Feeding And Digestion In Sponges

Because they are sessile, sponges cannot pursue food. Instead, most sponges feed by sieving food out of the water. The flagella of the choanocytes beat, drawing water through the ostia. The choanocytes trap plankton and other tiny organisms in their small, hairlike projections. This feeding method is called filter feeding. Other food of filter-feeding sponges includes bits of organic matter. However, scientists have discovered a sponge species that uses movable filaments covered with hooked spicules to snare small shrimp. The shrimp are then absorbed into the sponge's body.

The food that a sponge collects is engulfed and digested by the choanocytes. Nutrients pass from the choanocytes to cells that crawl about within the body wall and deliver the nutrients to the rest of the body. Scientists call these crawling cells amoebocytes (uh-MEE-buh-siets) because the cells resemble amoebas. Locate the amoebocytes in Figure 33-1. Carbon dioxide and other wastes produced by the sponge's cells diffuse into the water that passes through the sponge. The water carries these wastes as it flows out through the osculum, thus removing the wastes from the sponge.

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