Phylum Porifera: Sponges
Sponges are simple invertebrate animals that are made up of many cells. Sponges are made up of two layers of cells around a central cavity (the space in the center). The outside layer is the protective epidermis, which contains many pores (each pore is hollow. Itís like a short piece of pipe connecting the outside and the inside of the animal). The inside layer is made up of collar cells. The whip-like flagella on the collar cells wave back and forth creating currents. Food particles in the incoming water are trapped and digested by the collar cells. Wastes leave the sponge through the opening at the top of the central cavity.
The shape of a sponge is determined by the materials found between epidermis and the collar cells. The skeleton is located primarily in the mesenchyme, but spicules frequently project through the epidermis. Spike-like specula form a hard framework. Some sponges have a material called spongin instead of specula. Rubber-like spongin forms a soft, spongy framework.
Sponges reproduce sexually, and sperm and eggs are shed into the central cavity. They meet and fuse. The young sponge (larva) that develops from a fertilized egg leaves the central cavity through the opening and attaches itself to a surface. Then it begins to grow into an adult sponge.
Also the sponges can reproduce asexually. For example, freshwater sponges may form balls of cells called gemmules when the environment is not right. When the sponge dies, the gemmules are released. They are made up of certain cells and some stored food. Each gemmule may become a new sponge when the environment becomes better.
New sponges may also form in another way. Small pieces sometimes break off of large sponges. When this happens, each of the small pieces attaches itself to a stone or other surface and begins to grow into a new sponge. In fact, people often cut living sponges into small pieces, attach each piece to a cement block, and drop the blocks into the sea. Each piece will grow into a new sponge. After several years, the sponges will be large enough to be harvested and sold.
Sponges have been used since ancient times as bath aids, cleaning tools, and even as drinking vessels. When dried, the skeletons of certain species of sponges are soft and elastic yet still capable of absorbing large amounts of water. Natural sponges are still preferred by artists and craft workers and by hospital surgical teams.
Date: 2014-12-22; view: 1216