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Belong to the subregnum bilateria

20,000 species

Habitats: marine, freshwater, damp terrestrial habitats

Some free living, some parasitic

Subregnum Bilateria

The supergroup containing all animals except sponges, cnidarians and ctenophores

Flatworm Anatomy & Physiology

Bilateral symmetry




Organ system level of classification: distinct nervous system organized by ganglia

Incomplete digestive system

Reproduction: sexual and asexual

Excretory system

Flatworm: Excretory system

Most basic organism to possess this

Organized through flame cells

Osmoregulation, elimination of nitrogenous waste


Balance of salt and water within an organism

Class Turbellaria: The Planarians

Mostly free-living (not parasites) in fresh water

Mouth on ventral surface with extendable pharynx


Sexual - fertilization

Asexual - fission


Ciliated epithelium - integral part of locomotion

Triangular head

Eye spots - light sensitive structures

Auricle - Olfactory structures

Trematoda: The Flukes

All parasitic

Most adults are vertebrate endoparasites

Multiple species parasitize humans

Most go through 2-3 species throughout their life cycle

Have oral sucker to attach to host

The Flukes

Mouth that leads to pharynx, which divides into intestinal caeca of the gastrovascular cavity


Capable of sexual reproduction

Cestoda: The Tapeworms

All endoparasitic

So used to living in other organisms, lack their own digestive system

Usually have 2 organism life cycle

Cestoda: The Tapeworm Anatomy

Segments called proglottids

Scolex acts as a holdfast

Spread by releasing proglottids while most of the organism remains attached. These are full of eggs

Phylum Nematoda



Bilateral pseudocoelomates

25,000 - 500,000 species

Non-living cuticle

Have an organ system level of organization

Capable of sexual reproduction and infernal fertilization

Nematode: Pseudocoelomates

Have a body cavity

Room for organs

Complete digestive system

Allows for hydrostatic skeleton - gives nematodes great mobility


Round worms

Body covered with flexible cuticle made up of collaged

Dioecious: distinct male and female

Female larger than males

Males have spicules near anus

Ascaris lumbricoides common intestinal parasite in South East, US


Mature in small intestine

Larvae burrow through skin of new host, find way to intestine via heart, lungs and esophagus

Trichina Worms

Cause of trichinosis - usually caused by eating undercooked pork

Pin Worms

Most common nematode parasite in US

30% of children infected

Adults live in large intestine

Crawl out of anus at night and bite down, lay eggs

Filiarial Worms

Usually spread to vertebrates from mosquitoes

Microfilarids circulate in bloodstream until mosquito bites

Responsible for heart worm in dogs

Cause Elephantitis in humans

Phylum Mollusca



More than 90,000 species

Marine, freshwater & terrestrial

Evolved in Precambrian

Protostome coelomates

Triploblastic, true coelum

Bilateral symmetry

Organ-system organization

Sexual reproduction, usually dioecious

Covered with protective, muscular mantle

Class Polyplacophora: Chitons

Marine animals that live on rocks close to shore

Scrape algae and diatoms off or sufaces with radula

Hold onto rocks with suction from their muscular foot

Breath through lateral gills

Eight hard plates along dorsal surface for protection

Class Gastropoda

Orders: snails, slugs, whelks, conchs, sea hares

Gastropod Anatomy & Physiology

Torsion (twisting) of visceral mass has caused asymmetry

Most have univalve, coiling shell; many have hard operculum to cover the opening

Well developed head

Rough radula for feeding

Flat foot to feed on surfaces

Most are hermaphroditic

Breathe through gills and mantle

Have 2 or 3 chambered heart

Bivalva: Clams, Oysters, Scallops and Mussels

Marine and freshwater

Sedentary filter feeders

Lack distinct head

2 shell, laterally compressed and attached with a hinge ligament

Umbo is the bump at the base - growth origin

Inside of shell lined with nacreous layer

Bivalva Anatomy & Physiology

Mantle cavity contains gills and visceral mass

Water enters through incurrent siphon
Flows over the gills
Exits via the excurrent siphon

Circulatory system with 3 chambered heart

Separate sexes

Class Cephalopoda



Marine predators

Foot modified into tentacles and arms

Chitonous beack

Siphon for expelling water

Well-developed eyes

Large brains for invertebrates

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