Platyhelminthes

By: Thomas Sullivan

PlatyhelminthesFlatworms: Flatworms are only a few

millimeters thick. Flatworms are soft, flat, have

tissues and internal organ systems.

They are the simplest animals to have 3 embryonic germ layers, bilateral symmetry, and cephalization.

The three embryonic germ layers include ectoderm, endoderm, and mesoderm.

Facts on Flatworms

All Flatworms: Have Acoelomate, which means without coelom. A coelom is a fluid filled body cavity that is lined with mesoderm. Have only one body cavity, which is the digestive cavity is lined with

endoderm. Have Bilateral symmetry, which means that flatworms has 2 well-

formed sides that can be identified as left and right. The cells of a flatworm are close to the external environment that allows

materials to pass easily into and out of their bodies. Have Organ systems for digestion, excretion, response, and

reproduction.

Have Reliance on diffusion for respiration and circulation.

How & What Flatworms Eat Free-living flatworms can be carnivores

that feed off of tiny aquatic animals or they can be scavengers that feed off of recently dead animals.

Flatworms have a gastrovascular cavity with a single opening (a mouth) where food and waste can pass.

A muscular tube called the pharynx near the mouth, is extended out of the mouth, then pumps food inside.

The food is digested and is diffused into all other body tissues from the gastrovascular cavity.

E. epidermis

D. nematocysts

C. mesoglea

B. gastrodermis

A. gastrovascular cavity

Respiration, Circulation, and Excretion

A circulatory system is not needed by many flatworms because of their size.

Instead, they rely on diffusion to transport oxygen and nutrients to their internal tissues, and to remove carbon dioxide and other wastes.

Flatworms have no gills and no heart, blood vessels, or blood.

Some flatworms have flame cells that function in excretion. They are cells that filter and remove excess water, ammonia

and urea from body.

Response

More Complex than cnidarians or sponges.

Ganglia; Groups of nerve cells that control the the nervous system.

Two long cords run down sides of body and shorter nerves run across.

Eyespot; detects light change. Flatworms have specialized cells that

detect stimuli. The nervous system locates food.

Movement

Flatworms move in two way. Cilia on their epidermal cells help

them glide through the water and over the bottom of a stream or pond.

Muscle cells controlled by the nervous system allow them to twist and turn so that they can react quickly to environmental stimuli.

A. ciliated epidermis

Reproduction Flatworms are hermaphrodites that

reproduce sexually. Hermaphrodite is an individual that has

both male and female reproductive organs. During a sexual reproduction, two worms

deliver sperm to each other. Eggs are laid in clusters and hatch within a

few weeks. Asexual reproduction takes place by

fission. An organism splits in two and grows new parts.

Broken pieces form new worms.

Groups of Flatworms

Flatworms are enormously diverse group with many different worms. The three main groups are turbellarians, flukes, and tapeworms.

Tubellarians- Member of class Turbellaria Tubellarians are Free-living flatworms, live in marine or fresh water.

Flukes- Member of class Trematoda Flukes are parasitic flatworms. Most flukes infect the

internal organs of their host. Tapeworms- Member of class Cestoda Tapeworms are long, flat, parasitic worms that are adapted

to life inside the intestines of their hosts.