832 Mr. A. Hancock on the Excavating Powers of Sponges, 
ordinary currents, which setting in from the surface of the sponge 
would convey the reduced matter into the principal channels, by 
which it would soon find an exit through the efferent papillary 
apertures. I have before alluded to calcareous particles found 
strewed along the channels inhabited by C. celata. These par- 
ticles, measuring z4,th of an inch long, are apparently too large 
to escape through the pores of the sponge, and are evidently not 
the scourings of the excavation ; they are much too large to arise 
in this way; but are pieces probably cut out by a combmation 
of the minute drills just described. To understand how this 
may be effected, we have only to suppose that numerous punc-. 
tures are made through a thin, slightly attached plate or lamina 
of the oyster-shell, and that imterspaces are left between the 
punctures ; and it is clear that as the drilling goes on, many 
of these interspaces will become detached im the form of de-. 
pressed, many-sided, angulated bodies. And such are those that 
are found in the channels of the excavations. Those particles 
of a similar nature occurring in the chambers in limestone are 
undoubtedly produced much in the same manner. 
In the siliceous granules on the surface, and in the contractility 
of these sponges, we thus find an explanation of the excavating 
powers. 
We shall conclude this communication with the description of 
a few of the species, premising that the figures of the spicula 
represent them drawn to a scale, so that at a glance a pretty 
correct idea may be obtained of their relative sizes. And it is as 
well perhaps to observe, that in every instance the full-developed 
spiculum has been measured and figured. 
Cliona celata, Grant. Pl. XIII. figs. 3 & 4. 
C. celata, Grant, Edin. New Phil. Journ. vol.1. p. 78. 
C. celata, Johnston, Br. Sponges, p. 125? | 
Sponge of a clear yellow-ochre colour occasionally inclining to 
olive, composed of a large open network of branches ; the meshes 
irregularly angulated, frequently five- or six-sided, and oceasion- 
ally half an inch wide ; the branches stout, often ;4,ths of an inch 
thick, distinctly nodulous and generally depressed : papille large, 
some measuring ;4,th of an inch in diameter*; for the most 
part in a single row along the branches, but penetratmg the 
surface of the matrix without much apparent order, and placed 
rather far apart from each other: terminal twigs rather short, 
delicate, almost linear, and generally bifurcated. Spicula very 
long, measuring upwards of ;',th of an inch in length, a little 
* Tn this and in the following descriptions the diameter of the papillze has 
been determined by that of the papillary punctures. 
