Flatworms move by beating cilia

Members of the phylum Platyhelminthes, or flatworms, the simplest lophotrochozoans (Figure 32.15), are bilaterally symmetrical, unsegmented, acoelomate animals. They lack organs for transporting oxygen to internal tissues, and they have only simple cells for excreting metabolic wastes. Their lack of transport systems dictates that each cell must be near a body surface, a requirement met by their dorsoventrally flattened body form.

The digestive tract of a flatworm consists of a mouth opening into a blind sac. However, the sac is often highly branched, forming intricate patterns that increase the surface area available for the absorption of nutrients. Flatworms either feed on animal tissues (living or dead), or absorb nu-tients from a host's gut. Free-living flatworms glide over surfaces, powered by broad bands of cilia. This form of movement is very slow, but it is sufficient for small, scavenging animals.

The flatworms that are probably most similar to the ancestral bilaterians are the turbellarians (class Turbellaria), which are small, free-living marine and freshwater animals (a few live in moist terrestrial habitats). At one end they have a head with chemoreceptor organs, two simple eyes, and a tiny brain composed of anterior thickenings of the longitudinal nerve cords.

Although the earliest flat-worms were free-living (Figure 32.15a), many species evolved a parasitic existence. A likely evolutionary transition was from feeding on dead organisms to feeding on the body surfaces of dying hosts to invading and consuming parts of living, healthy hosts. Most of the 25,000 species of living flatworms—including the tapeworms (class Cestoda) and flukes (class Trematoda; Figure 32.15b)—are internal parasites. These flatworms absorb digested food from the digestive tracts of their hosts, so many of them lack digestive tracts. They inhabit the bodies of many vertebrates; some cause serious human diseases, such as schistoso-

32.15 Flatworms Live Freely and Parasitically (a) Some flatworm species are free-living, like this marine flatworm photographed in the oceans off Sulawesi, Indonesia. (b) The flatworm diagrammed here, which lives parasitically in the gut of sea urchins, is representative of parasitic flukes. Because their hosts provide all the nutrition they need, these intestinal parasites do not require elaborate feeding or digestive organs and can devote most of their bodies to reproduction.

Adhesive Organ Turbellarians

(a) Pseudoceros bifurcus

The flatworm's body is filled primarily with sex organs.

Pharyngeal opening Intestine

Egg capsule Testis

Yolk gland

Seminal receptacle

Ovary

Vagina

Pharyngeal opening Intestine

Egg capsule Testis

Yolk gland

Pseudoceros Body Parts

The flatworm gut has a single exterior opening. The pharyngeal opening serves as both "mouth" and "anus."

Posterior

The flatworm gut has a single exterior opening. The pharyngeal opening serves as both "mouth" and "anus."

Posterior

(a) Pseudoceros bifurcus

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