World Builders™
World Builders™
    Session Six  -- Water Animals     
Session Six  -- Water Animals    

Cnidaria - Sea Anemones      
Cnidaria - Sea Anemones   

Cnidaria  - Jellyfish

Cnidaria -- Coral

The Phylum Cnidaria is made up
of members who's body plan is a bag within a bag.  The outside skin, or exoderm, has nematocysts, cells that can sting and protect the animal:  the inside. or ectoderm, holds the food that the animal has captured and digests this food.  It expels what it
cannot digest through the opening
in the top of its body.

     Between the ectoderm and the endoderm there is a space filled with a soft jello-like material called the mesoglea.  This material is not inside cells, it is a sort of filler that can absorb oxygen and nutrients and redistribute these substances by diffusion.  It makes the walls of the anenome thicker, which strengthens them.  The mesoglea also has collagen in it, which strengthens the walls and helps the animal to maintain its shape.

     Cnidarians introduced something new: tissues, which are groups of cells that have a specific, differentiated function.  The cnidaria were the first animals to have simple muscles and nerves to allow them to contract the sides of their bodies.  They need to do this in order to expel non-food items from their bodies.

     This was a great advance in the evolution of animals!

  Their simple body plans work!

    Starting from this basic plan, many different animals evolved.  All the animals that descended from the distant common ancestor continued to have the nematocysts, the stinging cells.  Their physical forms evolved to find different ways to live in the ocean.  We will look at three of them:  sea anemones, corals, and jellyfish.

Sea Anemones

     Sea anemonies clearly have the bag within a bag shape.  Although they look like flowers when their tentacles are open, they are really quite fierce predators, spearing their prey with the stinging cells (nematocysts) in their tentacles.  The ones that feed on plankton have delicate, feathery tentacles. those that seek larger prey have stronger tentacles that can pull small fishes into the anemone's digestive cavity.  The anemone can close off its top and digest its meal, then it spits out whatever is left over.

Sea anemones are found in all ocean environments, even in the great depths of the abyssal plain.  They are more plentiful near the shores.

Anemones reproduce sexually.  The eggs are fertilized in the gastric cavity and later released as free-swimming larvae.  The young anenomes later attach themselves to rocks or other solid objects.  Anemones can also reproduce by budding.  A little anemone can just bud off from the side of an adult.

Anemones absorb oxygen from the water around them.

Sea anemones can move by creeping on their basal disks, but this is such a slow process that it is not really visible if you sit down to watch it.

There are over 1000 species of anemones in earth's oceans today.  That's a success story!

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Header by Viau from
Olympic National Park         Photos by Viau and NOAA
© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at .