• They are triplobastic
    • They are bilaterally symmetrical
    • They are flatworms
    • They have reached an organ level of organization
    • Possesses a poorly developed sense organ
    • Mostly endoparasites e.g tapeworm, liverfluke
    • Few are free living e.g planaria
    • Lack special organ for gaseous exchange
    • Gaseous exchange is by diffusion through body surface
    • No separate sexes (i.e they are haemaphrodites)
    • No special organ for locomotion (no cilia or flagella)
    • Most do not digestive tract except the free living ones like planaria
    • The parasite ones absorbs nutrients from their host for survival
    • The parasitic ones can survive in anaerobic condition
    • Examples include tapeworm (Taenia solium) planaria, etc.
    • Unsegmented (like nematode worms, unlike annelid worms)
    • They are acoelomate
    • They are flattened shaped
    • Possess mouth but lacks anus
flatworm diagram

The triploblastic condition

This is the condition in which a thirdlayer, called the mesoderm developsin the embryo. This separates the ectoderm from the endoderm. The presence ofmesoderm in the body is significant for several reasons.

  • It allows triploblasticorganisms to increase in size and this results in considerable separation ofthe alimentary canal from the body wall.
  • It has been used to form avariety of organs, which may combine together and contribute towards an organsystem level of organisation. Examples of such system include the centralnervous system and digestive, excretory and reproductive systems.
  • It enables the improvement ofthe muscular activity of triploblastic organisms. This is necessary as theirincreased size renders the ciliary or flagellar mode of locomotion inadequate.

This increase in size, however, posesproblems of transport of materials between the endodermal and ectodermallayers. In some animals the mesoderm completely fills the space between theendoderm and ectoderm (the acoelomatecondition) in which case the transport problems are overcome by aflattening of the body, so maintaining a large surface area in relation tovolume.

This diffusion of materials betweenenvironment and tissues is rapid enough to satisfy metabolic requirements. Inother animals a space (the coelom)develops within the mesoderm (the coelomatecondition) and transport systems are developed which carry materials fromone part of the body to another.

The platyhelmiths are constructed on thetriploblastic body plan and are the earliest animals to have developed organsand organ systems from the mesoderm. They are acoelomate and therefore haveflat bodies – hence their common name of flatworms.

Much of the mesoderm remains undifferentiatedand forms a packing tissue, the mesenchyme,which supports and protects the organs of the body.

The phylum is divided into three classes;two of these are completely parasitic, whereas the other class, the mosttypical, contains free-living forms. The platyhelminths possess a clearlydifferentiated ‘head’ situated anteriorly(at the front), and a distinct posterior(back) end. There are clearly defined dorsal(upper) and ventral (lower)surfaces.

Many structures (such as eyes) aresymmetrically arranged on the right and left-hand sides of the body. Suchorganisation, where the right side is approximately the mirror image of theleft and where there is a distinct anterior end, is called bilateral symmetry.

No transport system has developed, becausein the basic body structure all parts are in close proximity to food and oxygensupplies.

All platyhelminths are thin and flat,providing a large surface area to volume ratio for gaseous exchange.

Many forms possess a much-branched gut,which ramifies throughout the body to facilitate absorption of food materials.In addition, excretory material is collected from all parts by a branchedsystem of excretory tubes.

Classification of Platyhelminthes (flatworms)

Class turbellaria – Turbellarians

Free living; aquatic
Delicate, soft body
Suckers rarely present
Outer surface covered with cilia for locomotion; cuticle absent
Enteron present
Sense organs in adult
e.g. planaria

Planaria is a free-living, carnivorousflatworm found in freshwater streams and ponds. It remains under stones duringthe day, emerging only at night to feed.

It is black in colour and can measure up to15mm in length. It has an elongated, extremely flattened body, with arelatively broad anterior ‘head’ possessing a pair of eyes in the dorsalsurface, and aa posterior end that is clearly tapered. Planaria is bilaterallysymmetrical, a body design associated with an active mode of life.

There is a single gut opening, the mouth,which is located on its ventral surface towards the posterior end of the body.Planaria feeds on small worms, crustacean and on the dead bodies of largerorganisms.

Class trematoda – Flukes

Class Trematoda (flukes)
Endoparasitic (live inside host) or ectoparasitic (live on outer surface of host)
Leaf-like shape
Usually ventral sucker in addition to sucker on ‘head’ for attachment to host
Thick cuticle with spines (protection); no cilia in adult (locomotion not needed because not parasitic)
Enteron present
Sense organs only in free-living larval stages
e.g. Fasciola (liver fluke) schistosoma (blood fluke) – cause of schistosomiasis (bilharzia) in many tropical countries

Fasciola hepatica, the liverfluke, belongs to the class trematoda, which is one major groups of parasitesin the animal kingdom.

It is endoparasitic, meaning it livesinside its host. It lives in the bile ducts of sheep, its most important, orprimary, host. Other primary hosts are cattle and, occasionally, humans.

Many differences exists between fasciolaand the free-living panaria. These differences can be attributed to theadaptations that fasciola has evolved in order to survive as an endoparasite.

