Phylum platyhelminthes | general characteristics of platyhelminthes | class 11

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This lecture is about
GRADE - BILATERIA
Triploblastic animals -The Acoelomates
Phylum: Platyhelminthes -The Flatworms
General Characteristics
The name Platyhelminthes means “latworms”. The body of these animals is soft and dorsoventrally
compressed.
The Platyhelminthes are triploblastic acoelomates. There is development of a third layer, the
mesoderm, which separates the ectoderm and endoderm. The Platyhelminthes exhibit bilateral
symmetry, and body is unsegmented.
With few exceptions the Platyhelminthes are parasites, mostly endoparasites, i.e., live inside their
hosts. The most common examples are Taenia solium (tapeworm), Fasciola hepatica (liver luke)
and Schistosoma (blood luke). The parasites are more common in tropics. Some of these cause
diseases in humans. A few species are free living and found in freshwater, for example Dugesia
(planaria).
Their size ranges from few millmeters (10 mm in case of Planaria) to several meters (tapeworm).
Much of the body space is taken up by a branching sac type digestive system. The digestive system
is poorly developed in some species or may be absent as in the tape-worms.
The excretory system consists of branching tubes ending in bulb-like cells, the lame cells.
A well developed nervous system is present in Platyhelminthes. It is in the form of either a simple
network of nerves or ganglia. The sense organs are present at the anterior end. Respiratory and
circulatory systems are absent.
The parasitic species absorb nutrients from the hosts. The free-living species (Planaria) feed on
small animals and bodies of dead and decaying animals.
The free-living forms are motile. They move by cilia present on their undersides (Planaria). In
parasitic forms the movement is restricted.
The Platyhelminthes reproduce both by sexual and asexual means of reproduction. Asexual
reproduction is by ission in which the animal constricts in the middle into two pieces, each of
which regenerates the missing part. The sexually reproducing species are hermaphrodite, i.e., both
male and female reproductive organs are present in the same individual. Larval form is sometimes
present.
The common examples of latworms are:
(i) Dugesia (Planaria) : A free-living latworms with a ciliated outer surface.
(ii) Fasciola (Liver luke) : It is an endoparasite in sheep and occasionally in human beings. It has
suckers used for attachment to host tissue. It completes its life cycle in two hosts, a snail, sheep or
man. It lives in the bile duct of its hosts.
(iii) Taenia (Tape worm) : An endoparasite of humans, cattle and pig, that completes its life cycle in
two hosts. The intermediate host is pig or cattle. The body is ribbon-like and divided into segments
called proglottids which contain mainly sex organs. The segments continue to break of and are
passed out from the intestine along with faeces.
Adaptations for parasitic mode of life
The parasitic Platyhelminthes have completely adapted themselves to parasitic mode of life by the
development of the following characteristics:
1. The epidermis is absent and there is the formation of resistant cuticle for protection.
2. They have developed adhesive organs, such as suckers and hooks, for attachment to the host.
3. There is degeneration of muscular system and nervous system.
4. The digestive system has become simpliied due to increased dependence on host.
5. The reproductive systems are complicated and the ova are produced in huge numbers to ensure
continuity of the species.
6. The complexity of life cycle and presence of more than one host during the life cycle is also an
important parasitic adaptation.