Platyhelminthes (Flatworms)• General characteristics:

– Body plan: acoelomate, bilateral symmetry

• Habitat: aquatic• Nutrition: many are parasites,

feed off host(blood, tissue, or predigested materials from host’s intestines) feed on dead animals

• Feeding: Planarians extend pharynx from mouth

• Examples: Turbellaria (Planarian)Trematoda (Fluke)Cestoda (tapeworm)

• Digestion: extracellular, food is pumped into digestive cavity or gut and cells digest and absorb nutrients, digested food is then diffused into the other body tissue

• Respiration and Circulation: Diffusion of gases through the skin, skin must remain moist

• Excretion: diffusion through the body wall– planarians have flame cells to

excrete water and wastes through pores

Freshwater 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

The Anatomy of a Flatworm

• Nervous: Planaria: ganglia (mass of nervous tissue), eyespots, sensory lobes respond to touch and chemicals– Demostrates cephalization, nervous tissue is

concentrated in the head region.

• Reproduction: Hermaphrodites– Asexual reproduction fission (worm splits into two )– Sexual reproduction exchange sperm, internal

fertilization– Parasitic worms require hosts to reproduce

Tapeworm

Section 27-1

Flukes mature and reproduce sexually in the blood vessels of human intestines. Embryos are released and passed out with feces.

Once in the water, embryos develop into swimming larvae that infect an intermediate host (snail).

After asexualreproduction, newlarvae are releasedfrom the snail into the water. They then infect humans, the primary host, by boring through their skin.

Primary host (human)

Intermediate host(snail)

Humanintestine

Adultfluke

Embryo

Ciliatedlarva

Tailedlarva

Other information:Other information:Schistosome Life CycleSchistosome Life Cycle

Shistosomiasis

• Mobility:– Cilia for gliding– Muscle cells controlled by nervous system to react

to stimuli

Nematoda (Roundworms)• General characteristics:– Pseudocoleomate – long and slender, tapered at both ends– Roundworms are protostomes– bilateral symmetry– Body is covered in a complex cuticle

• Habitat/Nutrition: Parasites that live in the intestines

• Feeding– Both mouth and anus connected by

digestive tube• Examples: hookworms, pinworms, guinea

worms

• Digestion: takes place in digestive cavity, digested food diffuses into other body tissue. – May also have digestive sacs that food is pumped into for

digestion.• Circulation and Respiration: Diffusion of gases through the skin• Excretion: Canal system, diffusion through body wall or out

through mouth • Nervous: simple nervous system, with a main ventral (belly)

nerve cord and smaller dorsal (back) nerve cord

• Reproduction:– Sexual, internal

fertilization– usually separate sexes– Genders differ in size,

shape and color– Parasitic require host

• Over 1/3 of human population potentially infested with these worms!

• Mobility: Muscles along body wall for movement

Hookworm egg Male Whipworm

Pinworm eggs

Annelidia• General characteristics:– Coelomates– Known for segmented body– Earthworms, leeches, and marine worms are common

examples.– Best developed sense organs in free living marine

organism

blood vesselsmuscle

heartsbrain

mouthdigestive tract

nerve cordsegment

• Habitat: aquatic and moist soil• Nutrition: heterotrophic, filter feeders,

parasites, scavengers– Feed on recently dead animals, blood, host’s body

• Feeding: Pharynx used to obtain food– Carnivorous species have two or more sharp jaws

• Digestion: extracellular, takes place in intestine– Earthworm: pharynx → esophagus→ crop

(storage) → gizzard (ground) → intestines (absorption)

• Circulation: – closed (blood contained in vessels)

• Respiration:– Aquatic: breathe through gills– Land dwellers: diffusion through skin (must stay

moist) typically secrete a mucus to keep skin moist

• Excretion: – Digestive wastes pass out

through the anus– Cellular wastes containing

nitrogen is passed out by the nephridia (simple kidney)

Section 27-3

Anus

Clitellum

Circular muscle

Longitudinalmuscle

Nephridia Ganglia Ringvessels

Reproductiveorgans

Ventralblood vessel

Ganglion

Brain

Mouth

Dorsalblood vessel

CropGizzardBody segments

Setae

• Nervous:– Well developed nervous systems consisting of brain

and spinal cord– Two or more pairs of eyes, sensory tentacles,

chemical receptors and statocysts that help detect gravity

• Reproduction–Most sexually– Some use external fertilization and have

separate sexes– Earth worms and leeches are hermaphroditic,

but rarely fertilize their own eggs

• Mobility– Two major groups of

muscles that function as part of the hydrostatic skeleton

– Longitudinal and circular muscles, move by alternating contractions

– Marine worms have parapodia to use for swimming and crawling