Worms and Mollusks

1. Worms and Mollusks Biology 112

2. Flatworms • Phylum Platyhelminthes • Soft, flattened worms with tissues and internal organ systems • The simplest animals with three germ layers, bilateral symmetry and cephalization • Acoelomates • No coelom between the tissues of flatworms • A coelom is a fluid-filled body cavity that is lined with tissue in the mesoderm

3. Body Functions of Flatworms • All use diffusion for distribution of nutrients and gases • Some have specialized flame cells used to remove excess water from the body • Contain a digestive cavity with one opening – a mouth • A muscular tube, called a pharnyx, pumps food into the digestive cavity

4. Classification of Flatworms • Tubellarians • Free-living flatworms, can live in fresh and salt water • Ganglia control the nervous system • Some contain an eyespot to detect light • Reproduce asexually and sexually • Planarians are the most familiar species

5. Classification of Flatworms 2. Flukes • Parasitic • Infect the internal organs of their host • Reproduce sexually in the primary host and asexually in the intermediate host

6. Classification of Flatworms 3. Tapeworms • long, flat parasitic flatworms • Live in the intestines of their hosts

7. Roundworms • Phylum Nematoda • Unsegmented worms that have pseudocoeloms • A body cavity that lies between the endoderm and the mesoderm tissues • Also have a digestive system with a mouth and an anus

8. Roundworms and Body Functions • Rely on diffusion for respiration, circulation and excretion • Muscles and fluid in the pseudocoelom act as a hydrostatic skeleton • Reproduce sexually by internal fertilization

9. Characteristics of Roundworms • Most are free-living but some are parasitic • Parasitic include: • Trichinosis-causing worms • Filarial worms • Ascarid worms • Hookworms

10. Annelids • Phylum Annelida • Earthworms • Segmented bodies • Have a true coelom that is lined with tissue from the mesoderm • Internal walls called septa separate the segments that divide the body • Most segments are similar to one another • A few are specialized with eyes or antennae • Many have bristles called setae attached to each segment

11. Internal Structure of Annelids • Complex organ structures • Many contain a pharynx that is used to get their food • Food moves through the crop, where it is stored • Then moves to the gizzard, an organ that grinds it into smaller pieces • Have a closed circulatory system in which blood is contained in vessels

12. Earthworms • Class Oligochaeta • Hermaphroditic • Has both male and female reproductive organs • When eggs are ready to be fertilized, a clitellum, a band of thickened segments, secretes a mucous ring in which fertilization takes place • Streamlined body with few setae • Can live in fresh water or soil

13. Mollusks • Phylum Mollusca • Soft-bodied animals that often have an internal or external shell • Many have a larval stage called a trochophore • Can be herbivores, carnivores, detritophores or parasites

14. Body Parts of a mollusk • Foot • Used for crawling, burrowing and capturing prey • Mantle • A thin layer of tissue that covers most of the body • Visceral mass • Makes up the internal organs • Shell • Glands secrete calcium carbonate

15. Body systems of the mollusk • Either closed or open circulatory systems • When it is closed, blood is pumped through vessels into a large saclike structures called sinuses • When it is open, blood is contained in blood vessels throughout the body • Nervous systems varies from simple to complex

16. Classification of mollusks • Gastropods • Includes pond snails and land slugs • Single-shelled or shell-less • Move by using a muscular foot on their left ventral side • Use a flexible tongue called a radula to eat • Bivalves • Clams, oysters, mussels and scallops • Two-shells held together by muscles • Cephalopods • Includes octopus and squid • Soft-bodied mollusks in which the head is attached to a single foot • Foot is divided into tentacles • Small internal shells or no shell at all • Have complex sense organs