Class Turbellaria

The majority of the approximately 4,500 species in the class Turbellaria live in the ocean. However, the most familiar turbellar-ian is the freshwater planarian Dugesia, shown in Figure 34-2. Planarians have a spade-shaped anterior end and a tapered posterior end. They move through the water by swimming with a wavelike motion of their body. Over solid surfaces, planarians glide on a layer of mucus that they secrete, propelled by the cilia that cover their bodies.

Digestion and Excretion in Planarians

Planarians feed by scavenging for bits of decaying plant or animal matter. They also prey on smaller organisms, such as protozoa. Food is ingested through a muscular tube called the pharynx (FAR-eenks), which the planarian extends from the middle of its body. As Figure 34-2a indicates, the pharynx leads to the highly branched gastrovascular cavity. Cells lining the cavity secrete digestive enzymes and absorb nutrients and small pieces of food. The nutrients then diffuse to other body cells.

Organisms that live in fresh water must deal with the water that constantly enters their bodies by osmosis. Planarians eliminate excess water through a network of excretory tubules that run the length of the body. Figure 34-2b shows that each tubule is connected to several flame cells, which are so named because they enclose tufts of beating cilia that resemble flickering candle flames. The beating of cilia in the flame cells draws in the excess water. The water is then transported through the tubules and excreted from numerous pores scattered over the body surface.

testes and ovaries.

testes and ovaries.

(a) DIGESTIVE SYSTEM (b) EXCRETORY SYSTEM (c) NERVOUS SYSTEM (d) REPRODUCTIVE SYSTEM

Word Roots and Origins pharynx from the Greek pharynx, meaning "throat"

figure 34-2

The organ systems of a planarian allow it to maintain its free-living existence. (a) The digestive system consists of the pharynx and gastrovascular cavity, which has many branches. (b) In the excretory system, flame cells collect excess water, which travels through excretory tubules to pores on the surface of the body.

(c) The nervous system is a ladderlike arrangement of nerves with two cerebral ganglia at the anterior end.

(d) Since planarians are hermaphrodites, their reproductive system includes both

Neural Control in Planarians

The planarian nervous system is more complex than the nerve net of cnidarians, as Figure 34-2c illustrates. Two clusters of nerve cells at the anterior end, the cerebral ganglia (suh-REE-bruhl GAN-glee-uh), serve as a simple brain. They receive information from sensory cells and transmit signals to the muscles along a ladderlike arrangement of nerves. A planarian's nervous system gives it the ability to learn. For example, a planarian normally moves away from light, but it can be trained to remain still when illuminated.

Planarians sense the intensity and direction of light with two cup-shaped eyespots located near the cerebral ganglia. You can see the eyespots in Figure 34-2c. Other sensory cells respond to touch, water currents, and chemicals in the environment. These cells are distributed over the body, but most are concentrated at the anterior end.

Reproduction in Planarians

Because planarians are free-living and motile, they can encounter and mate with other individuals of the same species. As shown in Figure 34-2d, planarians are hermaphrodites—they have both male sex organs (testes) and female sex organs (ovaries). When two planarians reproduce sexually, they simultaneously fertilize each other. Their eggs are laid in protective capsules that stick to rocks or debris and hatch in two to three weeks.

Planarians also reproduce asexually, generally during the summer. During asexual reproduction, the body constricts just behind the pharynx. While the posterior part of the worm is attached to a solid surface, the anterior part moves forward until the worm splits in two. This type of asexual reproduction is known as fission. The two halves then regenerate their missing parts to produce two complete planaria. During regeneration, each part of the planarian retains information about its original orientation in the body. If a piece is cut from the middle of a planarian, the anterior end of the piece will always regenerate a head and the posterior end of the piece will regenerate a tail.

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