AjN’D phtsiologt of the spongiad^. 
1093 
sea*water, and examined with a power of about five or six hundred linear by trans- 
mitted light. The cilia will be seen in rapid action just within the oscula which termi- 
nate each of the large angular interstitial cells of the sponge. This action, and the 
mode of the disposition of the cilia within the cells, I have described at length in the 
Transactions of the Microscopical Society of London, vol. iii. p. 137, plate 19. In 
accordance with these variations in structure I purpose dividing the British species into 
four genera. 
Class PORIPERA, Grant. 
Order I. CALCAREA. 
Genera. Grantia. 
Leucosolenia. 
Leuconia. 
Leucogypsia. 
Geantia, Fleming. 
Sponge furnished with a central cloaca, parietes constructed of interstitial cells, 
more or less regular and angular in form, disposed at right angles to the external 
surface, and extending in length from the outer to very near the inner surface of 
the sponge, where each terminates in a single osculum. 
Type, Grantia compressa, Johnston. 
The cloaca varies in its form and proportion. In some species it has invariably one 
large terminal mouth, while in others it is furnished with several mouths, from which 
the excurrent faecal streams are discharged. 
The interstitial structures of the sponges of this genus assume a greater amount of 
regularity than is found to exist in any other genera of these animals. The whole of 
the parietes of the sponge are formed of somewhat angular cells, the sides of which 
belong to the individual cell, and are not common to the adjacent cells. The length of 
the cells in proportion to their diameter varies in different species, and also in the same 
species in proportion to the age and thickness of the parietes of the sponge. The cell- 
walls are formed of comparatively stout transparent membrane, strengthened and 
supported by numerous triradiate spicula ; and the whole length of the cell, from the 
inner edge of the osculum to near the outer surface of the sponge, is closely studded 
with tessellated nucleated cells, each of which is furnished with a long attenuated cilium. 
Each interstitial cell terminates in a single osculum, slightly within the plane of the 
inner surface of the sponge. I do not remember to have ever seen these oscula entirely 
closed. When the inhalant action of the sponge is in vigorous operation, the excurrent 
streams may be seen issuing from them with considerable force, and the cilia appear in 
action immediately within them. 
Hitherto the mouths of the great cloacal cavity of the sponges of this tribe have been 
described as oscula ; but if we carefully examine the structure of these and similarly 
