Phylum Platyhelminthes1

Phylum Platyhelminthes

Flatworms

Flukes

Tapeworms

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Phylum Platyhelminthes

The phylum consists of four classes– Turbellaria – Trematoda– Cestoda

Triclad Flatworm

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Reproduction

Almost all are simultaneous hermaphrodites

Parasitic species (flukes and tapeworms) have complex lifecycles, with various hosts and several different larval stages

Incredible powers of regeneration

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Nervous System

Free-living species usually have a well-developed sensory system

Parasites generally have less elaborate systems.

WHY?

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Taxonomic Summary

Phylum Platyhelminthes (Flatworms)– Class Turbellaria– Class Cestoda– Class Trematoda– Class Monogeneans

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Class Turbellaria

Most are free–living, tiny and inconspicuous – Primarily marine and

freshwater Free-living

Flatworm

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Flatworm Body Plans

Exhibit bilateral symmetry, acoelomate, and are triploblastic

Parenchyma tissue

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Digestion and Excretion

Mouth is usually located at the anterior end or mid-body on ventral surface

Excretory system – Contains flame

bulbs

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Regeneration

Many species posses remarkable powers of regeneration and repair wounds

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Turbellarian Lifestyles

Most are benthic

Posses several nerve cords with a centralized ganglia (brain)

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Turbellarian Habits

Some species also have a protrusible pharynx that captures food and transfers it into the mouth– Can be carnivores or scavenge on dead animals and

detritus

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Reproduction

Reproduction occurs with the reciprocal exchange of sperm

Fertilized eggs are released and usually develop directly into flatworms– Muller's larva

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Class Cestoda (Tapeworms)

Defining characteristics– Scolex– Proglottids

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Proglottids

Proglottids– Each animal can be

3,000 – 4,000 per animal

– Amazing reproductive output

– Each may contain several ovaries and 1,000 distinct testes

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Beef tapeworm

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Problems of a parasitic existence

Reproduce within the definitive host Get fertilized eggs out of the host Contact a new and appropriate host Obtain entrance into the host Locate the appropriate environment within the host Maintain position within the host Withstand an often anaerobic environment Avoid digestion or attack by the hosts immune system Avoid killing the host, at least until reproduction is completed

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Class Trematoda (Flukes)

All are external and internal parasites of other animals

Leech-like bodies with a sucker at each end

Have a gut and well-developed reproductive system, never segmented

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Trematoda Lifecycles

The lifecycle is complex with up to 4 different hosts and several larval types

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Trematodes of Concern

Schistosomiasis – Deadly prominent disease in

many regions of the world – Bores into the skin of the

definitive host and travels in the circulatory system to the heart the lungs and the kidneys where it feeds and grows

– Inflammation is caused by eggs becoming trapped in the hosts tissues

Chinese liver fluke– Lives in the bile duct of

humans, cats, and dogs (1-2 cm.)

– Goes through two intermediate hosts

– Infection occurs from ingestion of raw fish