Section 1 Platyhelminthes Chapter 34 Structure and Function of Flatworms • The phylum Platyhelminthes includes organisms called flatworms. • They are more complex than sponges but are the simplest animals with bilateral symmetry. • Their bodies develop from three germ layers: • ectoderm • mesoderm • endoderm • They are acoelomates with dorsoventrally flattened bodies. • They exhibit cephalization. • The classification of Platyhelminthes has undergone many recent changes.

Section 1 Platyhelminthes Chapter 34 Characteristics of Flatworms Click below to watch the Visual Concept. Visual Concept

Section 1 Platyhelminthes Chapter 34 Class Turbellaria • The majority of species in the class Turbellaria live in the ocean. • The most familiar turbellarians are the freshwater planarians of the genus Dugesia. • Planarians have a spade-shaped anterior end and a tapered posterior end.

Section 1 Platyhelminthes Chapter 34 Class Turbellaria, continued Digestion and Excretion in Planarians • Planarians feed on decaying plant or animal matter and smaller organisms. • Food is ingested through the pharynx. • Planarians eliminate excess water through a network of excretory tubules. • Each tubule is connected to several flame cells. • The water is transported through the tubules and excreted from pores on the body surface.

Section 1 Platyhelminthes Chapter 34 Class Turbellaria, continued Neural Control in Planarians • The planarian nervous system is more complex than the nerve net of cnidarians. • The cerebral ganglia serve as a simple brain. • A planarian’s nervous system gives it the ability to learn. • Planarians sense light with eyespots. • Other sensory cells respond to touch, water currents, and chemicals in the environment.

Section 1 Platyhelminthes Chapter 34 Class Turbellaria, continued Reproduction in Planarians • Planarians are hermaphrodites that can reproduce sexually or asexually. • Their eggs are laid in capsules. • During asexual reproduction, their body undergoes fission and the two halves regenerate missing parts.

Section 1 Platyhelminthes Chapter 34 Planarian

Section 1 Platyhelminthes Chapter 34 Anatomy of a Planarian Click below to watch the Visual Concept. Visual Concept

Section 1 Platyhelminthes Chapter 34 Exploration of a Flatworm

Section 1 Platyhelminthes Chapter 34 Development of Flatworm Embryo

Section 1 Platyhelminthes Chapter 34 Classes Trematoda and Monogenea • The classes Trematoda and Monogenea consist of parasitic flukes. • Some are endoparasites; others are ectoparasites. Structure of Flukes • A fluke clings to the tissues of its host by an anterior sucker and a ventral sucker. • A fluke’s nervous system is similar to a planarian’s, but simpler. • The external surface of a fluke is covered by a protective layer called the tegument.

Section 1 Platyhelminthes Chapter 34 Tegument

Section 1 Platyhelminthes Chapter 34 Classes Trematoda and Monogenea, continued Reproduction and Life Cycle of Flukes • Most flukes have highly developed reproductive systems and are hermaphroditic. • Fertilized eggs are stored in a fluke’s uterus until they are ready to be released. • Flukes have complicated life cycles that involve more than one host species. • For example, the trematode blood flukes of the genus Schistosoma use humans as a primary hosts and snails as intermediate hosts. They can cause schistosomiasis.

Section 1 Platyhelminthes Chapter 34 Life Cycle of Schistosoma

Section 1 Platyhelminthes Chapter 34 Life Cycle of Flukes Click below to watch the Visual Concept. Visual Concept

Section 1 Platyhelminthes Chapter 34 Class Cestoda • About 5,000 species of tapeworms make up the class Cestoda. • Tapeworms can live in the intestines of almost all vertebrates. Structure of Tapeworms • Tapeworms are surrounded by a tegument. • They attach to the host with a scolex. • The body is a series of many sections called proglottids. • Tapeworms have no light-sensing organs, no mouth, no gastrovascular cavity, and no digestive organs.

Section 1 Platyhelminthes Chapter 34 Anatomy of a Tapeworm Click below to watch the Visual Concept. Visual Concept

Section 1 Platyhelminthes Chapter 34 Class Cestoda, continued Reproduction and Life Cycle of Tapeworms • Nearly all tapeworms are hermaphrodites. Each proglottid contains male and female reproductive organs and little else. • New proglottids are added to the front of the tapeworm. Older proglottids grow, mature, and begin producing eggs. • Eggs in one proglottid are usually fertilized by sperm from a different proglottid, possibly a different individual. • An example is the beef tapeworm, Taenia saginatus. Its primary host is a human and its intermediate host is a cow. Its larvae form cysts in the muscle tissue of the cow.

Section 1 Platyhelminthes Chapter 34 Life Cycle of Beef Tapeworm

Section 1 Platyhelminthes Chapter 34 Life Cycle of Tapeworms Click below to watch the Visual Concept. Visual Concept

Section 2 Nematoda and Rotifera Chapter 34 Objectives • Describethe body plan of a nematode. • Outlinethe relationship between humans and parasitic roundworms. • Describethe anatomy of a rotifer.

Section 2 Nematoda and Rotifera Chapter 34 Phylum Nematoda • The phylum Nematoda is made up of roundworms with long, slender bodies that taper at both ends. • Roundworms are among several phyla of pseudocoelomates. • Roundworms have a digestive tract with two openings. • Most roundworms have separate sexes and are covered by a protective cuticle. • Most species are free-living; some are parasites.

Section 2 Nematoda and Rotifera Chapter 34 Phylum Nematoda, continued Ascaris • The genus Ascaris infects pigs, horses, and humans. • The eggs enter hosts through contaminated food or water, develop into larvae in the intestines, and can infect the lungs. • The eggs are spread in the hosts’ feces. Hookworms • Hookworms are intestinal parasites that feed on blood. • The eggs produce larvae in soil, and the larvae enter hosts through the feet. • Hookworms infect about one billion people worldwide.

Section 2 Nematoda and Rotifera Chapter 34 Phylum Nematoda, continued Trichinella • The genus Trichinella infects humans and other mammals. • Adults live in intestines and larvae form cysts in muscles. • People usually become infected from undercooked pork. • Infection causes the disease trichinosis. Other Parasitic Roundworms • Pinworms, genus Enterobius, are common parasites of humans. They do not cause any serious disease. • Filarialworms infect many people in tropical countries. The most dangerous ones infect the lymphatic system and may cause elephantiasis.

Section 2 Nematoda and Rotifera Chapter 34 Exploration of a Roundworm

Section 2 Nematoda and Rotifera Chapter 34 Phylum Rotifera • Members of the phylum Rotifera are called rotifers. • Most rotifers are tiny, transparent, free-living animals that live in fresh water. • Some can survive without water for long periods. • Although tiny, they are truly multicellular and have specialized organ systems. • They use the crown of cilia around their mouth to sweep food into the mastax. • The digestive, reproductive, and excretory systems empty into the cloaca. • Some species reproduce by parthenogenesis.

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