OF THE SPONGIA1XE. 163 



assumes 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 each other. The lengths of the cells 

 in proportion to their diameters vary 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 spi- 

 cula, 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 tesselated 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 

 formed sponges, we shall find in all cases that those organs 

 exist only on the inner surface of the great cloacal cavities. 



The construction of the interstitial cells is best demon- 

 strated in a longitudinal section of a dried specimen of 

 Grcmtia ciliata, mounted in Canada balsam, and in a 

 specimen so prepared spaces are seen between the cells 

 which are often nearly half the size of the cells. These 

 spaces are most probably produced by the contraction of 

 the tissues induced by the mode of the preparation of the 

 object, and do not exist in the living sponge, but they 

 serve admirably to demonstrate the fact that each inter- 

 stitial cell has its own special parietes, and that the 

 divisions between the cells are not common to each other. 

 Pigs. 31:2, 313, Plate XXJ, and figs. 345, 346, a, 

 Plate XXVI. 



