Clade Lophotrochozoa Phylum Platyhelminthes

(Flatworms)

Kingdom Animalia

The clade Lophotrochozoa was identified by molecular data

Some develop a lophophore for feeding, others pass through a trochophore larval stage, and a few have neither feature

Lophotrochozoa includes the flatworms, rotifers, ectoprocts, brachiopods, molluscs, and annelids

Clade Lophotrochozoa

Phylum Platyhelminthes Flatworms

Members of phylum Platyhelminthes live in marine, freshwater, and damp terrestrial habitats

Although flatworms undergo triploblastic development, they are acoelomates

They are flattened dorsoventrally and have a gastrovascular cavity with one opening

Gas exchange takes place across the surface, and protonephridia regulate the osmotic balance

Flatworms are divided into two lineagesCatenulida, or “chain worms,” reproduce

asexually by buddingRhabditophora are more diverse and

include both free-living and parasitic species

Flatworms are divided into three classes The mostly free-living Turbellaria include

the planarian, Dugesia; these are found in the oceans, in fresh water, and in moist terrestrial habitats, and a few are parasitic.

The Trematoda, or flukes, are all parasitic, and have complex life cycles specialized for parasitism in animal tissues.

The Cestoda, or tapeworms, are intestinal parasites in vertebrates, and they also show anatomical and life history modifications for parasitism.

The best-known rhabditophorans are planarians

Planarians live in fresh water and prey on smaller animals

Planarians have light-sensitive eyespots and centralized nerve nets

The planarian nervous system is more complex and centralized than the nerve nets of cnidarians

Planarians are hermaphrodites and can reproduce sexually, or asexually through fission

Class Turbellaria

Pharynx

Gastrovascularcavity

Gastrovascular cavity

Mouth

EyespotsVentral nerve cords

Ganglia

Dugesia (Planaria)

Class Trematoda

Trematodes parasitize a wide range of hosts, and most species have complex life cycles with alternating sexual and asexual stages.

Many trematodes require an intermediate host in which the larvae develop before infecting the final hosts (usually a vertebrate) where the adult worm lives.

The blood fluke Schistosoma causes schistosomiasis, a disease that infects 200 million people, leading to pain, anemia, and dysentery.

The intermediate host for Schistosoma is a snail.

Larval stages of some medically important species include miracidium, redia, cercaria, and metacercaria.

Class TrematodaCecaria Redia

Fasciola hepatica

Fasciola hepatica

Clonorchis sinensis

Tapeworms are a large and diverse group of parasitic rhabditophorans, with adults that live mostly in vertebrates, including humans.

Suckers and hooks on the anterior end, or scolex, anchor the tapeworm in the digestive tract of the host.

Tapeworms lack a mouth and gastrovascular cavity, and absorb food from their hosts across their body surface.

A long series of proglottids, sacs of sex organs, lie posterior to the scolex.

Mature proglottids, loaded with thousands of fertilized eggs, are released from the posterior end of the tapeworm and leave with the host’s feces.

Class Cestoda

Proglottids withreproductivestructures

Hooks

Sucker

100 m

Scolex

Taenia pisiformis

Tapeworm Mature Proglottid

Lab Instructions: View all organisms listed in the chart above in the microscope, stereoscope, preserved or on the observation tray. Draw each organism in your lab notebook and note the distinguishable characteristics of each. Review internal anatomy of each organism.

Organism (Scientific Name)

Common Name Class

Dugesia (sl) Planaria TurbellariaClonorchis sinensis (sl) &

(sp)Chinese Liver

FlukeTrematoda

Fasciola hepatica (sl) & (sp) Sheep Liver Fluke TrematodaTaenia pisiformis (sl) & (sp) Tapeworm Cestoda

Trematoda redia, cercariaTrematoda, cercariaTaenia immature proglottidsTaenia mature proglottids