46 INVERTEBRATE ANIMALS. 
have beens observed in the fresh-water sponge (Spongilla 
fluviatilis). If a specimen of Sfongilla be observed towards 
the approach of winter, its deeper portions will be found to be 
filled with numerous small rounded bodies like seeds, which 
have been called “‘gemmules.” Each gemmule (fig. II, a@, 6) 
exhibits at one point a small aperture or “hilum ;” and is found 
to be composed of a leathery membrane, surrounded by a layer 
. of sarcode, in which are imbedded a number of spicula. These 
spicula are called ‘‘amphidiscs,” and consist each of a central 
rod or axle carrying a toothed wheel or disc at each end (fig. 
11, c). In the interior of the capsule thus formed is a mass of 
cells, of which the central ones contain numerous reproductive 
germs. When the spring comes, these masses are discharged 
into the water through the aperture or hilum of the gemmule, 
and become developed into fresh Sfongil/z. In addition to 
this method of reproduction, the fresh-water sponge during the 
summer months has the power of producing true eggs or ova, 
and sperm-cells. The impregnated ova develop themselves into 
embryos, which are provided with numerous cilia or vibrating 
hairs, by means of which they swim about freely. Finally, 
upon finding a suitable locality, they fix themselves to some 
solid object, lose their cilia, and grow up into Spongille. 
Indeed, as a general if not universal rule, the embryos of the 
sponges are provided with cilia, and are thus capable of active 
locomotion. In this way is secured the extension over a wide 
area of these otherwise fixed and plant-like organisms. 
Distribution of Sponges tn Space——l\t remains only to add a 
few words on the distribution of sponges in space. With the 
single exception of Spozgzl/a, all known sponges are inhabitants 
of the sea; but the former is to be found in lakes and rivers in 
most parts of the world. The marine sponges are found mostly 
attached to stones and other foreign objects between tide-marks 
and in deep water. The sponges of commerce are mostly 
obtained from the Grecian Archipelago, but inferior kinds are 
imported from the Bahama Islands. One common sponge 
(Cliona), instead of incrusting other objects, inhabits branching 
cavities in shells, which it excavates for itself. It apparently 
lives upon the animal matter contained in the shell, and few 
oyster-shells can be picked up upon our shores which do not 
exhibit the perforations and mines of some species or other of 
this genus. Fossil shells, also, often occur, which show that 
these mining sponges have enjoyed a vast antiquity. 
