WHAT IS A CNIDARIAN?

Cnidarians are soft bodied animals that have stinging tentacles arranged in circles around their mouth. All cnidarians exhibit radial symmetry and have specialized cells and tissues. Most have life cycles with two different stages. One of those stages is called polyp, which is sessile and flower like, and the other stage is called medusa, which is motile and bell-shaped. Some cnidarians live in groups of dozens or thousands connected together called a colony. Other cnidarians live individually.























General Anatomy
The body wall of consists of three layers. Epidermis is the layer of cells that covers the outer surface of the body. The mesoglea is a thin non-cellular membrane in polyps and a thick jellylike material that may contain amebocytes in medusae. The gastroderm is the layer of cells that covers the inner surface, lining the gastrovascular cavity. The body wall surrounds the gastrovascular cavity where food is digested.

Some Examples
Class Hydrozoa contains hydra, obelia, and physalia
ex. Portuguese man-of-war









Portugese man-of-war



Class Scyphozoa contains aurellia (jellyfish)
ex. Lion's Mane, Box Jellies, Moon Jellies





Moon Jelly



Class Anthozoa contains sea anemones and corals
ex. Sea fans, Sea Pens, Gorgonian Coral and Brain Coral












Sea Pen


Defining Characteristics
Cnidarians have stinging cells called nematocysts in their tentacles. Nematocysts are a poison-filled sac outfitted with a dart that shoots into the victims skin. Other characteristics of cnidarians is that they have no centralized nervous system, are radially symmetric, are aquatic, and have only a single opening to the body cavity.


Habitat
Cnidarians live in mostly marine environments at varying depths. A few species live in freshwater.


Adaptations to their environment
Jellyfish have the ability to swim which makes it easier to find and catch prey, instead of having to wait for the prey to come to you like in polyps. Cnidarians have also developed stinging cells called nematocysts which paralyze their victims to make them possible to eat.


Comparisons to other phyla
Cnidarians are an advancement over sponges. Sponges are totally sessile, have no gastrovascular cavity, and have no specialized tissues. More highly evolved phyla have a centralized nervous system which cnidarians lack.


Vocab
All underlined words are important and are defined within the paragraph they are found in.