Platyhelminthes are a Phylum of Bilaterian, Unsegmented, Soft-Bodied, usually Flattened Invertebrate Animals. They are also known as Plathelminthes or simply Flatworms. Platyhelminthes include Animals like Flukes, Tapeworms, Planarians, Turbellaria etc. There are more than 20000 Species of Platyhelminthes. Platyhelminthes come in various shapes and sizes, many are microscopic and the large Species have flat ribbon-like or leaf-like shapes. About 80 percent of all Flatworms are parasitic i.e., they live on or in another organisms and derive nourishment from them, rest are are free-living Flatworm Species i.e. they are living independently of any other organism.

• Classification
  • Classes
    

• Characteristics
  • Symmetry
  • Body Cavity
  • Nervous System
• Locomotion
• Feeding
• Reproduction

The oldest known Platyhelminth specimen is a fossil preserved in Eocene age baltic amber and placed in the monotypic Species 'Palaeosoma balticus', while the oldest sub-fossil specimens are Schistosome eggs discovered in ancient Egyptian mummies. Various Species of Flukes and Tapeworms cause enormous harm to humans and their livestock. Schistosomiasis (bilharzia, bilharziosis or snail fever), caused by one genus of Trematodes is the second most devastating of all human diseases caused by parasites.

 Platyhelminthes Classification

Kingdom - Animalia.
Sub Kingdom - Eumetazoa.
Superphylum - Platyzoa.
Phylum - Platyhelminthes.

Platyhelminthes Classes

Classes, Orders and Subclasses of Platyhelminthes

The classification Platyhelminthes is a varied one as there is no unanimity regrading the classification. Below mentioned classification is a provisionary one.

1.  Turbellaria - There are about 4,500 Species.

Orders

• Acoela
• Neorhabdocoela
• Catenulida
• Macrostomida
• Polycladida

2.  Monogenea - There are about 1,100 Species.

3.  Cestoda - There are about 3,500 Species.

Subclass - Cestodaria.

Orders

• Amphilinidea
• Caryophyllidea
• Gyrocotylidea

Subclass - Eucestoda

Orders

• Tetraphyllidea
• Lecanicephalidea
• Proteocephalidea
• Diphyllidea
• Trypanorhyncha
• Pseudophyllidea
• Nippotaeniidea
• Cyclophyllidea (Taenoidea)
• Aporidea
• Spathebothriidea

4.  Trematoda - There are about 11,000 Species. This Class includes Flukes.

Subclass - Aspidogastrea
Subclass - Digenea

Orders

• Strigeidida
• Echinostomida
• Plagiorchida
• Opisthorchiida

Characteristics of Platyhelminthes

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• Bilaterally Symmetrical - Platyhelminthes are Bilaterally Symmetrical i.e their left and right sides are mirror images of each other and they have distinct top and bottom surfaces and distinct head and tail ends.

• Unsegmented Body - Platyhelminthes have no segments in their body and the bulk of their body is occupied by Mesenchyme (spongy connective tissue).

• Triploblastic - Platyhelminthes body is composed of three fundamental cell layers. These are also known as embryonic layers, these are Endoderm, Mesoderm, and Ectoderm.

• Lack of Respiratory System -  The lack of Circulatory and Respiratory Organs limits Platyhelminths to sizes and shapes that enable oxygen to reach and carbon dioxide to leave all parts of their bodies by simple diffusion.

• Lack of Skeletal System - Platyhelminthes are soft-bodied  and lack cuticle or other rigid skeletal support. Amongst Platyhelminthes, the Mesenchyme, a connective tissue made of cells and reinforced by collagen fibres acts as a type of skeleton, providing attachment points for muscles.

• Lack of Circulatory System - Platyhelminthes lack a Circulatory System; these functions take place by absorption through the body wall. Non-parasitic forms have a simple, incomplete gut; even this is lacking in many parasitic Species.

• Lack of a Body Cavity - Flatworms have no body cavity other than the gut and lack an anus; the same pharyngeal opening both takes in food and expels waste. Because of the lack of any other body cavity, in larger Flatworms the gut is often very highly branched in order to transport food to all parts of the body. The lack of a cavity also constrains Flatworms to be flat; they must respire by diffusion, and no cell can be too far from the outside, making a flattened shape necessary.

• Locomotion - Movement in some Flatworms is controlled by longitudinal, circular, and oblique layers of muscle. Others move along slime trails by the beating of Epidermal Cilia. The development of directional movement is correlated with Cephalization. In some Flatworms, the process of Cephalization has included the development in the head region of light-sensitive organs called Ocelli. Other sense organs found in at least some members of this group (not necessarily on the head) include chemoreceptors, balance receptors (statocysts), and receptors that sense water movement (rheoreceptors).

• Nervous System -  In all Platyhelminths the Nervous System is concentrated at the head end. This is least marked in the acoels, which have nerve nets rather like those of Cnidarians and Ctenophores, but densest around the head. Other Platyhelminths have rings of ganglia in the head and main nerve trunks running along their bodies.

