Phylum Rotifera

Another group of pseudocoelomates are the approximately 1,750 species in the phylum Rotifera. Members of this phylum are called rotifers. Most rotifers are transparent, free-living animals that live in fresh water, although some live in salt water and damp soil. They typically range in length from 100 to 500 pm, with males being much smaller than females. Many rotifers can survive without water for long periods, during which they dry up and look like grains of sand. When wet conditions return, they absorb water and resume their activities. Although rotifers are tiny, they are truly multicellular and have specialized organ systems.

As Figure 34-10 illustrates, rotifers have a crown of cilia surrounding their mouth. Under a microscope, the crown with its beating cilia looks like a pair of rotating wheels. Rotifers use their cilia to sweep food—algae, bacteria, and protozoa—into their digestive tract. As shown in Figure 34-10, food moves from the mouth to the mastax, a muscular organ that breaks the food into smaller particles. The food is further digested in the stomach, and the nutrients are absorbed in the intestine. Indigestible material passes from the intestine to the cloaca (kloh-AY-kuh), a common chamber into which the digestive, reproductive, and excretory systems empty. Like planarians, rotifers use flame cells and excretory tubules to collect excess water in the body. The excess water, along with wastes from the intestine and eggs from the ovaries of females, leaves the cloaca through the anus. Rotifers exhibit cephalization, with a pair of cerebral ganglia and, in some species, two eyespots at the anterior end of the body.

Rotifer reproduction is unusual because some species of rotifers reproduce by parthenogenesis. In the process of parthenogenesis (PAHR-thuh-NOH-JEN-uh-sis), unfertilized eggs develop into adult females. Other species produce two types of eggs. Eggs of one type develop into small males that produce sperm. The sperm fertilizes eggs of the other type, which then form tough-coated zygotes that can survive poor conditions.

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