curious contractile substance. At the root of the process 
bearing the hair-hke prolongation there is a small rounded ca- 
vity with an aperture in its posterior wall, exactly like that de- 
scribed in the corresponding position in C. reptans (fig. 7a). Each 
cell has four small hollow spies attached to its upper edge, two 
adhering to each angle. These spines are very considerably 
smaller than those in C, reptans, and in old specimens are gene- 
rally broken off. The position of the aperture in the cell through 
which the polype protrudes is similar to that in C. reptans, and is 
also provided with a short flexible tube, which acts as an operculum 
when the polype retires within its cell. Many specimens are pro- 
vided with ovary-capsules placed as in C. reptans. The polype 
has generally twelve tentacula of a light orange-colour, and has 
in other respects a great resemblance to that in C. reptans, and is 
provided with the same muscular bundles for effecting its move- 
ments and closing the operculum. 
Cellularia avicularis. 1 lately obtained a large and very per- 
fect specimen of this polype. The shape of the polype-cell, as 
Dr. Johnston remarks, is similar to that in Flustra avicularis. 
The bird-process is also exactly alike in both. It can, however, be 
readily distinguished from the latter by all the branches being 
composed of two rows of semi-alternate cells, and each cell having 
only two conical spines directed upwards or in the line of the 
long axis of the cells, and a little outwards and forwards, and at- 
tached to the angles of the superior margin of the cell. Ina 
small number of cells an additional small spine, making three in 
all, projected from the outer angle in the same direction as the 
normal one. On the other hand, almost all the cells in Plustra 
avicularis have four spines, which differ in appearance from those 
of Cellularia avicularis. This specimen when dried assumed 