• Digestive System - Digestion is both extracellular and intracellular. Digestive enzymes (biological catalysts), which mix with the food in the gut, reduce the size of the food particles. This partly digested material is then engulfed (phagocytized) by cells or absorbed; digestion is then completed within the gut cells.

• Restricted to Moist Environment - Respiration through the whole surface of the body makes Platyhelminthes vulnerable to fluid loss, and restricts them to environments where they don't have to face dehydration. Therefore they inhabit sea and freshwater; moist terrestrial environments such as leaf litter or between grains of soil; and as parasites within other Animals.

Size - Platyhelminthes range in length from much less than an inch (a fraction of a millimetre) to 50 ft (15 m).

Geographical Range and Habitat

Platyhelminthes live in a variety of marine, freshwater, and terrestrial habitats worldwide.

Platyhelminthes Feeding

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Diet - Free-living Platyhelminths like Turbellaria are mostly carnivorous. Some Species release ensnaring mucus threads to catch their prey. Because they have developed various complex feeding mechanisms, most Turbellarians are able to feed on organisms much larger than themselves, such as Annelids, Arthropods, Mollusks, and Tunicates like Sea Squirts. Flatworms with a simple ciliated pharynx are restricted to feeding on small organisms such as Protozoans and Rotifers, but those with a muscular pharynx can turn it outward, thrust it through the tegument (external covering or skin) of Annelids and Crustaceans, and draw out their internal body organs and fluids. Turbellarians with a more advanced type of pharynx can extend it over the captured prey until the Animal is completely enveloped.

Predators - Natural predators of Flatworms include the black and neon, velvet Nudibranch and various Species of the Fish family Wrasse which includes Yellow Coris, Green Coris, and the Mystery Wrasse.

History and Evolution of Platyhelminths

The origin of Platyhelminths and the evolution of the various Classes is not clear. There are two theories regarding their evolution. According to the widely accepted theory, the Turbellaria represent the ancestors of all other Animals with three tissue layers. The Other theory proposes that Platyhelminths may be secondarily simplified i.e. they may have degenerated from more-complex Animals by an evolutionary loss or reduction of complexity. It is generally believed that the parasitic groups are derived from the Turbellaria, many of which form close associations with other Animals. These associations often show great host specificity, a characteristic of truly parasitic forms.

There are a number of views regarding the evolutionary relationships among the various parasitic groups.

i)  One school of thought proposes that 'Rhabdocoel Turbellarians' gave rise to Monogeneans; these, in turn, gave rise to Digeneans, from which the Cestodes were derived.

ii)  The second school of thought proposes that the Rhabdocoel ancestor gave rise to two lines; one gave rise to Monogeneans, who gave rise to Digeneans, and the other line gave rise to Cestodes. A further modification to this theory which is based largely on the study of the larval forms, proposes that Cestodes were derived from Monogeneans.

With very few exceptions, Mollusks act as intermediate hosts in digenean life cycles. This condition has led to the widely accepted view that Digeneans were originally commensals (organisms living on or within another organism, and deriving benefit without harming or benefiting the host) of Mollusks that subsequently turned parasitic. Digeneans later formed an association with Vertebrates; the Vertebrates, in turn, became incorporated into the life cycle as definitive hosts.

Platyhelminthes Reproduction

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Apart from a very few exceptions, Platyhelminthes are mostly Hermaphroditic, and their reproductive systems are generally complex. They generally have numerous testes but only one or two ovaries. The Female system is unusual in that it is separated into two structures: the ovaries and the vitellaria, often known as the 'Vitelline Glands' or 'Yolk Glands'. The cells of the vitellaria form yolk and eggshell components. In some groups, particularly those that live primarily in water or have an aqueous phase in the life cycle, the eggshell consists of a hardened protein known as 'Sclerotin', or 'Tanned Protein'. Most of this protein comes from the vitellaria. In other groups, especially those that are primarily terrestrial or have a terrestrial phase in their life cycle, the eggshells are composed of another protein, Keratin, a tougher material that is more resistant to adverse environmental conditions.

In the Tapeworms, the tapelike body is generally divided into a series of proglottids, each of which develops a complete set of Male and Female genitalia. A rather complex copulatory apparatus consists of an evertible (capable of turning outward) penis, or cirrus, in the Male and a canal, or vagina, in the Female. Near its opening the Female canal may differentiate into a variety of tubular organs. Fertilized eggs are often stored in a sac-like uterus, which may become greatly distended; in Tapeworms, it may fill a whole segment. Each Male and Female reproductive system may have its own external opening, or gonopore, or the terminal regions of each system may join to form a common genital atrium, or passage, and a genital pore. Either cross-fertilization (i.e., involving two individuals) or self-fertilization may occur; self-fertilization is probably more common. Many Turbellarians clone themselves by 'Transverse' or 'Longitudinal Division', and others, especially Acoels, reproduce by 'Budding'.  Some free-living Flatworms perform a type of copulation known as 'Hypodermic Impregnation', whereby the penis of one Animal pierces the epidermis of another and injects sperm into the tissues.