'Physiology of some Zoophytes. 389 



stalk of a Pedicellina echinata, and press it aside. The tooth-like 

 process (iig. 7 c) is hollow, has an aperture in its upper edge, 

 and in several specimens I have observed it tilled 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-like 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 foui* small hollow spines 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 ofi*. 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. I 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. In a 

 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 Flustra 

 avicularis have four spines, which differ in appearance from those 

 of Cellularia avicularis. This specimen when dried assumed 



