Thursday, May 04, 2006

26.3 Platyhelminthes

Structure and Function of Flatworms
*The phylum Platyhelminthes inclides organisms called flatworms.
*They are more complex than sponges but are the simplest animals with bilateral symmetry.
*They are acoelomates (no body cavity)
*They exhibit cepholization (have a head)
Class Turbellaria-Planarians
Digestive and Excretion in Planarians
*The most familiar turbellarians are the freshwater planarians.
*Planarians feed on decaying plant or animal matter and small 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 where water is excreted.
Nueral Control in Planarians
*The Planarians nervous system is more complex than the nerve net of cnidarians.
*The cerebral ganglia serveas a simple brain.
*A planarian's nervous system gives it the ability to learn.
*Planarians sense light with eyespots.
*Others sensory cells resoond to touch, water currents, and chemicals in the environment.
Reproduction in Planarians
*Planarians are hermaphrodites that can reproduce sexually or asexually.
*During asexually reproduction, their body undergoes fission and the two halves regenerate missing parts.
Class Trematoda
*The class Trematoda consist of parasistic flukes.
*Some are endoparasites;others are ectoparasites.
Structure of Flukes
*A Fluke clings to the tissue of its host by on anterior sucker and a ventral sucker.
*A Fluke's nervous system is similar to a planarian's but simpler.
Reproduction and Life Cycle of Flukes
*Most Flukes have highly developed reproductive systems and are hermaphroditic.
*Flukes have complicated life cycles the involve more than one host species.
*For example, the trematocle blood flukes of the genus Schistosome use humans as a primary host. They can cause schistosomiasis.
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 w/a scoley.
*The body is a series of many.
Sections called proglotius
*Tapeworms have no light-sensing organs,no results,no gastrovascular cavity,and no digestive organs.
Reproduction and Life Cycle of Tapeworms
*Nearly all tapeworms are hermaphrodites.
*New proglottius are added to the front of the tapeworm. Older proglottius grew,mature, and begin producing eggs.
*Eggs in one proglottius,are usually fertilized by sperm from a different proglottius,possibly a different individual.
*An example is the beef tapeworm, Tuenia 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.

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