Nell Anders, age 9, of Winston Salem, North Carolina, for her question:
Why is a sponge classified as an animal?
Let's think of bathtubs and leafy green trees, frisky fishes and sunny seas because all these things come into the story. A sponge lives his life in the sea and often ends up in a bathtub. The tree belongs in the Plant Kingdom of lovely calm living things. The fish and a million or so other living things belong in the busy Animal Kingdom. In his very quiet way, the sponge belongs there too.
When it comes to sorting the plants and animals, you know just where to put the tiger and the tiger lily and also the horse and the horseradish. The plants on this list remain rooted in the soil and the animals are free to roam around. The plants wear greenery and the animals prefer other colors. As a rule, we can tell the difference at a glance but a sponge presents a problem.
He looks for all the world like a chubby little shrub, rooted firmly on the floor of the sea. But scientists have other clever tests. They prove that he does not belong in the Plant Kingdom at all. For one thing, the green plants use sunlight to create their own sugary food from air and water. Animals cannot perform this miracle. They must feed on foods that are created by plants or by other animals that feed on plants. The shrubby looking sponge cannot create his own food from air and water. He feeds on floating scraps of meat and vegetables.
His body is somewhat like a dark colored cauliflower, riddled with a network of tunnels. Water flows into and out of the pores in his tough, leathery skin. As it sweeps through the tunnels inside, it delivers oxygen and floating morsels of solid food. Everything he needs is served by the sea, which makes it possible for the sponge to be just about the laziest animal in the world.
As a rule, a plant produces its own seeds for the next generation. But baby animals need two parents. The stay home sponge cannot go dating to find a mate. But he can solve this problem by producing cells of both a male and a female parent. The babies form inside the tunnels. When they are ready to leave home, the streaming water washes them outside into the sea. These baby sponges have tiny tails, which they wave to swim freely through the water. No plant bears its children in this way. And nobody would mistake those frisky young sponges for plants. However, later, they settle down and stay rooted to the same spot for the rest of their lives.
Plants are plants mainly because almost all of them can create their own food from air and water. A sponge cannot do this, so he must be an animal. He also can bear animal type babies, even though he has to serve as both parents. However, things get somewhat confusing because a sponge also can produce baby buds. These may stay clinging to the parent sponge all their lives. Or they may break away and start life in new locations. Actually, his diet and his family life prove that the sponge is an animal, even though he does sprout buds and looks like a grubby underwater cauliflower.