Gemmules of Freshwater Sponges. 165 
is furnished with a remarkable closing-apparatus, as Carter 
figures it in a gemmule of S. Carter?, Bow. The envelope 
immediately surrounding the inner germ separates at one spot 
in such a manner as to form a globular hollow space, the outer 
wall of which projects somewhat beyond the outer surface of 
the compartment-layer, in the thickness of which the cavity is 
situated ; therefore this alone is penetrated. If the dry gem- 
mules be thrown into water they float with this capsule up- 
wards, so that its surface remains out of the water. It is 
only after from eight to ten days that they begin to sink; and 
if, as is probable, this is the case also with the living gemmules, 
the germs will only then awaken. This closing-vesicle seems 
to me to be a hydrostatic apparatus ; and that it maintains the 
gemmules so long at the surface of the water is perhaps not 
without significance, for if they are carried by the wind into 
shallow pools, which the power of the sun would soon dry up, 
their contents will not issue forth before the evaporation 
occurs, and will thus, by the delay that takes place, escape 
destruction. 
The formation of the germ-capsule will very probably take 
place as follows :—the portion of the parent animal separated as 
a germ first of all itself secretes on its surface a horny covering 
in layers (whence the concentric striation) as a cuticular for- 
mation; to this is then applied from without a system of tan- 
gential spicules; and upon this again, as a cuticular formation 
of the parent organism, the compartment-layer, which is finally 
coated by the external system of tangential spicules. The 
germ, which, as in all Spongille, consists in the dry state for 
by far the greater part of starch-corpuscles (probably reserve 
nutritive material), as already described and figured by Carter, 
is in this way admirably protected, but at the same time also 
in other respects most advantageously endowed. 
The gemmules in S. nitens are remarkably small (as also 
in S. Carterd), and therefore light, and all the lighter because 
the comparatively thick enveloping capsule contains such 
numerous cavities. ‘The importance of this remarkable archi- 
tecture of the capsule in my opinion lies in this, that by it the 
gemmules will find the widest possible distribution under the 
circumstances in which the stock or parent Spongilla appears 
to exist—the light capsule enclosing air-spaces acts as an 
aerostatic apparatus ! 
The Spongille in question which break up into such gem- 
mules are inhabitants of hot countries; they will frequently 
be liable, under the influence of the glowing sun, to be laid 
dry : most of them when this happens will die away ; but they 
live on in the parts of themselves in the protected gemmules, 
