Introduction to Phylum Platyhelminthes

Flatworms

Phylum PlatyhelminthesThere are hundreds of species of flatworms

that have been identified to date. They are assigned to phylum

Platyhelminthes.Prefix platy means “flat”Suffix helminth means “wormlike”

Phylum PlatyhelminthesFlatworms have a solid body construction

No body cavity between their gut and skinAnimals with no body cavity between their gut

and body wall are termed acoelomate. Acoelomate animals have a limited amount of organ

development and are considered simple animals.

Phylum PlatyhelminthesWhen they evolved, flatworms were the first

animals to have a middle embryonic cell layer, or mesoderm.

Thus, they (and higher phyla) have three layers in their embryo.

Phylum PlatyhelminthesAs you recall,

mesoderm makes possible the formation of muscle and other connective tissues, blood vessels, and organs such as the testes and ovaries.

Phylum PlatyhelminthesAs a result of their mesoderm, flatworms

possess a fair amount of muscle tissue and a reproductive system comparable to that of higher animals.

They do not however, have any blood vessels or circulatory system.

Materials are circulated through their body by diffusion.

Other systems flatworms do not include a respiratory system. Oxygen and carbon dioxide are exchanged across their skin by diffusion.

Phylum PlatyhelminthesFlatworms were the first animals to be

bilaterally symmetrical. They were also the first animals to develop a

distinct head region containing sensory organs.

Animals that have ahead containing a concentration of nerve cells are said to be cephalized.

Cephalization enabled them to adapt to their surroundings more successfully.Note: Zoologists credit them with the first

eyespots

Phylum PlatyhelminthesHaving no coelom nor circulatory system,

flatworms must be thin enough so that dissolved substances, such as oxygen and carbon dioxide, can pass through their solid body by diffusion.

A thin body shortens the distance these substances must travel to reach each cell.

To help shorten the distance food substances must travel, the gut in a flatworm is highly branched.

Phylum PlatyhelminthesPortions of it run close to

practically all of the flatworm’s tissues, giving each cell access to food molecules.

The gut has only one opening – the mouth – through which food enters and undigested food exits. Thus, like the animals we

studied before flatworms, the digestive system is primitive.

Phylum PlatyhelminthesThere are three main classes in phylum

Platyhelminthes:The latter two classes consist of flatworms

that live as parasites.1. Turbellaria2. Trematoda3. Cestoda

Phylum PlatyhelminthesTrematoda and Cestoda are parasites.

This means that they live in and obtain their nutrition from a host organism

They cause untold misery and bring early death to immense numbers of humans, especially in the poorer parts of the world.

Class Turbellaria (Planarians)Flatworms of the class Turbellaria are

described as independent free-living. This means that they move about and feed on

their own.Depending on the species, turbellarians can be

found living in aquatic environments, damp soil, or fresh water.

Dugesia tigrina Tricladida

Class Turbellaria (Planarians)The most commonly studied member of class

Turbellaria is the small freshwater flatworm planarians.

Several species of this flatworm have been identified.

They can be found under rocks and logs in streams and ponds.

They live partly as scavengers and partly as predators. They grow to 20mm or less.

Class Trematoda (Flukes)Flatworms of the class Trematoda are known as flukes.They are all parasitic and may be very damaging to

their hosts. The Asian liver fluke is very common in many parts of

Asia and is responsible for a great deal of sickness, disability and death.

People become infected by eating raw or improperly cooked meat carrying these fluke larvae.

Class Trematoda (Flukes)Other species of flukes invade the lungs, spleen,

intestines, or blood. Schistosomes, or blood flukes, infects at least three

hundred million humans in tropical regions.These tiny worms drill through a person’s skin, enter

the blood stream, and lay hundred of eggs.The eggs can damage or block small blood vessels and

often cause internal bleeding in the bladder and intestine.

Schistosomiasis kills approximately 600 thousand people in Asia, Africa, Latin America, and the Middle East.

Class Cestoda (Tapeworms)Flatworms of the class Cestoda are known as

tapeworms because of their shape.These parasitic flatworms infect nearly

almost all vertebrates.The adult stage lives in the intestine of its

host.It depends upon the host not only for food but

for the digestion process as well, as it lacks a digestive system.

Class Cestoda (Tapeworms)The head of a tapeworm is known as a scolex

and is equipped with one or more kinds of attachment organs, depending upon the species.

The segments making up the rest of the body are called proglottids.

Class Cestoda (Tapeworms)These structures contain both male and

female reproductive organs and produce packets of fertilized eggs.

As a tapeworm grows, it adds proglottids just behind the neck, pushing the older proglottids toward the end. The proglottids break off and pass out of the intestine with the feces.

Class Cestoda (Tapeworms)Tapeworms that can infect humans include

the beef tapeworm and pork tapeworm.People become infected when they eat

undercooked meat containing the larvae of these worms.

Venn DiagramSponges Cnidarian

sCnidarians Flatwor

ms

EndodermEctodermHermaphroditeRegenerationGastrovascular cavity

BilateralMesoderm: AcoelomateOrganOrgan system: reproductiveGanglia: eyespot

Radial symmetryMesogleaNo organs or organ systemsPolyp/medusaTentacles/stinging cells