The representatives of the phylum Platyhelminthes are commonly known as the flatworms or tapeworms. The word ‘Platyhelminthes’ is derived from the Greek word, ‘platy’ meaning flat and ‘helminth’ meaning worm. They are simple soft-bodied, bilaterian, unsegmented invertebrate animals. The Phylum Platyhelminthes makes up the 4th largest phylum among the animal kingdom. But among the acoelomate organisms, the phylum Platyhelminthes constitutes the largest phylum with more than about 20,000 known species throughout the world. Among them, around 80% live as parasitic life on humans and other animals and few are free-living.
Flatworms are acoelomate, triploblastic animals. They lack circulatory and respiratory systems, and have a rudimentary excretory system. This digestive system is incomplete in most species. There are four traditional classes of flatworms, the largely free-living turbellarians, the ectoparasitic monogeneans, and the endoparasitic trematodes and cestodes. Trematodes have complex lifecycles involving a molluscan secondary host and a primary host in which sexual reproduction takes place. Cestodes, or tapeworms, infect the digestive systems of primary vertebrate hosts.
Characteristics of Phylum Platyhelminthes
- These are mostly parasitic. Some are free-living. The free living forms are chiefly aquatic and the majority are marine forms. A few are terrestrial, confined to humid areas.
- They are triploblastic animals having three primary germ layers: ectoderm, endoderm and mesoderm. Mesoderm contributes to the development of true muscle tissue.
- They have dorso-ventrally flattened the un-segmented and tape-like or leaf-like body.
- The body is soft and does not contain any cilia.
- They lack skeletal, respiratory and circulatory systems.
- They respire by simple diffusion through the body surface.
- They show bilateral symmetry and cephalization.
- The digestive system is incomplete or absent with no anus. In this case, the true stomach is absent and pharynx opens into a complex intestinal structure.
- Their nervous system is primitive, ladder-like. The main nervous system consists of a pair of ganglia or brain and one or three pairs of longitudinal nerve cords connected by transverse nerves.
- They reproduce sexually by fusion of gametes and asexually by regeneration by fission and regeneration.
- Fertilization is internal but cross-fertilization in trematodes and self-fertilization in cestodes.
- The majority of them are white, colorless and some derive color from ingested food while free-living form are grey, brown-black or brilliantly colored.
- Mouth and genital pores on the ventral surface are well marked in turbellarians but less marked in cestodes and trematodes.
- In the majority of form, eggs are devoid of yolk. They are produced separately in the yolk or vitelline glands.
- The excretory system includes a lateral canal and a single or pair of protonephridia with flame cells or bulbs. Absent in some primitive form.
- Protonephridia or flame cells are primarily osmoregulatory and secondarily excretory in function. Most of the excretory wastes diffuse out through the body surface.
- They are acoelomates and lack large fluid filled body cavity. Connective tissue compartments between the gut and the body wall is called parenchyma.
- Gut is a blind sac. Mouth is used for ingestion and egestion. Anus is absent except in some turbellarians. Digestion is both external and internal.
- These animals are hermaphrodites i.e. both male and female organs are present in the same body.
- The life cycle of these organisms can be complex, especially if they are parasitic, as this may involve one or more host animals.
Phylum Platyhelminthes (flatworms) Classification
The Phylum Platyhelminthes is classified into the following classes:
- Class-1: Turbellaria
- Class-2: Monogenea
- Class-3: Cestoda
- Class-4: Trematoda
Class I: Turbellaria (L. turbella=stirring)
The class Turbellaria includes mainly free-living, marine species, although some species live in freshwater or moist terrestrial environments. The ventral epidermis of turbellarians is ciliated and facilitates their locomotion. Some turbellarians are capable of remarkable feats of regeneration in which they may regrow the body, even from a small fragment.
- This class includes planarians, acoels etc.
- They are usually free living and some forms are also commensals or parasites
- The body of these animals is unsegmented and covered by ciliated epidermis.
- Epidermal glands cells secrete rod-shaped inclusions called rhabdoids. When these inclusions are released to the surface of epidermis they form mucus.
- Adhesive glands help in temporary adhesion to the substratum.
- Mouth is ventral and the pharynx is protrusible.
- The branched gut facilitates the transport of nutrients to all parts of the body. So gut performs the functions as a gastro vascular system
- Planarians have remarkable ability of regeneration. Totipotent cells called as neoblast cells are important in this phenomenon of regeneration.
- Development is direct but larvae like Muller’s larva or Goette’s larva are present in some forms.
Examples: Pseudobiceros bedfordi, Pseudoceros dimidiatus
Class Turbellaria includes the following orders:
- Order: Acoela
- Order: Neorhabdocoela
- Order: Catenulida
- Order: Macrostomida
- Order: Tricladida
- Order: Proseriata
- Order: Polycladida
- Order: Lecithoepitheliata
- Order: Kalyptorhynchia
Class II: Trematoda (Gr. Trema=hole, eidos=form)
The trematodes, or flukes, are internal parasites of mollusks and many other groups, including humans. Trematodes have complex lifecycles that involve a primary host in which sexual reproduction occurs, and one or more secondary hosts in which asexual reproduction occurs. The primary host is almost always a mollusk. Trematodes are responsible for serious human diseases including schistosomiasis, a blood fluke. The disease infects an estimated 200 million people in the tropics, leading to organ damage and chronic symptoms like fatigue. Infection occurs when the human enters the water and a larva, released from the primary snail host, locates and penetrates the skin. The parasite infects various organs in the body and feeds on red blood cells before reproducing. Many of the eggs are released in feces and find their way into a waterway, where they are able to reinfect the primary snail host.
