Worms Mollusks Chapter 27 Flatworms Phylum Platyhelminthes Bilateral

Worms & Mollusks Chapter 27

Flatworms Phylum Platyhelminthes Bilateral symmetry All 3 germ layers Cephalization One digestive opening Acoelomate

3 groups of flatworms • 1)turbellarians – Class Turbellaria • 2)flukes – Class Trematoda • 3)tapeworms – Class Cestoda

turbellarians • Free living

Flukes • Parasitic • Live in internal organs

tapeworms • intestinal parasites of vertebrates, where they absorb predigested nutrients (they have no digestive tract).

Roundworms • Phylum Nematoda • Bilateral symmetry • All 3 germ layers • Cephalization • 2 digestive openings • Pseudocoelomate • Most are parasitic

Roundworms & Disease • Trichinosis (from undercooked pork) • Filarial worms (elephantiasis)

• Ascarid worms (intestine parasites) • Hookworms (25% of world population)

Segmented worms • Phylum Annelida • Bilateral symmetry • All 3 germ layers • Cephalization • 2 digestive openings • Coelomate • segmented

• Classes – Oligochaeta • earthworm – Polychaeta • many bristled worms – Hirudinea • leeches

Mollusks Phylum Mollusca Soft bodied invertebrates that usually have an internal or external shell. Bilateral symmetry All 3 germ layers Coelomates Cephalization 2 digestive openings “open” circulatory system

Groups of Mollusks • 1)Gastropods: (class Gastropoda) snails, slugs, limpets, nudibranchs • Move using a muscular “foot”

• 2)bivalves: (class Bivalvia) clams, mussels, oysters, scallops • Have 2 shells held together by powerful muscles.

• 3)cephalopods: (class Cephalpoda) octopus, squid, cuttlefish, nautilus • Soft bodied mollusk with head atteched to foot. Foot is divided into tentacles • Most intelligent invertebrates

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