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Comparative studies of natural groups of organisms
all forms of amoeba including parasitic fluids in the amoeba cell are relatively
and free living amoeba. Parasitic amoeba more concentrated, hence allowing
include Entamoeba hystolytica, which more water to flow into their body in
feeds on cells of the human colon, and response to osmotic gradient. Amoeba
cause amoebic dysentery also (amoebiasis). in turn pumps out this water using
The disease is characterised by abdominal their contractile vacuole.
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pains, nausea, vomiting, erosion of blood
vessels of the gut and diarrhoea containing Distinctive features of phylum
blood. Rhizopoda
Members of the group Rhizopoda have
General characteristics of phylum the following features, which distinguish
Rhizopoda them from the rest of the protoctists.
They are found in sea water, fresh water,
and in the soil. They can also be found (a) They have pseudopodia, which are
in a mud shallow pond and slow flowing used for both locomotion and feeding.
streams containing plenty of decaying Such pseudopodia are constantly
organic matter. changing as amoeba moves and feeds.
(a) They are single-celled eukaryotes and Food particles and small organisms
their cells have no definite shape. are engulfed using pseudopodia and
(b) They are unicellular whose bodies digested in the food vacuoles. The
are surrounded by membranes. Their digested food is assimilated into the
cytoplasms have two distinct regions, rest of the body. This type of feeding
the plasmasol (inner) and plasmagel is called phagocytosis. Ingestion is
(outer). by endocytosis and egestion is by
(c) Most of them are free living, forming exocytosis.
important links in the food chains. They (b) The food vacuole and the oil droplets of
feed on plant and animal materials. amoeba confer a granular appearance
In contrast, others are parasitic or to the endoplasm.
infectious to animals, including human
being; some human pathogens include (c) They possess a contractile vacuole,
Entamoeba histolytica that causes which carries out osmoregulation.
amoebic dysentery. When the contractile vacuole reaches
(d) They reproduce asexually by binary a certain size, it fuses with the cell
fission. membrane where water is released
(e) They move by using pseudopodia; the outside the cell.
temporary projections of the cytoplasm (d) Their cytoplasm has two distinct
hence, amoeba and other protoctists regions; viscous outer layer, ectoplasm
that use this mode of locomotion are (plasmagel) and more fluid internal
called pseudopods. endoplasm (plasmasol).
(f) They possess contractile vacuole that
carries out osmoregulation. Normally,
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