Animals are multicellular organisms with digestive tracts.  

     How simple this definition seems when we think of the complexity of animals, their body forms, their life styles, their behaviors, their many ways of surviving on this world!

Yet this is what the animals are all about: they capture and consume organic material in order to live.

     Complex animals began to develop in pre-cambrium times, more than 540 million years ago.  Unlike seaweeds, which remained simple in structure, all the basic body plans of animals appeared then, and no other basic body plans  have developed on earth since.  The amazing variety of life forms that we see now have evolved from the earliest forms.  

The basic body plans are called the phyla.

     In this lesson we will look at some of the phyla and how the basic structure of their members solves specific survival problems.

     You will see that once the body plan of a phylum has developed, an animal cannot evolve into another phylum.  For example, a crab cannot evolve into a horse, a cow cannot become a butterfly.  Keep this in mind when you watch your animals evolve!  They have to develop the assets that they have, and cannot acquire new characteristics without modifying some pre-existing structure.

     We will look at these phyla, which contain members that are familiar to us:

.     As we learn about each group, we will get some ideas about challenges that life forms must meet if they are to evolve and survive.