The Platyhelminthes include free-living flatworms, like the planarians, and the parasitic tapeworms and flukes. The term flatworm refers to the fact that the body is dorsoventrally flattened. Flatworms are the first organisms to have tissues organized into organs and the first to demonstrate bilateral symmetry. Bilateral symmetry means that one plane passing through the longitudinal axis of an organism divides it into right and left halves that are mirror images. It is characteristic of active, crawling, or swimming organisms and usually results in the formation of a distinct head (cephalization) where accumulation of nervous tissue and sensory structures occurs.
Classes of Platyhelminthes:- There are Five Classes of Platyhelminths
1. Class Turbellaria
2. Class Cestoda
3. Monogenea
4. Trematoda
5.Aspidogastria
Major Characteristics of Platyhelminths:
1. Bilateral Symmetry
These worm like animals are bilaterally symmetrical, which means that their right and left sides are the mirror images of each other. This also indicates that these animals have distinct head and tail ends. They are triploblastic, that is they possess three main cell layers: outer ectoderm, middle mesoderm and inner endoderm just like the other bilateral animals. The middle layer in these animals is made up of spongy mesoderm cells and is also called parenchyma.
2. Acoelomate
Unlike the other bilateral animals, these animals possess no internal body cavity. Coelomate means body cavity present and since body cavity or coelom is absent, they are termed as acoelomate.
3. Absence of Circulatory and Respiratory Organs
The animals of platyhelminthes lack circulatory and respiratory organs, which is why their bodies are so flat. The flat body shape permits the flow of oxygen and nutrients to reach all parts of the body by simple diffusion process. Carbon dioxide leaves the body by the same diffusion process. The gut of these animals is branched profusely in order to facilitate adequate diffusion of nutrients to all parts of the body.
4. Restricted to Wet Environment
Respiration occurs throughout the surface of the body, thereby making the Platyhelminthes animals susceptible to loss of precious body fluids. This can result into dehydration. Thus these animals are restricted to living in a wet environment, such as in freshwater, sea or in moist terrestrial environment. The different moist terrestrial environment comprise grains between the soil, in leaf litter or living as parasites in the bodies of other insects.
5. Possesses a Type of Skeleton
Mesenchyme, a connective tissue is seen to fill the space between the skin and gut in these animals. It consists of two kinds of cells: fixed cells and stem cells. Fixed cells have fluid filled vacuoles and stem cells are the cells capable of transforming into any other type of cell, thereby useful during the tissue regeneration and asexual reproduction. This mesenchyme is reinforced by collagen fibers that provide attachment points for the muscles, thereby acting like a skeleton. Mesenchyme comprises internal organs and permits the passage of nutrients, oxygen and waste products.
6. Digestion
The digestive system is incomplete with a single opening serving as the mouth and anus. A single layer of endodermal cells line the gut, whose role is to absorb and digest the food materials. Some species also feature the secretion of enzymes in the gut or pharynx to soften and break up the food. The undigested food material is regurgitated through the mouth as an anal opening is absent. However, there are exceptions to this. Large species do possess an anal opening and some with exceptionally profusely branched guts possess more than one anal opening. This is because solely excreting from the mouth would prove difficult for them.
7. Excretory System
The excretory system of these animals is well-developed and is called protonephredia. Protonephredia comprises a network of tubules within the animal's body tissues. One end of the tubule extends into an exterior pore on the surface of the body, while the other end leads into spherical structure called flame cells. These flame cells possess long cilia which carry out a beating function. When the cilia beats, it gives the flame cells an appearance of a flickering candle and this is where the cells get their name. Excess water and body wastes enter the flame cells, are pushed into the tubules by the movement of cilia and thrown out of the body from the pore on the surface.
8. Osmoregulation
The control or regulation of water balance and ion levels in the body is termed as osmoregulation. Platyhelminthes animals live in the environment with high concentrations of dissolved materials. All animals have to maintain the concentration of dissolved substances at a constant level. These animals let their body tissues have the same concentration level as that of the environment with the help of protonephredia network.
9. Nervous System
Nervous System marked at the head region is of the primitive type. It comprises pair of anterior ganglia or a nerve ring connected to 1-3 pairs of longitudinal nerve chords with transverse commissures. These ganglia serve as the brain.
10. Reproductive System
These animals can reproduce asexually and as well as sexually. They can reproduce asexually by transversal bipartition due to their wonderful ability to regenerate their tissues. They attach themselves to a substrate and induce a constriction in the body's mid region and get divided into two parts. The two divided body parts can regenerate into two different animal individuals. These animals are mostly hermaphrodite, that is both female and male reproductive organs are situated in one animal itself. Rarely animals with separate sex are found. Reproductive organs are highly developed. Self-fertilization (fusion of male and female gametes from the same animal) as well as cross fertilization (fusion of male and female gametes from different animals of the same species) is seen. The development is mostly direct, which means that the development does not include larval stage formation.
