WormsChapter 27

Flatworm characteristics

• Soft, flattened bodies

• Simplest animals with three embryonic germ layers

• Bilateral symmetry

• Cephalization: head formation

• Acoelomate: fluid filled body cavity made of mesoderm tissue

Flatworm feeding styles

• Carnivorous: feed on tiny aquatic animals

• Scavengers: feed on recently dead animals

• Parasites: blood, tissue fluids, cells within host’s body– Lack complex digestive system because host

partially digests food– Ex: tapeworm (no digestive system at all)

Feeding• Mouth only• Pharynx:

muscular tube, extended from mouth

• Digested food and nutrients absorbed in the digestive cavity

Branched digestive system

translucent species Aquaplana/Paraplanocera sp. extends throughout the entire body

Photo by Robert F. Bolland). 

Respiration, Circulation, & Excretion

• Flat and thin body shape requires little circulation.

• Diffusion transports oxygen and nutrients to internal tissues

• Diffusion removes CO2 and waste• Flame cells: specialized cells that remove

excess water from body– helps to remove metabolic waste (ammonia

and urea)

Flame-bulb excretory system

Response

• Ganglia: group of nerve cells with two long nerve cords

• Eyespots: group of cells that can detect changes in light intensity

Section 27-1

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

Figure 27–3 

Planarian with noticeable eyespots

Eyespots of a planarian

Movement

• Cilia on epidermal cells

• Nerve controlled muscle cellsMuscle filament staining of a turbellarian flatworm reveals a meshwork of longitudinal, circular and diagonal muscles. The large, bright ring with muscle fibers radiating outward is the muscular pharynx plicatus

Image by M. D. Hooge and S. Tyler, Department of Biological Sciences, University of Maine).

Groups of flatworms

• Turbellarians: free living, marine & freshwater

• Flukes: parasitic, infects internal organs of host

• Tapeworms: parasitic, adapted to life inside host’s intestines

Turbellarian

Brightly colored turbellarian

Turbellarian reproduction

Asexual - fission

Section 27-1

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

If they get into 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 burrowing through the skin.

Primary host (human)

Intermediate host(snail)

Humanintestine

Adultfluke

Embryo

Ciliatedlarva

Tailedlarva

Life cycle of fluke

Species: Schistosoma Mansoni

Brachylaima cribbi fluke

• (A) adult worm

• (B) two suckers and spiny coat

• (C) sperm deposited from pore

Tapeworm

Proglottid: tapeworm segment

• Proglottid with fertilized eggs bursts open to release zygotes

Parasitic tapeworm from fish

Human tapeworm

• Scolex: head with suckers or hooks

Intermediate hosts• Tapeworm larvae burrow into cow or fish

muscle tissue, which infects the animal that eats this tissue.

Rat tapeworm

Roundworms

C. elegans

Roundworm characteristics

• Unsegmented

• Tapering ends

• Variety of environments and climates

Roundworm characteristics

• Pseudocoelomate: body cavity lined partially with mesoderm tissue

• Two digestive tract openings, mouth and anus

Length of roundworm

• Microscopic to 1 m in length

Feeding

• Predators with grasping mouthparts

• Spines to catch and eat small animals

• Detritovores

parasitic hookworm

Respiration, Circulation, & Excretion

• Gas exchange through body walls

• Metabolic waste excretion through body walls

• No internal transport system

Response and movement

• Simple nervous system of ganglia

• Hydrostatic pressure to move around

• Muscular contractions to move through water

Reproduction

• Sexually

• Male or female reproductive system

• Internal fertilization: male deposits sperm into female

• Complex life cycles in parasitic roundworms

Guinea worm disease

Pinworm infection in child’s intestine

Trichinella life cycle

Filarial worms

• Primary cause of elephantiasis condition where parts of the body are swollen

• Requires arthropod intermediate host to complete life cycle

Ascarid worms

Hookworm life cycle

C. elegans and research

• First multicellular animal to have entire genome sequenced

• Every body cell’s developmental pathway is also recorded.

Segmentedworms

Annelid characteristics

• Segmented bodies

• True coelom: body cavity lined with mesoderm tissue

• Mouth and anus

Section 27-3

Anus

Clitellum

Circular muscle

Longitudinalmuscle

Nephridia Ganglia Ringvessels

Reproductiveorgans

Ventralblood vessel

Ganglion

Brain

Mouth

Dorsalblood vessel

CropGizzardBody segments

Setae

The Anatomy of an Earthworm

Figure 27–16 

Feeding and Digestion

• Filter feeders

• Predators

• Pharynx that may be collect or capture (if jawed) food

• Food moved into esophagus, crop for storage, gizzard for grinding, digestive tract for absorption

Circulation

• Closed circulatory system: blood contained within network of blood vessels

• Two major blood vessels running from head to tail– Dorsal vessel: blood towards head, functions

like heart– Ventral vessel: blood towards tail

Respiration

• Gills: specialized for underwater gas exchange in aquatic worms (ex: tube worms)

• Terrestrial annelids perform gas exchange through their moist skin.

Tubeworms

Excretion

• Nephridia: organs that filter fluid in the coelom

Response

• Most have well-developed nervous systems with brain and nerve cords

• Marine annelids have the best adapted nervous system.

Movement

• Marine annelids may have parapodia, paddlelike appendages.

• Hydrostatic skeleton: longitudinal and circular muscles– Longitudinal: contract to make worm shorter

and fatter– Circular: contract to make worm longer and

thinner

Reproduction

• Sexual reproduction

• External fertilization

• May be single sex or hermaphrodite

• Clitellum band thickens, secreting mucus ring with eggs where sperm are deposited

• Fertilized eggs slips off with the mucus ring, which forms a cocoon where the young worms hatch later.

Clitellum

Earthworm cocoon

Oligochaetes: streamlined bodies, few setae

Oligochaete worm with filamentous algae

Groups of annelids

• Leeches: external parasites that suck blood and body fluids from host

Medical leech used in surgeries

Leeches release a chemical that prevents the prey from forming scabs. A leech bite is less painful than it is just messy with the profuse bleeding.

Polychaetes

• Marine annelids with paired, paddlelike appendages with setae

Earthworms help compost matter.