Body Plan Of Sponges

Sponges are aquatic animals that make up the phylum Porifera (pohr-IF-uhr-uh). These simple organisms clearly represent the transition from unicellular to multicellular life. Because sponges are het-erotrophic, multicellular organisms that do not have cell walls, they are classified as animals. Sponges have no gastrula stage, exhibit less cell specialization than most other animals, and have no true tissues or organs.

But sponges do have a key property of all animal cells—cell recognition. A living sponge can be passed through a fine mesh, which separates the sponge into individual cells. These separated cells will then regroup to form a new sponge. There are about 5,000 named species of sponges. About 150 species live in fresh water, while the rest are marine.

Sponges are so unlike other animals that early biologists thought that sponges were plants. Most sponges do resemble plants in some ways. For example, adult sponges are sessile (SEHS-il), which means that they attach themselves firmly to a surface and do not move. Sponges grow in many shapes, sizes, and colors and often look like mossy mats, cactuses, or blobs of fungus. Sponges can be as small as 1 cm (0.4 in.) in length or as large as 2 m (6.6 ft) in diameter.

The basic body plan of a sponge consists of two layers of cells separated by a jellylike substance called mesohyl (MEHZ-oh-hil). In the simplest sponges, the body wall forms a hollow cylinder that is closed at the bottom and open at the top. The interior of the cylinder is lined with flagellated cells called choanocytes (koh-AN-oh-siets), or collar cells. By beating their flagella, choanocytes draw water into the sponge through numerous pores, called ostia (AHS-tee-uh) (singular, ostium), that penetrate the body wall. In fact, the name Porifera comes from a Latin word meaning "pore-bearer." The water that is pumped into the interior of the sponge leaves through the osculum (AHS-kyoo-luhm), the opening at the top of the sponge that you can see in Figure 33-1 on the next page.

A sponge would collapse without some type of supporting structure. In some sponges, support is provided by a simple skeleton made of a network of tough, flexible protein fibers called spongin (SPUHN-jin). Other sponges have skeletons consisting of spicules.

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