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Session Six  --  Water Animals
Session Six  --  Water Animals 



Chordata - Animals with Backbones
Chordata - Animals with Backbones

Fishes are vertebrates, with internal skeletons that grow inside the animal.

   
      The animals that we have been learning about so far are all marine invertebrates.  Invertebrates are animals without backbones, and they are organized around their intestinal tracts -- the sac that captures and processes food, or a tube that starts with the mouth and ends with an opening near the tail end of the animal.  This basic organizing feature allows for an amazing number of body forms.

     The phylum chordata is the one whose members are most visible to us.  Animals in this phylum are characterized by a supporting  rod of connective tissue running down the length of the body.  In the earliest chordates this was just a rod between a nerve cord and a digestive tract, all of which ran down the length of the body from the head  Some of these early chordates still have descendents alive today.

     The early chordates did not have bones -- they had cartilage.  Cartilage is made of  -- Guess what -- Collagen!!!   Do you remember which animal was the first to have collagen?  Click here to review

     However, the most successful and numerous representatives of this phylum are in the sub-phylum of the vertebrates.  Vertebrates are animals with backbones and bony. internal skeletons, animals such as fishes, amphibians, reptiles, mammals, and birds.  The notochord is seen in their embryos, and is encased inside the vertebrae as they develop

     Before vertebrates ventured onto the land, however, they were already a big success in the oceans.  The Devonian Age is called the Age of fishes, and the fossil record is rich in fishes of all sizes.

Here is a picture of a fossil of a fish.

Take a minute to look at this fossil fish.  See how the spine runs down the body and how the other bones attach to it.   There are more than 25,000 species of fish in the waters of the earth today.  In lesson 9 we will think about the vertebrates that live on land.


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Photos from archives at Biology Department, University of Bowling Green, Ohio
Header Fossil Photo by Viau when in Smithsonian Museum
© 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 eviau@earthlink.net .