For many years that question puzzled scientists, but in 1825 a scientist studied sponges under a microscope and saw water enter them through some openings and come out others, looking different. This was evidence that the sponge was an animal which captured tiny plants and animals from the water to use as food, digested them, and gave off wastes.
The sponge is one of the lowest forms of life in the animal kingdom. It lives, but does not move about. It does not have a head, mouth, eyes, ears, heart, stomach, nerves, or sense organs. If you touch a sponge, it doesn’t draw away or strike back; it doesn’t have any of the reactions seen in other low forms of life in the animal kingdom.
What, then, happens in a sponge? The outer layer of a sponge is made up of flat cells. Inside, are long canals with the most unusual cells ever found in an animal, for these cells work like a filter. They have tiny, whip-like threads, called lashers, which beat at the water, filtering out tiny plants and animals to use as food. The lashers then force that food and oxygen into the cells.
The inside of the sponge is a slimy, jellylike mass full of holes and tunnels. Cells wander through these tunnels, probably digesting the food, breathing in some fashion, and giving off some of the waste products. Although these waste products give sponges a bad smell, they also keep other sea creatures from eating them.
Sponges grow in a variety of colors: red, orange, brown, green, yellow, and white. They also have a wide variety of shapes: fans, bowls, branches, and balls.
The largest sponge ever found measured six feet in circumference and weighed between 80 and 90 pounds!