SUGGESTIONS.—In many parts of North America, fresh-water sponges may, by careful searching, be found growing on rocks and logs in clear water. They are brown, creamy, or greenish in colour, and resemble more a cushion-like plant than an animal. They have a characteristic gritty feel. They soon die after removal to an aquarium. [Illustration: FIG. 21.—FRESH-WATER SPONGE. ] A number of common small bath sponges may be bought and kept for use in studying the skeleton of an ocean sponge. These sponges should not have large holes in the bottom; if so, too much of the sponge has been cut away. A piece of marine sponge preserved in alcohol or formalin may be used for showing the sponge with its flesh in place. Microscopic slides may be used for showing the spicules. [Illustration: FIG. 22.—SECTION of fresh-water sponge (enlarged). ] The =small fresh-water sponge= (Fig. 21) lacks the more or less vaselike form typical of sponges. It is a rounded mass growing upon a rock or a log. As indicated by the Arrows, where does _water enter_ the sponge? This may be tested by putting colouring matter in the water near the living sponge. Where does the _water come out_? (Fig. 22.) Does it pass through _ciliated chambers_ in its course? Is the _surface_ of the sponge rough or smooth? Do any of the skeletal _spicules_ show on the surface? (Fig. 21.) Does the sponge thin out near its edge? [Illustration: FIG. 23.—EGGS and SPICULES of fresh-water sponge (enlarged). ] The _egg_ of this sponge is shown in Fig. 23. It escapes from the parent sponge through the _osculum_, or large outlet. As in most sponges, the first stage after the egg is ciliated and free-swimming. =Marine Sponges.=—The _grantia_ (Fig. 24) is one of the simplest of marine sponges. What is the _shape_ of grantia? What is its length and diameter? How does the free end differ from the fixed end? Are the _spicules_ projecting from its body few or many? [Illustration: FIG. 24.—Grantia. ] Where is the _osculum_, or large outlet? With what is this surrounded? The osculum opens from a central cavity called the cloaca. The canals from the pores lead to the _cloaca_. _Buds_ are sometimes seen growing out from the sponge near its base. These are young sponges formed asexually. Later they become detached from the parent sponge. [Illustration: FIG. 25.—Plan of a sponge. ]