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Retell one of the texts.
Read the texts using your dictionary.
Answer these questions to Text 1.
Give a title to each paragraph.
1. What phyla of invertebrates do you know?
2. What features distinguish mollusks from other invertebrates?
3. Tell about morphological characteristics of Cephalopods.
4. Describe the structure of Cephalopods.
5. Why and how do they change color?
6. What is the difference between arms and tentacles?
7. Tell about invertebrates` importance to humans.
The jellyfish is one of the oldest living creatures in the world. Jellyfish have existed on the face of this planet for over 650 million years. They have existed since before the dinosaurs and have survived long after the dinosaurs and million other species have gone extinct. The Jellyfish is amongst the most fascinating and intimidating creatures of the marine world. Jellyfish are found across all oceans in the world. Jellyfish exist at a wide range of depths and water conditions, from the ocean surface to floor. They look like shapeless blobs when they wash up on the shore, but look extremely graceful and dangerous when they are afloat in water. There are over 2000 species of jellyfish known in the world, with many more species being discovered as man searches the depths of the ocean. Jellyfish are a member the phylum Cnidaria, to which the coral, sea whip and sea anemones also belong. They are simple invertebrates and are mobile unlike all other members of the phylum Cnidaria.
The movements of jellyfish are usually subject to winds and currents of the ocean, although they are capable of vertical movement.
The body parts of a jellyfish radiate from the central axis and are perfectly symmetrical. The perfectly symmetrical body of the jellyfish allows it to respond to danger from all sides. It also helps it in being able to detect food from every direction. The body of a jellyfish contains only a nervous system and the nerve receptors themselves formulate appropriate reaction to detection of light, odor, pressure and other external stimuli. The jellyfish does not have a brain or any specialized functional system. It has an outer layer called the epidermis, and an inner layer which is gastrodermis which lines the gut. In between is the thick, elastic-like jelly known as mesoglea. In a simple digestive cavity the gullet, stomach and the intestine lies. On one end of the cavity is the mouth; on the other is the anus. There are four to eight oral arms near the mouth that help in bring the food near the mouth, along with the tentacles. In fact, only 5% of the body of a jellyfish is solid matter. Jellyfish have different shapes, sizes and colors. They can be smaller than an inch or larger than 7 feet in diameter, with tentacles spanning more than a 100 feet! Irrespective of their size, jellyfish can only have control over their vertical movement. Horizontally, their movement is completely controlled by the wind and the current. Their bell shape allows them contract and force outer outwards to get a push upwards. Jellyfish have the ability to sting with their tentacles. Their tentacles have a number of capsule like apparatus that are filled with paralysis causing toxins and are fired when the tentacles comes in contact with something. Jellyfish use this facility mainly for hunting; However, jellyfish also use this facility to defend themselves. The toxins in the tentacles are powerful enough to paralyze their victim and buy enough time to escape, if not to kill it. However, in the case of humans, most jellyfish stings will only result in minor discomfort. The severity of stings is high in jellyfish found in warm and temperate oceans and these can be dangerous to humans.
Jellyfish are amongst the most abundant inhabitants of marine waters. Most jellyfish sting and are equipped with stingers to procure food and protect themselves. While jellyfish do not actively attack humans, when their tentacles come in contact with human flesh, their stingers are released as part of their natural defense mechanism. Even though most jellyfish stings are completely harmless to humans and do not pose a serious risk, it is important to know the basic first aid required for a jellyfish sting before you step into jellyfish infested waters to swim, surf or dive. Please be aware of the specific species of jellyfish known to inhabit the waters you are entering, so that you can be prepared with the treatment required in case of a jellyfish sting.