Danny Van Meter, age 12, of Fort Wayne, Ind., for his question:
IS A SPONGE A PLANT OR AN ANIMAL?
At one time, people believed that sponges were plants because they were attached to the bottoms of oceans and did not move around. Although many sponges look like a type of plant, zoologists classify them as animals.
Sponges make up an animal classification called porifera, which means pore bearers. The surface of a sponge's body is covered with tiny pores.
Although a few of them live in fresh water, most sponges live in the oceans of the world. They can be found in both shallow and deep water.
Although they also live in cold water seas, more kinds and numbers of sponges live in the warm temperate and tropical waters than anywhere else. The largest sponges, including the well known commercial sponges that people use for cleaning, grow in these warmer waters.
A sponge does not look like any other animal. Some sponges are shaped like vases but most species have no definite shape. Some may be thin and flat while others become round masses. Still others may look like branching shrubbery or treelike bushes.
Sponges also vary in size. Some grow to be less than an inch across while others grow to be more than four feet.
You won't find a head, mouth or internal organs in a sponge. It depends on a system of water canals in its body to bring in food and oxygen. This system also carries away waste products.
Very small pores in the surface of the sponge's body lead to the tiny canals. The canals lead to chambers called flagellated chambers. Each cell that lines the chambers contains a flagellum, which is a long thread that whips around to aid movement.
The flagellated chambers drain into other small chambers. These canals join a network of small canals that eventually lead to the outside through a large opening in the sponge's body called the osculum.
The beating, whiplike movements of the flagella circulate water through the sponge. When tiny plants and animals are swept through a sponge by water, the cells that line the chambers surround and digest them.
Sponges have several types of skeletons. Some sponge skeletons consist of tiny needles called spicules. Other skeletons are made of fibers called spongin.
Some sponges have skeletons that consist of both spicules and spongin.
Some skeletons have spicules made of calcium carbonate, or limestone. Others have spicules made of silica, or glass.
All sponge skeletons form a supporting meshwork throughout the sponge's body.
Most of the so called sponges sold in stores today are not true sponges. They are synthetic materials made to look and to clean like true animal sponges.