• 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 third layer, called the mesoderm develops in the embryo. This separates the ectoderm from the endoderm. The presence of mesoderm in the body is significant for several reasons.

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

This increase in size, however, poses problems of transport of materials between the endodermal and ectodermal layers. In some animals the mesoderm completely fills the space between the endoderm and ectoderm (the acoelomate condition) in which case the transport problems are overcome by a flattening of the body, so maintaining a large surface area in relation to volume.

This diffusion of materials between environment and tissues is rapid enough to satisfy metabolic requirements. In other animals a space (the coelom) develops within the mesoderm (the coelomate condition) and transport systems are developed which carry materials from one part of the body to another.

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

Much of the mesoderm remains undifferentiated and 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 most typical, contains free-living forms. The platyhelminths possess a clearly differentiated ‘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) are symmetrically arranged on the right and left-hand sides of the body. Such organisation, where the right side is approximately the mirror image of the left and where there is a distinct anterior end, is called bilateral symmetry.

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

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 branched system 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, carnivorous flatworm found in freshwater streams and ponds. It remains under stones during the day, emerging only at night to feed.

It is black in colour and can measure up to 15mm in length. It has an elongated, extremely flattened body, with a relatively broad anterior ‘head’ possessing a pair of eyes in the dorsal surface, and aa posterior end that is clearly tapered. Planaria is bilaterally symmetrical, 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 larger organisms.

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 liver fluke, belongs to the class trematoda, which is one major groups of parasites in the animal kingdom.

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

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

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

The large numbers of offspring produced in this way help to offset the high mortality rate that inevitably occurs during infection of new hosts.

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

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

Adult liver fluke:

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

A hermaphrodite (male and females sex organs in the same organism) reproductive system ensures that self-fertilisation or cross-fertilisation can occur. The fluke can survive anaerobically if there is a shortage of oxygen.


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

The miracidium is attracted to its secondary host, the freshwater snail, by chemotaxis (locomotion in a particular direction in response to a chemical stimulus). It attaches at its anterior end to the snail’s foot, and a gland secretes protein-digesting enzymes onto the surface 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 to wriggle through the tissues of the host. In this way it migrates to the digestive glands. There are special germ cells present inside the miracidium which give rise to the next larval forms.


The function of this stage is to increase numbers to compensate for wastage of larvae that do not find hosts. It is an immobile, 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. Muscle cells multiply into more rediae, or into cercariae. There is a pore for the escape of the new rediae or the cercariae.


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

Limnea is an amphibious snail inhabiting ponds, muddy tracks and damp vegetation. It is able to withstand adverse conditions. Therefore the sporocyst and redia stages of fasciola life history, which develops within the snail, are themselves directly protected from such unfavourable conditions. Indeed, in conditions of low temperature, rediae remain within the snail and can overwinter within the host, only producing cercariae when warmer weather returns in the spring.

Limnea is also a very rapid breeder. It has been 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 chances of cercariae escaping from the snails and entering new, uninfected primary hosts will be considerably increased.

The amphibious mode of life of limnea ensures that when the cercariae escape there is water available in which to disperse.

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

The young flukes burrow through the intestinal 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 the bile ducts.

Fasciola can have several effects on its primary host. A heavy infection can cause death. Liver metabolism of the host is interfered with when the young flukes migrate through it. Cells are destroyed 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 can affect digestion, and the excretory wastes of fasciola can have a toxic effect on the host.

The following measures can be taken against fasciola.

  1. Drainage of the pasture land and introduction of snail-eating geese and ducks to the pastures (a method of biological control) will help to remove the secondary host Limnea.
  2. The filling of the ponds and use of elevated drinking troughs will also help to achieve this.
  3. Use of lime on the land will help to prevent the hatching of the eggs of the parasite, as they will not hatch in water with a pH of more than 7.5
  4. For sheep which are already infected, 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)

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