PHYLUM-
PLATYHELMINTHES
Phylum—Platyhelminthes (Platy — flat; helminth — worm)
includes flatworms. The group includes the first simplest
triploblastic group of animals. Gegenbaur coined the term
‘Platyhelminthes’.
It includes about 12000 species of
animals. They have leaf-like or ribbon-like body.
Some important general features of
phylum—Platyhelminthes are discussed below
(i) Habit and Habitat Majority of forms are parasitic
(tapeworms, liver flukes. blood flukes). etc., and
free-living forms (planarians). Hooks and suckers are
present in parasitic forms.
(ii) Symmetry The body is bilaterally symmetrical with
definite orientation like anterior and posterior end.
(iii) Cephalisation Primitive cephalisation is present in
free-living flatworms.
(iv) Germ Layers They are triploblastic.
(v) Body Cavity They are acoelomate. The space
between the body wall and body organ is filled by
characteristic connective tissue called parenchyme
or mesenchyme.
(vi) Body Organisation They show organ system grade
of organisation.
(vii) Digestive System It is simple and incomplete with
only one opening called mouth. The anus is absent.
(viii) Respiration It occurs through general body surface.
(ix) Excretory System It consists of peculiar flame cells
that can help in osmoregulation and excretion.
Circulatory and skeletal systems are absent in
platyhelminthes.
(x) Nervous System It is ladder-like and consists of a
brain and two main longitudinal nerve cords jointed
at intervals by the transverse commissures.
xi) Reproduction Flatworms
are hermaphrodite
monoecious or bisexual, having both male and female
reproductive system. Fertilisation often includes one
or more larval stages. Some also can regenerate, e.g..
Planaria, Taenia (tapeworm) and Echmococcus (dog
tapeworm).
Knowledge Plus
• They have organ system level of organisation, a head With cense
organs In the anterior end, three germ layers, organisation
excretory system and reproductive organs with ducts
present.
Dugesja is used In experiments on regeneration
• Fasciola causes liver rot disease in sheeps and goats
• Taema solium causes taeniasis in man