Many people wrongly think that sponge is a sort of seaweed. This is not surprising, for sponge was once regarded as one of Nature’s puzzles. One old writer imagined that sponges were made out of the foam of the sea. Another thought that they were worm houses, built by worms much as bees build honeycombs and wasps build nests.
The truth is that sponge is an animal. The dry material that we use as ‘sponge’ is a horny skeleton. When it is alive this fibrous skeleton of the animal has tiny cells plastered all over its surface in such a way that it forms a porous mass. The mouths and tiny pores on the outside lead into a network of tubes large and small. The cells that are set around the pores and mouths have little finger-like processes which all move together. A stream of water flows in at the pores and out again at the mouths. In this way food and gasses in the water are made to circulate in all the cells.
Some sponges reproduce their kind by growing buds on the parent animal. Among other species there are males and females, and the female sponges produce eggs that develop into single cells. At first these go swimming through the water but afterward they take root and form elaborate compound structures as described. There are sponges of all shapes, sizes and colors. Some are the size of pinhead, some as tall as a man; some are fan-like, some tree-like, some cup-like, and some basket-like; some are built on a horny framework; some are made of lime; some have glassy frame. Some are snow-white, some grass-green, some sky-blue, some red and some yellow. There are in fact many thousands of species of sponges!