Jellyfish is regarded as the most primitive life forms still in existence. It is found in most of the oceans and seas all over the world. These are known more for their sting which is similar to electric shock and their bell shaped structure. This primitive life form is composed of up to ninety-five percent water!
Jellyfish, as the name suggests, has no fixed body structure. They do not have bones and a skeletal system. They do not even have a protective exoskeleton. This gives them the undefined shape. They can be damaged easily and are extremely delicate. The extent of delicateness can be understood by the fact that they cave in and perish when removed from water.
The body of the jellyfish is symmetrical about the vertical axis. The top portion of the body is shaped like a bell and the lower portion has numerous tentacles. Unlike most other animals, reptiles and mammals, they do not have bilateral symmetry. This implies that their right side is identical to their left side.
The jellyfish has a mouth underneath the bell. This is used to take the food in and discard the waste out as well. The food is broken down in the coelenteron, which is a sac. It uses the currents in the water when it is required to move over long distances. While in water, the jellyfish makes use of propulsion in order to move in the forward direction. It takes in water under its bell and then expels it just behind its body. This propels the jellyfish in the opposite direction.
They do not have any nervous system or brain. The sense of light and dark is made out using the eye spot and neural net while chemo-sensory pit is used to detect the presence of preys.
Their body is divided in to 3 distinct layers. The outer layer called epidermis, middle layer, which is essentially a thick material called mesoglea, and inner layer called gastrodermis. This inner layer forms the lining for the coelenteron.
The sensors used for detecting a probable prey are called nematocytes. These line the entire body all along the feeding arms, mouth and tentacles. Each nematocyte is made up of a small capsule in which a barbed coil, chemosensitive trigger and venom sac are located. Once the would-be prey comes in contact with the nematocyte, the trigger is activated. This causes expulsion of the barbed coil. When the contact is made, venom from the sac is injected in to the prey which immobilizes it. The jellyfish then makes use of its arms to physically push the prey in to its mouth.
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