Cnidarians: Freshwater Hydra, Jellyfish and Corals

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“Cnidarians” is Greek for “stinging nettle” (“Introduction to Cnidaria"). Phylum Cnidaria include freshwater hydra, jellyfish, and corals. Each of these invertebrates go through transitions in body forms. Jellyfish are the most unusual and complex out of the phylum. Jellyfish are in the class Scyphozoa, which means true jellyfish.
Cnidarians are found in the Mediterranean and in the oceans. They can live in the ocean as well as coastal waters. But the primary habitat for Cnidarians are in the open ocean. Jellyfish can live in any oceans. There are deep water and shallow water jellyfish. Certain jellyfish, such as the Pelagia Noctiluca, can live anywhere that the ocean currents decide to carry it ("Phyla Cnidaria, Platyhelminthes, and Nematoda").
A jellyfish’s unique body structure helps it move, feed, and protect itself. Their body is known as the bell. During the bell stage, medusa, the jellyfish is mobile and is specialized for swimming. But in some Cnidarians, the bell stage only occurs in the embryo stage ("Phyla Cnidaria, Platyhelminthes, and Nematoda"). The medusa then produces an egg, which grows into a polyp (vase stage) which is sessile (or immobile) life. Jellyfish do not have a back or front, left or right. They don’t have a heart, head, legs, or fins (Zimmer). They have a stomach pouch to hold food, the bell (body), oral arms which bring the captured food to the mouth, tentacles which sting and kill prey, a mouth connected to digestive structures to digest food, and gonads which are reproductive organs. Jellyfish are also 95% water (Northeaster University).
Jellyfish move by expanding and contracting to push the water behind them. In between each contraction and expansion is a pause so that a vortex can be created ...

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... The jellyfish don’t have an excretory system, so the hole for the mouth serves as an anus as well (“Pelagia Noctiluca”).
A jellyfish doesn’t have a circulatory system because it is radially symmetrical, meaning they have a symmetrical arrangement of parts about a central point. (“Pelagia Noctiluca”)
Jellyfish are a unique and interesting species. Even though jellyfish look complex they are simple. They have very little internal systems, such as respiratory, circulatory, excretory, and nervous systems. Even though they don’t have a nervous system they have the ability to sting and kill prey to survive. Chemicals can cause them to glow. They can move by expansions and contractions. They can reproduce both asexually and sexually. They are simple invertebrates but yet are still unique and interesting. Jellyfish are fascinating and more complex than they appear to be.
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