CHAPTER III
SPONGES
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.
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.
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?
Fig. 23.—Eggs and SPICULES of fresh-water sponge (enlarged).