both ova and spermatozoa are found throughout the sponge, though more
concentrated in the interior. The ova consist of spherical cells, while
the spermatozoa resemble an arrow-head in shape. It has not yet been
ascertained whether two sexes exist in sponges, or whether the ova and
spermatozoa are produced at different periods by the same sponge. When
the embryo has become partly developed, it detaches itself from the
parent sponge, and, issuing from the oscula, propels itself through the
water by means of a number of flagella.

Silicious spicules next appear in its structure, and it then attaches
itself to a rock and assumes its mature form. Sponges are most numerous
in the waters of the temperate and sub-tropical zones, and the
salt-water varieties are by far more numerous than the fresh water.
Thus, while there are not more than ten fresh-water species known, Dr.
Ledenfeld remarked that about one thousand species of salt-water sponges
had been recognized. Each species of the salt-water sponge is, however,
generally found only in limited areas, and very few, all of which
inhabit deep water, are cosmopolitan. This is the more remarkable as Dr.
Ledenfeld asserts that all the sponges inhabiting the rivers of
Australia are identical with the fresh-water sponges of Europe, and in
order to explain this fact he put forward a rather interesting theory.
He assumes that sponge life in rivers has been originally generated by
the introduction of a single, or at most two or three germs by means of
aquatic birds. The inbreeding consequent upon this paucity of sponge
life has produced a certain fixity of character in fresh-water sponges,
and is in direct opposition to the effects of hybridization in the
salt-water sponges, by which they have acquired the capacity of adapting
themselves to local circumstances.

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