write a short note on plathehelminthis...??
Answers
Answer:
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Explanation:
Platyhelminthes include the flatworms, their bodies are compressed dorsoventrally and show bilateral symmetry. They are the lowest triploblastic acoelomate Metazoa, but they are more advanced than Coelenterata because their tissues are organised into organs.
Answer:
Platyhelminthes are the flat worms that are unsegmented invertebrates with soft body.
Since they have no respiratory nor circulatory organs, all respiration and blood circulation is done by diffusion.
They have no body cavities except the internal, digestive one that is using only one opening on their bodies to eat and fecate: their mouth.
By eating and excrementing through their mouths such similar feature can be found in succinic polyps.
They are bilaterians (animals with two symmetrical body sides, having head and tail and synonym for them is Triploblasts) that having both sides of body perfectly symmetrical with distinctive head and tail ends.
They have endoderm, mesoderm and ectoderm as three chief cell layers covering their bodies.
Since they lack respiration and circulation (organ lacking), they use diffusion (as random walk of particles from the regions of their higher concentration to the regions with lower concentration by concentration, temperature and pressure in reference with distance as bulk motion) to circulate oxygen and nutrients across their bodies.
Their only bodily cavity is a digestive one that has tubes as branches that are sending nutrients from this cavity to the rest of the body through diffusion.
The respiration as diffused is conducted through whole body where oxygen and carbon dioxide intake and departure is done by their ribbon like and leaf like body shapes that have a body walls that absorb these gases, some have ribbon like and some leaf like shapes.
This type of absorption (through whole body) is what makes them sensitive to dehydration or liquid loss so they usually live in humid to very humid environments.
Their guts are connected to the skin by mesenchyme (a type of connective tissue that is found in embryos) and is perfused with collagen fibres which consists their skeleton which is further connected with muscles that can have oblique, longitudinal and circular shapes.
These muscles are used to control the movements and walking while some kinds of flatworms are using a sort of motile cilia (large organelles) that form epidermis and inside of which there are dynein motors that are making the cilia to move and produce the fluid flow; their ciliated epidermis is excreting slime like fluid which allows them to glide over the surfaces similar as snails do.
Their nervous system is located at the back of the head and is cephalized that has one ganglion with longitudinal nerve cords going through the whole body and are interconnected with transversal branch like nerves.
Since they have all internal organs except respiratory and circulatory ones, the excretion and osmoregulation (process of regulation of osmotic pressure of body fluids in invertebrate organisms) is done by a type of flame cells or excretory cells located in protonephridia as primitive excretory organ.
They have numerous receptors in their bodies that are used for sensing such as rheoreceptors through which they can sense the movement of the water.
All of these functions, excretion, movement, sensing are governed by their heads that are consisted of head ganglion, mouth, sensing organs which gathered through the evolutionary process called the Cephalization.
Since they have no anus, they do excrementing on their mouth but there are some types of flatworms are having one and more anuses and with guts.
To digest food, platyhelminthes are having guts encased in endodermal cells that are digesting the food while some types are softening the food before they eat by secreting the enzymes formed in their guts through the throat.
The reproduction is sexual and asexual with them being monoecious which means that both male and female reproductive organs are within one individual.
The hatching can be Direct: worms identical to adults hatch from the eggs only smaller and Indirect Hatching: as larvae.
The main sub types are: Turbellaria, Trematoda, Monogenea and Cestoda.
The sub types are:
– Catenulida, Rhabditophora, Trepa
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