Associated with its parasitic mode of lifeis a complex life history, involving three larval stages (the miracidium, rediaand cercaria), which provide opportunities for increasing its numbers duringthe life cycle.

The large numbers of offspring produced inthis way help to offset the high mortality rate that inevitably occurs duringinfection of new hosts.

For a part of its life history Fasciolainfests a secondary host, the freshwater snail (Limnea), in which some of itslarval stages are able to live and multiply.

Each stage in the life history of fasciolashows structural, physiological and reproductive adaptations suited to its modeof life. Some of these are listed below.

Adult liver fluke:

The body is thin and flat and attaches tothe lining of the bile duct. The body wall protects the fluke against thehost’s enzymes. The gland cells situated here also secrete material whichprotects the parasite against the host’s antitoxins.

Ahermaphrodite (male and females sex organs in thesame organism) reproductive system ensures that self-fertilisation orcross-fertilisation can occur. The fluke can survive anaerobically if there isa shortage of oxygen.


This is the first of the larval stages offasciola. It main function is to find the secondary host, for which it needssense organs and the ability to move. It also produces more larvae(sporocysts). It has a ciliated epidermis which allows it to swim in water orin moisture on vegetation.

The miracidium is attracted to itssecondary host, the freshwater snail, by chemotaxis (locomotion in a particulardirection in response to a chemical stimulus). It attaches at its anterior endto the snail’s foot, and a gland secretes protein-digesting enzymes onto thesurface of the snail to help in the penetration of the host’s tissues.Penetration is further helped by the muscle cells which help the larva towriggle through the tissues of the host. In this way it migrates to thedigestive glands. There are special germ cells present inside the miracidiumwhich give rise to the next larval forms.


The function of this stage is to increasenumbers to compensate for wastage of larvae that do not find hosts. It is animmobile, closed sac containing germ cells which multiply to form many rediae,the next larval stage


This is a multiplication and feeding stage.It has musclular pharynx to suck in fluids and tissues from its host. Musclecells multiply into more rediae, or into cercariae. There is a pore for theescape of the new rediae or the cercariae.


This bears many features in common with theadult fluke, which include oral and ventral suckers for anchorage to suitablesubstrates such as grass. There is also a tail to assist in locomotion throughwater or moisture on vegetation. Glands are present which secrete a cyst wall.The encysted cercaria undergoes no further development until it is swallowed bya sheep. It has considerable powers of resistance to low temperatures, but issusceptible to dessication.

Limnea is an amphibious snail inhabitingponds, muddy tracks and damp vegetation. It is able to withstand adverseconditions. Therefore the sporocyst and redia stages of fasciola life history,which develops within the snail, are themselves directly protected from suchunfavourable conditions. Indeed, in conditions of low temperature, rediaeremain within the snail and can overwinter within the host, only producingcercariae when warmer weather returns in the spring.

Limnea is also a very rapid breeder. It hasbeen estimated that on snail may produce upto 160, 000 offspring in 12 weeks.If all of these offpring contain developmental stages of fasciola, then the chancesof cercariae escaping from the snails and entering new, uninfected primaryhosts will be considerably increased.

The amphibious mode of life of limneaensures that when the cercariae escape there is water available in which todisperse.

The release of young adult flukes from theencysted cercaria takes place in the gut of the sheep or cow. The process isinitiated in the stomach by high carbon dioxide levels and temperature ofaround 39oC. Under these conditions the parasite releasesprotein-digesting enzymes which digest a hole in the cyst wall. Emergence ofyoung flukes is triggered off by the presence of bile in the digestive juice ofthe small intestine.

The young flukes burrow through theintestinal wall and migrate to the liver. For a time they feed on liver tissue,but about six weeks after infection they become permanently attached in thebile ducts.

Fasciola can have several effects on itsprimary host. A heavy infection can cause death. Liver metabolism of the hostis interfered with when the young flukes migrate through it. Cells aredestroyed and bile ducts may be blocked; large scale erosion of the liver(liver rot) will cause dropsy. Little, or absence of, bile in the gut canaffect digestion, and the excretory wastes of fasciola can have a toxic effecton the host.

The following measures can be taken against fasciola.

  1. Drainage of the pasture landand introduction of snail-eating geese and ducks to the pastures (a method ofbiological control) will help to remove the secondary host Limnea.
  2. The filling of the ponds anduse of elevated drinking troughs will also help to achieve this.
  3. Use of lime on the land willhelp to prevent the hatching of the eggs of the parasite, as they will nothatch in water with a pH of more than 7.5
  4. For sheep which are alreadyinfected, the administration of carbon tetrachloride kills flukes in the liver.

Class cestoda – Tapeworms

Class Cestoda (tapeworms)
Endoparasitic (live inside host)
Elongated body divided into proglottides which are able to break off
Suckers and hooks on ‘head’ (scolex) for attachment to host
Thick cuticle (protection); no cilia in adult
No enteron (no digestion required – absorbs pre-digested food from host)
Sense organs only in free-living larval stages
e.g. Taenia (tapeworm)