36 
LIFE, IN ITS LOWER FORMS. 
more rarely, the elegant Sack Sponge ( Grantia compressa). 
It takes the form of a little flattened bag of angular out- 
line, and of a whitish hue, with an orifice at each angle. 
The bags, which arc frequently clustered, hang by a slender 
base from the stalks of sea-weeds, or from the naked rocks 
(PI. II. fig. 3). When examined they are found to be 
hollow, with thin walls ; and if a small portion be torn off 
and placed beneath a microscope, it will exhibit well the 
structure of a spicular Sponge. The substance will appear 
crowded with, and almost composed of, calcareous crystals, 
most of which are stars of three radiating points, but some 
are linear needles, and on the exterior are many which are 
pointed at one end, and terminate in a bent, club-like knob 
at the other. 
It was this species which, under Mr Bowerbank’s expe- 
rienced eye and delicate manipulation, revealed the moving 
cilia. By tearing specimens in pieces (for the use of the 
keenest cutting instruments so crushed the texture as to 
destroy the parts), and examining the separated edges with 
high powers, he found that the sides are composed of a 
number of hexagonal cells, defined by the peculiar arrange- 
ment of the triradiate spicula, and having their walls 
formed by a multitude of nucleated granules (fig. 4). 
These angular cells are laid at right angles to the long 
axis of the Sponge, extending from the outer surface to 
the inner ; and they are crossed, near the middle, by a 
thin partition, perforated in the centre. In this perfora- 
tion, several long, whip-like cilia were seen lashing with 
energy, and the same organs were afterwards found to be 
connected with the granules of which the cell-walls were 
composed. By means of the wavings of these cilia, then, 
