"Well, Paul," the boy answered, "the young of sponges are larvæ 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 larvæ, 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."