1. Class Turbellaria
2. Class Cestoda
3. Monogenea
4. Trematoda
5.Aspidogastria
Major Characteristics of Platyhelminths:
1. Bilateral Symmetry
These worm like animals are bilaterally symmetrical, which means that their right and left sides are the mirror images of each other. This also indicates that these animals have distinct head and tail ends. They are triploblastic, that is they possess three main cell layers: outer ectoderm, middle mesoderm and inner endoderm just like the other bilateral animals. The middle layer in these animals is made up of spongy mesoderm cells and is also called parenchyma.
2. Acoelomate
Unlike the other bilateral animals, these animals possess no internal body cavity. Coelomate means body cavity present and since body cavity or coelom is absent, they are termed as acoelomate.
3. Absence of Circulatory and Respiratory Organs
The animals of platyhelminthes lack circulatory and respiratory organs, which is why their bodies are so flat. The flat body shape permits the flow of oxygen and nutrients to reach all parts of the body by simple diffusion process. Carbon dioxide leaves the body by the same diffusion process. The gut of these animals is branched profusely in order to facilitate adequate diffusion of nutrients to all parts of the body.
4. Restricted to Wet Environment
Respiration occurs throughout the surface of the body, thereby making the Platyhelminthes animals susceptible to loss of precious body fluids. This can result into dehydration. Thus these animals are restricted to living in a wet environment, such as in freshwater, sea or in moist terrestrial environment. The different moist terrestrial environment comprise grains between the soil, in leaf litter or living as parasites in the bodies of other insects.
5. Possesses a Type of Skeleton
Mesenchyme, a connective tissue is seen to fill the space between the skin and gut in these animals. It consists of two kinds of cells: fixed cells and stem cells. Fixed cells have fluid filled vacuoles and stem cells are the cells capable of transforming into any other type of cell, thereby useful during the tissue regeneration and asexual reproduction. This mesenchyme is reinforced by collagen fibers that provide attachment points for the muscles, thereby acting like a skeleton. Mesenchyme comprises internal organs and permits the passage of nutrients, oxygen and waste products.
6. Digestion
The digestive system is incomplete with a single opening serving as the mouth and anus. A single layer of endodermal cells line the gut, whose role is to absorb and digest the food materials. Some species also feature the secretion of enzymes in the gut or pharynx to soften and break up the food. The undigested food material is regurgitated through the mouth as an anal opening is absent. However, there are exceptions to this. Large species do possess an anal opening and some with exceptionally profusely branched guts possess more than one anal opening. This is because solely excreting from the mouth would prove difficult for them.
7. Excretory System
The excretory system of these animals is well-developed and is called protonephredia. Protonephredia comprises a network of tubules within the animal's body tissues. One end of the tubule extends into an exterior pore on the surface of the body, while the other end leads into spherical structure called flame cells. These flame cells possess long cilia which carry out a beating function. When the cilia beats, it gives the flame cells an appearance of a flickering candle and this is where the cells get their name. Excess water and body wastes enter the flame cells, are pushed into the tubules by the movement of cilia and thrown out of the body from the pore on the surface.
8. Osmoregulation
The control or regulation of water balance and ion levels in the body is termed as osmoregulation. Platyhelminthes animals live in the environment with high concentrations of dissolved materials. All animals have to maintain the concentration of dissolved substances at a constant level. These animals let their body tissues have the same concentration level as that of the environment with the help of protonephredia network.
9. Nervous System
Nervous System marked at the head region is of the primitive type. It comprises pair of anterior ganglia or a nerve ring connected to 1-3 pairs of longitudinal nerve chords with transverse commissures. These ganglia serve as the brain.
10. Reproductive System
These animals can reproduce asexually and as well as sexually. They can reproduce asexually by transversal bipartition due to their wonderful ability to regenerate their tissues. They attach themselves to a substrate and induce a constriction in the body's mid region and get divided into two parts. The two divided body parts can regenerate into two different animal individuals. These animals are mostly hermaphrodite, that is both female and male reproductive organs are situated in one animal itself. Rarely animals with separate sex are found. Reproductive organs are highly developed. Self-fertilization (fusion of male and female gametes from the same animal) as well as cross fertilization (fusion of male and female gametes from different animals of the same species) is seen. The development is mostly direct, which means that the development does not include larval stage formation.
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