G, Fig. 1, is a spicule of fresh-water sponge, spongilla fluviatilis. Sponges belong to the animal kingdom.

Group 1. Ceratosa, with horny fibres. In this group come the common sponges of commerce, marine.

2. Calcispongia, with calcareous spicules.

3. Silicea, aquatic, with spicules of silica like our cut.

The office of these spicules is not settled, though thought to be what answers to the skeleton of the animal. They are, however, characteristic of sponges when found.