Sponges are the lowest of slimy, multicellular animals. Their tissues are primitive and not differentiated as in other multicellular animals (Metazoa). They are therefore placed under an isolated division - Parazoa. All sponges except a single fresh-water family live in the sea attached to rocks, pilings, plants, etc. They grow much as plants do. Some are branched, some shaped like cups, some like gloves or domes. In size they range from one millimetre to a metre in diameter and forty centimetres in thickness. Despite varioation their essential structure is the same.

The form of a typical simple sponge is that of a vase or cylinder, closed and fixed at one end and with an opening called the osculum at the free end. The cylinder encloses a cavity - the gastric cavity or paragaster. The wall of the cavity is of two layers - an outer layer of flat cells and an inner, lining layer of collared cells (cells with cilia or flagella). The cells in both these layers are but loosely connected and not fitted firm as in other multicellular animals.

Sponges feed on minute organic particles living or lifeless existing in the surrounding water which circulated in their body through a canal system which falls under three types. On the outside of a sponge are a number of minute pores. In simple sponges like Leucosolenia - a common seashore species attached to rocks - these pores open directly into the gastric cavity. Water enters into the cavity through the pores. The collared cells lining the cavity digest the food matter contained in the water and throw out the waste. The refuse water is finally pushed out through the osculum. The inflow of the water through the pores, its circulation within the cavity and its final ejection through the osculum are all due to the harmonious lashing of the vibratile cilia of the collared cells in the direction of the osculum. This simple canal system is called `ascon`.

In the next higher type, which exists in the genus known by the name Sycon, the wall of the cavity is folded into a series of flagellated chambers. Outside water enters these chambers through special in-current canals connecting the pores and the chambers. Digestion takes place in the chambers. The used water then flows into the cavity and thence out through the osculum. In the third type, `leucon` or Rhagon, the flagellated chambers become further folded into a number of smaller chambers each one connected to the pore by an incurrent canal and the gastric cavity by an excurrent canal. This elaborate system is found in all fibrous (bath and freshwater) sponges.

All sponges have a gelatinous layer of mesoglea between the two layers of cells. Embedded in this mesoglea is a skeletal framework. The skeleton consists of lime (calcareous), glass (siliceous) needles or spicules or horny fibre sponging. Calcareous spicules are generally single, three or four-layered. The siliceous spicules are much more varied in shape and in one and the same kind of sponge there may be different forms of spicules each form having a special place in the skeleton of the various parts of the sponge body.

Spongin is a substance closely allied to silk. In the common bath sponge the skeleton is entirely of spongein threads which branch and form a compact but extensive supporting structure. The skeleton is not only supporting but also protective. The sharp spicules and the bitter taste of sponging keep predacious enemies off. Sponges are grouped according to their skeletal peculiarities.

In the cavities of large-sized sponges small shrimps, sea worms and molluscs enter and live for shelter, feeding on the waste matter. Species of Cliona bore the surface of bivalve shells and grow on them. Some get fixed on the back or legs of brabs. This association benefits both. The sponge is inedible and with it as a cover the mollusc and crab are protected from their enemies. The sponge is benefited by its being carried from place to place, enabling it to obtain adequate oxygenation and food.

Sponges reproduce asexually by budding or sexually by ova and sperms which unite to form a zygote. The zygote divides and becomes a spherical mass of cells successively called the blastula and the amphiblastula. Later it invaginates (or tucks), develops collared cells within, fixes itself to a holdfast and grows into an adult. The growth of the sponge in magnitude and direction is to a great extent dependent on the nature of the substance.