Platyhelminthes:

Flatworms

Phylum Platyhelminthes• Phylum Platyhelminthes - Flatworms• Soft, unsegmented, flattened worms that have tissues and internal

organ systems• Have three embryonic germ layers• Simplest animals with bilateral symmetry• Exhibit cephalization• More developed organ systems than Poriferans and Cnidarians• Acoelomates – “without a coelom” – a coelom is a fluid-filled body

cavity that is lined with mesoderm

Anatomy of a FlatwormFreshwater flatworms have simple ganglia and nerve cords that run the length of the body. The excretory system consists of a network of tubules connected to flame cells that remove excess water and cell wastes.

Most flatworms are hermaphrodites, having male reproductive organs (testes) and female reproductive organs (ovaries) in the same organism.

Flatworms use a pharynx to suck food into the gastrovascular cavity. Digested food diffuses from the cavity into other cells of the body. Eyespots in somespecies detect light.

Eyespot

Head

Gastrovascularcavity

Ganglia

Nervecords

Excretorysystem

Mouth Pharynx

Ovary

Testes

Excretorytubule

Flame cell

Feeding in Flatworms

• Can be carnivores or scavengers

• Gastrovascular cavity with one opening at the end of a pharynx

• Digested food diffuses to other body tissues

Respiration, Circulation, and Excretion in Flatworms

• No circulatory system or respiratory organs – rely on diffusion to transport nutrients and waste

• Expel undigested material through pharynx

• Flame cells filter and remove excess water and wastes from body

Response in Flatworms

• Nervous system much more developed than Sponges or Cnidarians

• Definite head

• Simple ganglia control nervous system

• Two nerve cords run from ganglia along both sides of body

• Ocelli, or eyespot, detects changes in the amount of light in their environment

Movement in Flatworms

• Cilia on epidermal cells

• Muscle cells allow to twist and turn to be able to react rapidly

Reproduction in Flatworms

• Sexual – hermaphrodites – two worms join as a pair and deliver sperm to each other

• Asexual – fission – splits in two

Penis Fencing in Flatworms

Groups of Flatworms

• 3 major classes of flatworms– Class Turbellaria– Class Trematoda– Class Cestoda

Class Turbellaria

• Free-living flatworms found in marine or freshwater

• Bottom dwellers

• Planarians – 1/2 cm to 6 m long

Examples of Class Turbellaria

Parasitic Flatworms

• Feed on blood, tissue, or cells; some just absorb nutrients through their “skin”

• Examples: Tapeworms, Flukes

• Very complicated sexual reproductive life cycles

Class Trematoda – Flukes

• Parasitic and infect internal organs of host

• Blood flukes Schistosoma mansoni

• Humans – primary host – burrow through exposed skin

• Many more eggs than free-living flatworms

• Eggs pass out with feces of host

• Tropical areas – Southeast Asia, Africa

Examples of Class Trematoda

Class Cestoda – Tapeworms

• Long, flat parasitic worms• Head of an adult called a scolex – used to attach to

intestinal wall where it absorbs nutrients• 18m long, host may lose weight and become weak• Behind scolex is a narrow neck region that produces

proglottids that break off the tail and rupture in intestine or feces and release embryos

• Eggs consumed by intermediate host• Larvae form cysts in muscle tissue• Tissue then consumed by humans

Life Cycle of Tapeworm

Examples of Class Cestoda

Nematoda:

Roundworms

Phylum Nematoda• Phylum Nematoda - Roundworms• Slender, unsegmented worms• Pseudocoelom – body cavity only partially lined with

mesoderm• Two openings, mouth and anus, in digestive system –

“tube-within-a-tube” – food moves only in ONE direction• Microscopic in size to 1m long• Can be decomposers or dangerous parasites• Might be the most common multicellular animal• Most are free-living, found everywhere

Feeding in Roundworms

• Mainly carnivores, some eat algae or decaying organic matter

Respiration, Circulation, and Excretion in Roundworms

• Exchange gases and excrete metabolic wastes through diffusion

• No internal transport system

Response in Roundworms

• Simple nerve network, ganglia

Movement in Roundworms

• Muscles and fluid = hydrostatic skeleton

Reproduction in Roundworms

• Sexual – internal fertilization

• Ascarid worms – complex life cycles involving two or three different hosts or several organs within a single host

• Stomach – lung – throat – intestine - feces

Examples of Nematodes

Roundworms and Human Disease

• Hookworms – similar to Ascaris lifecycle, except they enter through the skin

• Filarial Worms – elephantiasis (swelling of body) – transmitted by mosquitoes

ORGANISM

Trichinella

Filiarial worms

Ascaris

Hookworms

ORGANISM

Trichinella

Filiarial worms

Ascaris

Hookworms

DISEASE

Trichinosis

Elephantiasis

Ascaris infection/infestation

Hookworm infection/infestation

DISEASE

Trichinosis

Elephantiasis

Ascaris infection/infestation

Hookworm infection/infestation

ROUTE OF INFECTION

Eating undercooked meat containing larval cysts

Bite of insect carrying filarial worms

Eating unwashedfood contaminatedwith Ascaris

Bare skin in contact with contaminated soil

ROUTE OF INFECTION

Eating undercooked meat containing larval cysts

Bite of insect carrying filarial worms

Eating unwashedfood contaminatedwith Ascaris

Bare skin in contact with contaminated soil

DESCRIPTION

Larvae burrow into tissues of host, causing pain

Worms block passage of fluids within lymph vessels, causing tissues to swell

Worms in intestine block normal passage of food and absorption of nutrients

Worms attach to intestinal wall and suck blood, causing weakness and poor growth

DESCRIPTION

Larvae burrow into tissues of host, causing pain

Worms block passage of fluids within lymph vessels, causing tissues to swell

Worms in intestine block normal passage of food and absorption of nutrients

Worms attach to intestinal wall and suck blood, causing weakness and poor growth