a man.
The sponge, as you use it in a bath, is just an animal's skeleton, or it
may be of several animals that have grown together."

"Yo' suah o' that, boss?" asked the boatman. "I allus hear' dat a sponge
was a plant--not any animal."

"It's an animal," Colin said shortly.

"But I thought," interjected Paul, "that the difference between a plant
and an animal was that an animal can move around and a plant can't."

"Well, Paul," the boy answered, "the young of sponges are larvae which
swim in the water by threshing with short hairs until they find
something suitable to stick on. Lots of animals which become fixtures
are free-swimming when young, oysters, for instance."

"Then a sponge doesn't seed itself, like a plant?"

"No, Mr. Murren," said Colin; "so far as I understand, the larvae, though
of a very simple type, have a certain amount of choice. A seed has got
to grow where it falls, or not at all, but a sponge larva, if it doesn't
find a suitable place on the first thing it touches, can swim about
blindly until it finds one that will do."

"Now about these sponges, Colin," his host said, impressed by the boy's
clear though crude way of explaining himself, "look through the glass
and tell me what you think about the bed."

"There are quite a lot of sponges there," the boy answered after a few
minutes' examination, "some of good size, too, but a number of them are
dead. See, the sand has drifted half over them. There's too much sand
and too little rock."

"Should they have a rock bottom?" the manufacturer queried.

"Rock am de bes', suah," the owner of the ground put in, "but a li'l bit
o' san' don' do no hahm. It shows dat de wateh am runnin'."

"Yes," said the boy, "the boatman is right there, Mr. Murren, sponges
must be in a current after they have once taken hold. They can't swim
around to get their food, so, like all the fixed forms of life, their
food must come to them. If there is no current there is not enough food
carried past for them to live on. If the current is too strong the
sponge has to make an extra tough skeleton to brace itself against the
rush of water and then it becomes too coarse for commercial use. Some of
the polyps live on tiny animals with a lot of flint in their shells and
the skeleton gets like glass. They call them glass sponges. Conditions
have got to be just right for their development, they're a most
particular sort of creature."

"But how do they feed?"

"A sponge is a