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