Phylum PlatyhelminthesThe “flat” “worms”

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Section 21.4

Phylum Platyhelminthes (flatworms):Planarians, flukes, and tapeworms

Flatworms Have Bilateral Symmetry

Figure 21.2

Section 21.4 Figure 21.12

Key features

Flatworms Have Bilateral Symmetry

Section 21.4 Figure 21.12

Diversity

Flatworms Have Bilateral Symmetry

Flatworm: ©Leslie Newman & Andrew Flowers/Science Source; planarian: ©Carolina Biological Supply

Company/Phototake; fluke: ©Volker Steger/Science Source; tapeworm: ©Biophoto Associates/Science Source

Class Turbellariaincludes: Planarians

Class CestodaIncludes: Tapeworms

Class TrematodaIncludes: Flukes

Section 21.4 Figure 21.13

Anatomy

Flatworms Have Bilateral Symmetry, Dorsoventrally Flattened, and lack a skeleton

• They have three germ layers:• Ectoderm: Outside

• Endoderm: Inside

• Mesoderm: Middle layer of tissue between the ectoderm and the endoderm

• Though Flatworms have three germ layers they are acoelomates and have no body cavity

(think of your normal sandwich)

• Flatworms do have true organs and primitive organ systems that are used for digestion and excretion

• Flatworms are also the most primitive organisms to show cephalization.

• The head region of Flatworms contains a concentration of nerve tissue called ganglia (singular=ganglion) that resembles a primitive brain.

Cephalization

They have two nerve cords that run from the ganglia in the head region along the ventralside of the worm to the tale region

In the head region of Flatworms are two eye-spots

They also have lobes on the side of their head called auricles (Primitive ear)

Auricles

Feeding • Flatworms have a mouth/anus which is

connected to the gastrovascular systemthrough a long muscular pharynx.

• Small invertebrates or the remains of dead animals are taken into the mouth/anus by the muscular pharynx

• The food is then digested in the highly branched gastrovascular cavity

• The nutrients moves from the gut into the body cells by diffusion

Respiration

• Respiration occurs by diffusion due to acoelomate body plan (super flat)

Internal Transport

• Nutrients and oxygen in the gut are simply absorbed into the body cells by the process of diffusion

Excretion

• Most undigested food is released directly out of the anus/mouth: Pharynx

• A flame cell is a specialized excretory cell

• Flame cells function like a kidney, removing waste materials.

• The beating of cilia resembles a flame, giving the cell its name.

Nerve Response Flatworms are able to sense and respond to at least

three forms of stimuli: Sense and respond to light: The eye-spots can detect

light and allow the Flatworms to respond to it Sense and respond to chemicals: Pits on the side of

their head regions can sense chemicals in the water and allow the Flatworms to respond (like “smelling”)

Sense and respond to touch: The auricles on either side of the head region can sense touch and allow the Flatworms to respond

• The ganglia in the head region relay messages from the sensory organs down the nerve cords to the rest of the body. The nerve cords can control muscles in the body which allow the Flatworms to move or eat.

Auricles

Movement

• The flatworms move across a surface using cilia on their ventral surface

• They can also move by contracting circular and longitudinalmuscles that lay just below the ectoderm. These muscles are controlled by the nerve cords.

Reproduction

• Asexual Reproduction:

• Flatworms can asexually reproduce through a process called fission. The anterior and posterior ends hold a surface and the midsection constricts. This results in two new flatworms, one from the anterior end of the original flatworm and the other from the posterior end of the original flatworm.

Reproduction

• Flatworms can also regenerate parts they have lost.

Regeneration video

Planaria Regeneration

• Planarians will spontaneously detach the tail end of their bodies and each half will regenerate into a full size flatworm

• Planarians can be cut either transversally (shown above) or dorsally and most will regenerate into a full size worm

Super Cool Fact: the smallest piece of planarian to ever regenerate in a lab into a new planarian was 1/279th of a planarian! That’s approximately 10,000 cells!

• Sexual Reproduction:• Flatworms are hermaphroditic

• After two flatworms have copulated they release sacs of fertilized eggs and attach them to a surface

Parasitic Flatworms: Tapeworms

• The tapeworm has an anterior end called a scolex with complicated hooks for attaching to the intestines of its host.

• The tapeworm does not have a mouth or digestive system. Instead they bath in the pre-digested fluids of their host and absorb nutrients directly into their body cells

Platyhelminthes: Cestoda (Tapeworm)General Anatomy

Taenia

Proglottids

Hooks

Scolex

Suckers

Rostellum

Lifecycle of a Tapeworm

Summarizing Identifying Characteristics of the Phylum Platyhelminthes

• Acoelomate

• Exhibit bilateral symmetry

• Have a bilateral nervous system with cephalization at the head end. Some species exhibit eyespots sensitive to light

• Possess a Gastrovascular Cavity (GVC) and primitive organ systems for digestion and excretion

• Do not have a circulatory or respiratory system but do take in O2 through their body surface (integumentary exchange)

• Hermaphroditic : can reproduce sexually (do not self fertilize) or asexually by regeneration

• Are motile and utilize an undulation form of motion

• Found in marine, freshwater and damp terrestrial habitats

Parasitic FlatwormsThe pork tapeworm – Taenia solium

• Infects pigs and humans

• Lives in the intestine of its host and passes eggs through the feces

• Highly adapted to constant internal environments

• Lacks sensory organs, coordination for mobility and a digestive system (more room for reproductive structures!)

• Have a modified epidermis “tegument” which protects against the digestive enzymes and the immune systems of the host

• Can reach 7m in length in humans!

• Are flat and long which maximizes absorption of nutrients from the host

The tapeworm “up close and personal….”

• The head end has a scolex with four suckers (two seen here) that help it attach to the intestine of its host

• The body is separated into sections called “proglottids” that house highly developed reproductive systems (darkened areas) capable of producing hundreds of thousands of eggs and sperm

Super cool Flatworm Fact• the largest tapeworm ever reported was in a sperm

whale and was 30 meters in length!

Tapeworm

Scolex