General Characters of Platyhelminthes
- The name Platyhelminthes means “flat worm”. Their body is soft and dorso-ventrally compressed.
- They are acoelomate i.e. body cavity or coelom is absent in them.
- Their body is composed of three layers. They develop a third layer between ectoderm and endoderm. It is called mesoderm.
- They show bilateral symmetry and body is unsegmented.
- Most of the Platyhelminthes are parasite. Mostly, they are endoparasite, i.e. live inside their hosts. These parasites are more common in tropics. The most common example are:
- Taenia solium (tape worm)
- Fasciola hepatica (liver fluke)
- Schistosoma (blood fluke)
A few species are free living. They live in freshwater, for example Dugesia (planaria).
- Their size may be from few millimeters (10mm in case of planaria) to several meters (tape worm).
- They have branching sac like digestive system. It occupies most of the space of the body. The digestive system is poorly developed in parasites. It may be absent in tape-worms. The parasite species absorb nutrients from the hosts. The free living species like planaria feed on small animals and bodies of dead and decay animals.
- Excretory system consists of branching tubes. The tubes end in bulb like cells called flame cells.
- A well-developed nervous system is present in Platyhelminthes. It is in the form of simple network of nerves or ganglia. The sense organs are present at the anterior end.
- Respiratory and circulatory systems are absent.
- The free living forms like planaria are motile (can move). They move by cilia. The cilia are present under their surface. The parasitic forms show limited movement.
- They reproduce both sexually and asexually.
- Asexual reproduction: It takes place by binary fission. In this case, the animal constricts in the middle and divides into two pieces. Each of which regenerates the missing part.
- Sexual reproduction: The sexually reproducing species are hermaphrodite. In this case, both male and female reproductive organs are present on the same animals. In some cases, larvae are also developed.
The common examples of flat worms are:
- Dugesia (planaria): It is a free living flatworm. It has ciliated outer surface.
- Fasciola (liver fluke): It is an endoparasite in sheep and sometimes, in man. It lives in the bile duct of its host. Platyhelminthes has sucker for the attachment to host tissues. It completes its life cycle in two hosts, snail, sheep or man.
- Taenia (tape worm): It is an endoparasite of humans, cattle and pig. It completes 1ts life cycle in two hosts. The intermediate host is pig or cattle.
Their body is ribbon like and it is divided into segments called proglottids. These segments contain many sex organs. The segments continuously break from the body and pass out of from the intestine of host with faeces.
Adaptation for Parasitic Mode of Life
The parasitic Platyhelminthes shows following parasitic adaptation for parasitic mode of life:
- The epidermis is absent in them. They develop a resistant cuticle for protection.
- They have developed adhesive organs (organ for attachment), like suckers and hooks. These organs are used for attachment to hosts.
- There is degeneration of muscular system and nervous system.
- They depend on host for food. So their digestive system has become simplified.
- They have complex reproductive system. They produce a large number of ova (eggs). So that their generations remain continue.
- They show complex life cycles. They spend their life in more than one host during their life cycle. It is an important parasitic adaptation.
Tania or tapeworm show following life cycle:
- In Man: The development of a new Tania (tape worm) begins inside in, the uterus of female tapeworm. The uteri of last segments or proglottids contain completely developed embryos. The embryo is rounded ¡n shape. It has sex chitinous hooks. The fully developed proglottids break from the body. They pass out of the body of man along with faeces. The embryo shows limited movements of contractions.
- In cow: It reaches a second host cow for further development. The parasite remains embedded in the voluntary muscles of cow.
If improperly cooked beef (meat) of the cow is eaten by a person, the parasite enters into the intestine of man. It starts a new life cycle.
Following measures can be taken for saving our body from infestation:
- If the parasite enters the intestine of man, it becomes difficult to remove it. So, beef should be cooked properly before eating it. It will reduce the chance of entry of parasite into our intestine.
- If parasite enters, then medicines should be taken for removing it. The parasites should be removed completely. It only a head of a parasite remains in the body, it can grow to form new tapeworm.
- The physicians also give anema to the patient for fully removing the parasite.