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?
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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?
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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.
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Fig. 25.—Plan of a sponge.
Commercial "Sponge."—What part of the complete animal remains in the bath sponge? Slow growing sponges grow more at the top and form tall, simple, tubular or vase-like animals. Fast growing sponges grow on all sides at once and form a complicated system of canals, pores, and oscula. Which of these habits of growth do you think belonged to the bath sponge? Is there a large hole in the base of your specimen? If so, this is because the cloaca was reached in trimming the lower part where it was attached to a rock. Test the elasticity of the sponge when dry and when wet by squeezing it. Is it softer when wet or dry? Is it more elastic when wet or dry? How many oscula does your specimen have? How many inhalent pores to a square inch?
