STRUCTURE AND LIFE-HISTORY OF A SPONGE. 
21 
another to form a cylindrical tube. The surface of the sponge is 
covered all over by erectly projecting spicules, which render it 
hirsute. 
General Movements . — With the exception of the movements of 
the spicules in the corona, the sponge gives very few signs of life, so 
that at first sight one might almost regard it, as indeed the older 
naturalists did regard it, as a plant. It is, however, in every respect, 
a true animal, lively enough in its way, and of wonderfully complex 
structure. 
That it is not quite so inert as it seems may be easily shown by 
putting a little finely powdered indigo into the water in which a 
healthy specimen is confined. The particles of coloring matter will 
then be observed to make their way towards the general surface of 
the sponge, over which they spread themselves, and then disappear 
below it. After being lost to sight for a little while they re-appear, 
not over the surface where they went in, but streaming out of the 
central mouth in a powerful current. From this we may infer that 
minute currents are entering the sponge throughi ts general surface, 
passing through its walls into the central cavity, and then outwards 
by way of the mouth. 
General Structure . — So far the sponge has been regarded simply 
as a hollow sac, but the w^alls of the sac possess a somewhat 
complicated structure, which we must now describe. Commencing 
from the inner face we find first a membranous lining, perforated 
by a great number of small holes, which are called mouths, or ostia, 
and because they open into the stomach, stomachal mouths or 
gastral ostia. Each is the open end of a thin-walled tube, which is 
closed and conical at the other end, and except that it is hexagonal 
in section, somewhat similar in form to a chemist’s test-tube. These 
tubes radiate from the gastral ostia to the exterior of the sponge, 
and constitute, lying side by side, joined close together, the greate 
part of the sponge wall. By holding together a number of test- 
tubes, and supposing them to be joined along their lines of contact, 
