........How is life organized? ... How many phyla are there?

These and similar questions have fascinated nature-watchers for centuries. About 150 years ago, Charles Darwin revealed the basic principles for the development of species, and indeed for the organization of all life. Biologists then recognized that all organisms are related to one another through descent from common ancestors – ancestors that evolved at various times in the long (3-billion-plus) year history of life. They visualized a great 'tree of life' full of millions and even billions of branches (equating to species). Fossils in rocks such as the Burgess Shales showed that the animal portion of the tree of life is ancient – at least 500 million years old.

Looking at this Tree of Life, biologists realized that phyla are deep branches where at the very beginning of the branch ancestral organisms developed a fundamentally new and different body plan. They concluded that all the descendants of this ancestor, living and extinct, form a group that can be called a Phylum.

Even with this understanding, it has not been easy to group animals into a definitive pattern of phyla. Although most biologists now recognize about 30 animal phyla in total, there are still many questions such as: how the organisms in any one phylum evolved, how the phyla are related, and even whether some groups that have traditionally been recognized as distinct phyla should be combined together with other groups into another phylum.

In this gallery we will develop a series of short, illustrated essays addressing key questions about the animal phyla. The essays will be developed over time so bookmark this page and check back often. You can even contribute to these essays or to other questions if you like. Send your comments and ideas by e-mail to david@ebiomedia.com.

About Animal Phyla (These will eventually be linked to essays):

What is a phylum?
Are all phyla really equivalent levels of organization?
How many animal phyla are there?
Traditional views of animal phyla classification are changing.
What does molecular genetics reveal about animal classification?
What are protostomes and deuterostomes?
What are the Ecdysozoans and Lophotrochozoans?
Is the Arthropoda one phylum or more? the Chordata? the Annelida?
What phyla might not be phyla?
How are animals related to other organisms?

The Big Nine

The vast majority of all animal species (over 95%) belong to the phyla listed here, sometimes called the ‘Big Nine’. These groups are covered by our Biology Of series of video and DVD programs. Check these links for concise information about each group in our online program study guides.

Phylum Porifera

Phylum Cnidaria

Phylum Platyhelminthes

Phylum Nematoda (with coverage of other groups)

Phylum Mollusca

Phylum Annelida

Phylum Arthropoda

Phylum Echinodermata

Phylum Chordata