![]() Physical Structure of Sponges The physical structure of a typical simple sponge is that of a vase or cylinder, closed and fixed at one end. There is an opening called the osculum at the free end. The cylinder encloses a cavity - the gastric cavity or paragaster. The wall of the cavity has 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 loosely connected and not fitted firmly as in other multi-cellular animals. Sponges are grouped according to their skeletal peculiarities. 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 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 sponge body. Sponging is a substance closely allied to silk. In the common bath sponge, the skeleton is entirely made up of sponging threads, which branch and form a compact and an extensive supporting structure. The skeleton is not only supportive but also protective. The sharp spicules and the bitter taste of sponging keep predacious enemies off. Digestive and Excretory System of Sponges ![]() Reproduction in Sponges 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. In the cavities of large-sized sponges, small shrimps, sea worms and molluscs enter and live for shelter and feed on the waste matter. The sponge is inedible and with it as a cover, the molluscs and crabs are protected from their enemies. The sponge is benefited by it being carried from place to place, enabling it to obtain adequate oxygen and food. |
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Indian Sponges