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Biology for Advanced Level Secondary Schools

water snail called Lymnea sp.).
Mouth
Oral sucker They have several larval stages
Head Genital pore (miracidium, sporocyst, redia, cercaria
Acetabulum and metacercaria) which increase the
(ventral sucker) chance of survival and perpetuation.

Flattened body
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Economic importance of flatworms
Some members of the phylum
Platyhelminthes play an important role
in marine, freshwater and terrestrial
Excretory
pore ecosystems. Several species of flatworms
are harmful parasites to humans and
Figure 3.43: Structure of a liver fluke
other animals.
Adaptations of the liver fluke to its
mode of life Advantages of phylum Platyhelminthes
Liver fluke possesses the following features (a) Some species of Platyhelminthes feed
that facilitate a parasitic mode of its life in on dead bodies of large organisms;
the host: therefore, they decompose organic
(a) It has suckers that provide a means of matter in the soil; example, free living
attachment to the host. Planaria.
(b) It has tegument with spines that (b) Some members of Platyhelminthes
prevent it from being washed away by such as Taenia sp. and Fasciola sp.
are used in biological studies as
bile; and also they help the parasite to specimens.
erode liver cells.
(c) The tough tegument and secretions Disadvantages of phylum Platyhelminthes
from the glands prevent the worm Some members are parasites, which cause
from the effects of the host’s toxins. diseases to human beings and domestic

(d) The parasite secretes enzymes that animals. Examples include:
help it to penetrate the liver cells at (a) Taenia sp. causes taeniasis; an intestinal
various stages of its life cycle. tapeworm infection caused by eating
(e) It has mechanisms and chemicals raw or undercooked contaminated
beef (Taenia saginata) or pork (Taenia
that suppress the actions of the host’s solium).
digestive enzymes. (b) Fasciola hepatica and Fasciola

(f) It has a high reproductive potential gigantica cause fascioliasis, which
and different multiplication phases results into liver rot in sheep, cattle,
that balance its high mortality rate. and humans.
They have two hosts, namely primary (c) Schistosoma haematobium and
hosts (example: sheep and humans) Schistosoma mansoni cause
and secondary host (example: fresh urinary and interstinal bilharzia
(schistosomiasis), respectively.

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