- Their alimentary canal and excretory system are well developed. In this case, the alimentary canal is highly branched with no anus where mouth as the only opening.
- They bear a well developed muscular system.
- Sexes are separate with the complex reproductive system. Their life cycle involves two types of the host such as intermediate and main host.
- They can live in blood for several years. In this case, adult flukes feed on the blood of the host.
- This class is characterized by a complex hermaphroditic reproductive system and a life cycle that involves intermediate and main hosts.
- The body bears oral and ventral suckers by which they make them attach with the host body for feeding easier.
- Life cycle includes miracidium larva, sporocyst, cercaria larva, metacercaria and adult.
Example:Multicotyle purvisi, Lobatostoma manteri
Class III: Cestoda (Gr. Kestos=girdle, eidos=form)
The cestodes, or tapeworms, are also internal parasites, mainly of vertebrates (Figure 3). Tapeworms live in the intestinal tract of the primary host and remain fixed using a sucker on the anterior end, or scolex, of the tapeworm body. The remaining body of the tapeworm is made up of a long series of units called proglottids, each of which may contain an excretory system with flame cells, but contain reproductive structures, both male and female. Tapeworms do not possess a digestive system; instead, they absorb nutrients from the food matter passing them in the host’s intestine.
- The member of the class is commonly known as tapeworms.
- They contain over 4,000 species which lead endoparasitic life forms.
- They have long flat and tape-like bodies which can grow up to 18 meters long.
- They have no digestive, circulatory and respiratory systems.
- The body bears large numbers of male and female reproductive structures which are known as proglottid that are capable of reproducing independently.
- They pick up nutrition using saprozoic method due to their lack of digestive system. In this case, their body surface is covered by small microvillus-like projections which effectively absorb nutrients.
- They can capable of producing thousands of eggs which hatch to produce larvae, known as coracidium.
- The body bears well-developed muscle.
- The body surface also contains modified cilia which are used as sensory endings.
- The nervous system consists of a pair of lateral nerve cords.
Example:Gyrocotyle rugosa, Amphilina foliacea
Class IV: Monogenea
The monogeneans are ectoparasites, mostly of fish, with simple lifecycles that consist of a free-swimming larva that attaches to a fish to begin transformation to the parasitic adult form. The parasite has only one host and that host is usually only one species. The worms may produce enzymes that digest the host tissues or simply graze on surface mucus and skin particles. Most monogeneans are hermaphroditic, but the male gametes develop first and so cross-fertilization is quite common.
- They have flattened cylindrical and leaf-shaped body with oral suckers.
- The body bears a large posterior adhesive disk which is known as opisthaptor. It helps to attach to the body of the host. By using these organs, they feed off the outer epidermal layer of the body of the host.
- The head is located at the anterior region which contains eyespots with pigments.
- The anterior region also contains poorly developed oval-shaped pharynx.
- The body does not contain an anal opening. To eliminate waste product, they use the protonephridia system.
- The respiratory and circulatory systems are absent.
- The nervous system consists of a nerve ring and nerves.
- They are hermaphrodites and fertilization is external with free-swimming larva.
Example: Diplozoon paradoxum, Gyrodactylus adspersi
Examples of Platyhelminthes
The examples of organisms belonging to phylum Platyhelminthes are:
- Taenia solium: It is also known as the pork tapeworm and is found in all the countries where pork is consumed. They live as parasites in the small intestine of human beings and their larva are found in the muscles of the pigs. It is a hermaphrodite and undergoes self-fertilization. Taenia solium causes taeniasis where the patient experiences abdominal pain, anaemia, indigestion, restlessness and false appetite.
- Schistosoma: It is found in the mesenteric blood vessels and hepatic portal system of humans and is therefore known as blood fluke. It shows well-marked sexual dimorphism. Schistosoma causes Schistosomiasis which spreads through contaminated water. The patient suffers from anaemia, pain, fever, liver and spleen enlargement, and diarrhoea.
- Dugesia (Planaria): These are commonly found in freshwater habitats. Dugesia species have an elongated body with a slightly triangle-shaped head. Usually they have grey, brown or black colors on the dorsal surface, the ventral surface uses to be paler.
- Fasciola: Fasciola commonly known as the liver fluke. It resides in the liver and bile duct of sheep and goat. It is a hermaphrodite but cross-fertilization takes place. It causes fascioliasis in animals. In this, the liver of the animal enlarges and the bile ducts are blocked. The infection weakens the muscles of the animals resulting in muscular pain. It might also prove fatal for the animals.
- Taenia saginata (Beef tapeworm)
- Echinococcus granulosus – The dog tapeworm