Triploblastic animals – The Acoelomates
Phylum: Platyhelminthes – The Flatworms

General Charactteristics:  The name platyhelminthes means “ flatworms ”. The body of these animals is soft and dorsoventrally compressed.

The platyhelminthes are triploblastic acoelomates. There is development f a third layer, the mesoderm, which separates the ectoderm and endoderm. The playhelminthes 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 fluke) and Schistosoma (blood fluke). The parasites are more common in tropics. Some of these cause diseases in humnas. 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 flame 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 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 fission 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 flatworms are:

(i)                  Dugesia (Planaria): A free-living flatworms with a ciliated outer surface.
(ii)                Fasciola (Liver fluke): It is an endoparasite in sheep and occassionally 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 off and are passed out from the intestine along with faeces.


Fig. 10.6 Examples of animals of platyhelminthes.

Adaptations fro parasitic mode of life

The parasitic platyhelminthes have completely adapted themselves to parasitic mode of life by the development fo 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 simplified 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.


In Taenia (tape worm), the development of the zygote begins while it is still inside the uterus of female. The last segments or proglottids and their uteri contain completely developed embryo. The fully mature proglottids break of from the body and pass out of the body of man along with faeces 9undigested waste). The embryo inside the egg is round in shape and has six chitinous hooks. It shows limited movement of contraction.

In order to develop further it must reach a second host which may be a cow. The parasite remains embedded in the voluntary muscles of cow. If an improperly cooked beef is eaten by a person, the parasite which has not been killed begins to develop further in the intestine of man.


Once the parasite has entered the intestine of man it is difficult to remove it completely. In this respect care should be taken to cook beef properly before eating it. So that there is no chance of the parasite entering the digestive system but if it has entered then certain medicines are taken to remove it. Its complete removal is necessary because if only head remains inside the intestine it can grow into new tape-worm once again. Besides treatment with drugs, physicians also give anema to the patient, to fully remove the parasite.