Bob Vaughan, age 9, of Brecksville, Ohio, for his question:
How can sponges eat?
Think of all the time and trouble it takes to get our food. Farmers, ranchers and dairymen toil to produce it. We shop for it, cook it and have to wash the dishes after we eat it. A quiet sponge has none of these problems. He squats on the ocean floor and the water serves his food right into his many little mouths, free of charge.
On a lazy day, it might seem nice to be a sponge. He does not have to market, shop or do the dishes. True he is not a very handsome creature. But the living sponge doesn't notice that he looks somewhat like a mass of raw liver. He has no mind to think, no eyes to enjoy the scenery, no arms or legs to go any place, even if he wants to. His lazy life might seem nice but no doubt we would find it dull and boring.
Actually, the sponge does do a little work for his dinner. But his body is built to make this as easy as possible. The part we use as a bath sponge is buried inside soft fleshy layers, all riddled with tunnels. His tough skin is speckled with tiny pores. To get the bath sponge, we must dry out the skin and fleshy layers, then scrape them away. This is a messy job.
The whole body of the living sponge works like a filter system. It pulls water through the pores in his skin, down through the tunnels and then jets it out through an exit hole where his head would be, if he had one. The exit hole has a tunnel down to a sort of hollow tummy. This center pocket is lined with special cells that run his filter system. They are called collar cells and they have hairy threads somewhat like tiny tails. Thousands of these hairs wave and wag to keep a current of water flowing through the filter system.
The collar cells also sift scraps of food from the water. They digest some of these morsels in tiny pockets called food vacuoles. Some scraps may be partly digested and sent to other cells. There they are completely digested and passed along to cells that need food.
He is not picky about his food, so long as it is served in very tiny helpings. The water teems with tiny plants and animals, many of them too small for our eyes to see. It also teems with scraps of fishes and other creatures that have died and decayed.
The sponge sifts out all these morsels and the circulating water keeps serving him all he needs. He squats there on the sea floor, his teams of tiny hairs waving day and night to keep currents of water steaming through his body. As it passes, he sifts out food, absorbs oxygen, and also takes the minerals he needs to build the bath sponge skeleton inside his body.
Many different sponges live their lazy lives in the salty sea. A few types prefer to live in fresh water lakes and rivers. All of them have bodies riddled with pores and tunnels with filter systems to sift their food from the water. No other animals in the world are like them. For this reason they are classed in a group of their own. All the sponges, and only the sponges, belong in the animal Phylum Porifera. This fancy name means the pore bearers.