What Is a Sponge?
A sponge is an extremely simple kind of water animal with a soft, elastic skeleton. It has no head, mouth, or brain. Sponges hatch from eggs. The baby sponge attaches itself to a firm surface under the water, such as a rock, and begins to grow.
For the rest of its life it stays in the same place, something like a plant. It feeds by filtering out bits of food matter from the water since it is able to capture and eat particles as small as bacteria as well as much larger particles.
The scientific term for sponges is Porifera which literally means “pore-bearing.” A sponge is covered with tiny pores, called ostia, which lead internally to a system of canals and eventually out to one or more larger holes, called oscula.
Within the canals of the sponge, chambers are lined with specialized cells called choanocytes, or collar cells. The collar cells have a sticky, funnel shaped collar and a hairlike whip, called a flagellum.
The collar cells serve two purposes. First, they beat their flagella back and forth to force water through the sponge. The water brings in nutrients and oxygen, while it carries out waste and carbon dioxide.
Second, the sticky collars of the collar cells pick up tiny bits of food brought in with the water. Another type of cell, called an amebocyte, takes the food to other cells within the sponge.
The “skeleton” of the sponge is composed of tiny needle-like splinters called spicules, a mesh of protein called spongin, or a combination of both.
The soft skeleton of a dead sponge can hold a lot of water, so people have long used sponges for cleaning. Probably, though, the sponges you use were made in a factory.