Connie Goodwin, age 11, of Annapolis, Md., for her question:
IS A SPONGE A PLANT OR AN ANIMAL?
A sponge is a water animal. Many people believe that sponges are plants because they are attached to the bottom of the ocean and they do not move around. Although many sponges look like a type of plant, zoologists definitely classify them as animals.
Most sponges live in the ocean but there are a few fresh water kinds. They can be found in both deep and shallow water. They inhabit all the seas, but more kinds and numbers of sponges live in the warm temperate and tropical waters than anywhere else.
A sponge does not look like any other animal. It has no head, mouth or internal organs. It depends on a system of water canals in its body to bring food and oxygen. This system also carries away waste products.
Tiny pores in the surface of the sponge's body lead to the tiny canals. Food and oxygen enter the body through the pores and are carried through the canals into small chambers. These are called flagellated chambers because each cell that lines them contains a "flagellum," or long thread, that whips around to aid movement.
The flagellated chambers drain into other small canals. 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.
A piece of sponge as small as a marble contains thousands of flagellated chambers and canals.
Symmetrical sponges with vase shaped or goblet shaped bodies have only one osculum. Sponges with bodies that are not symmetrical have many oscula, and each one provides an exit from the thousands of flagellated chambers and canals.
Many zoologists believe that a sponge with many oscula is not a single animal, but a colony made up of many sponges.
Whiplike movements of the flagella circulate water through the sponge. The water sweeps in food particles. Cells that line the chambers engulf and digest the particles.
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. There are two kinds of spicules in sponge skeletons. 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. Glass sponges contain skeletons made of evenly arranged glass spicules. After the sponge cells are removed, these beautiful skeletons look like glass wool. The delicate Venus' flower basket is a glass sponge.
Other less attractive sponges leave only the rough spongin fibers after the cells have been removed.
Most 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.