Phylum – Platyhelminthes Characters and Classification

Phylum – Platyhelminthes
Platyhelminthes are sutposed to have evolved from a coelenterate rike ancestor change from the radial to
bilateralsymmetry. Primitive flatworm share many characteristics with the planula larva or coelenterates.
Gagenbaur - suggested the name platyhelminthes
Phylum Platyhelminthes: flatworms are dorsoventrally flattened acoelomates
The members of the phylum Platyhelminthes differ from the phylum Cnidaria in that
they:
Exhibit bilateral symmetry with moderate cephalization
Are triploblastic (develop from three-layered embryos: ectoderm, endoderm and mesoderm)
Possess several distinct organs, organ systems, and true muscles
Although more advanced than cnidarians, two things point to the early evolution of platyhelminths in
bilateria history.
A gastrovascular cavity is present.
They have an acoelomate body plan.
They have dorso-ventrally flattened body, hence are called flatworms.
These are mostly endoparasites found in animals including human beings.
Flatworms are bilaterally symmetrical, triploblastic and acoelomate animals with organ level of
organisation. Hooks and suckers are present in the parasitic forms
.Some of them absorb nutrients from the host directly through their body surface.
Specialised cells called flame cells help in osmoregulation and excretion.
Sexes are not separate.
Fertilisation is internal and development is through many larval stages.
Some members like Planaria possess high regeneration capacity.
Examples:
• Taenia (Tapeworm),
• Fasciola (Liver fluke).
There are more than 20,000 species of Platyhelminthes which are divided into four
classes
• Class Turbellaria
• Classes Trematoda and Monogenea
• Class Cestoda
1. Class Turbellaria
e.g.,Dugesia (Planaria) Stenostomum, NotoplanaMostly free-living, marine species; a few species are found in
freshwater and moist terrestrial habitats. Planarians are familiar and common freshwater forms
Carnivorous, they feed on small animals and carrion
Lack specialized organs for gas exchange or circulation
Gas exchange is by diffusion (flattened body form places all cells closeto water).
Fine branching gastrovascular cavity distributes food throughout theanimal.
Flame cell excretory apparatus present which functions primarily t o maintain osmotic balance of the
animal.
Nitrogenous waste (ammonia) diffuses directly from cells to the water.
Move by using cilia on the ventral dermis to glide along a film of mucus. Muscular contractions produce
undulations which allow some to swim.
On the head are a pair of eyespots which detect light and a pair of lateral auricles that are olfactory
sensors.
Possess a rudimentary brain which is capable of simple learning.
Reproduce either asexually or sexually.
Asexually by regeneration: mid-body constriction separates the parent into two halves, each of which
regenerates the missing portion
Sexually by cross-fertilization of these hermaphroditic forms
2. Classes Monogenea and Trematoda
Examples: Gyrctdactylus, Dactylogrus, Polystorna, Diplozoom, Faciola, Schrslosorna, Opisthorchis (Clonorchis),
Asprdogaster, Cotytapsis. Shichocotyle.
All members of these two classes are parasitic.
Flukes are members of the class Trematoda.
Suckers are usually present for attaching to host internal organs.
Primary organ system is the reproductive system; a majority are hermaphroditic.
Life cycles include alternations of sexual and asexual stages with asexual development taking place in an
intermediate host.
Larvae produced by asexual development infect the final hosts where maturation and sexual reproduction
occurs
Schistosoma spp. (blood flukes) infect 200 million people worldwide.
Members of the class Monogenea are mostly external parasites of fish.
Structures with large and small hooks are used for attaching to the host animal.
All are hermaphroditic and reproduce sexually.
3. Class Cestoda
Eg.Amphilina . Endoparasite of fish. Haplobothrium : Teleosts and Terrestria I verteb rates.
Echinococcus granulosus : Dog Tapeworm or Hydatid worm.
Taenia saginata : Beef tape worrn.
Adult tapeworms parasitize the digestive system of vertebrates.
Possess a scolex (head) which may be armed with suckers and/or hooks that help maintain position by
attaching to the intestinal lining ..
Posterior to the scolex is a long ribbon of units called proglottids.
A proglottid is filled with reproductive organs.
No digestive system is present.
The life cycle of a tapeworm includes an intermediate host.
Mature proglottids filled with eggs are released from the posterior end ofthe worm and pass from the body
with the feces.
Eggs are eaten by an intermediate host and a larva develops, usually inmuscle tissue.
The final host becomes infected when it eats an intermediate hostcontaining larvae.
Humans can become infected with some species of tapeworms by eating undercooked beef or pork
containing larvae.