Bilateria
Triploblasts, Bilaterally symmetrical animals with three germ layers
This tree diagram shows the relationships between several groups of organisms.
The root of the current tree connects the organisms featured in this tree to their containing group and the rest of the Tree of Life. The basal branching point in the tree represents the ancestor of the other groups in the tree. This ancestor diversified over time into several descendent subgroups, which are represented as internal nodes and terminal taxa to the right.
You can click on the root to travel down the Tree of Life all the way to the root of all Life, and you can click on the names of descendent subgroups to travel up the Tree of Life all the way to individual species.
For more information on ToL tree formatting, please see Interpreting the Tree or Classification. To learn more about phylogenetic trees, please visit our Phylogenetic Biology pages.
close boxThis tree represents the view of bilaterian relationships as it is currently emerging from analyses based on molecular data (mostly 18S rRNA sequences). For an alternative hypothesis of bilaterian relationships based on morphological data, see the Discussion of Phylogenetic Relationships.
Discussion of Phylogenetic Relationships
Due to new evidence from developmental biology and molecular phylogenetics, ideas about bilaterian relationships have undergone a major paradigm shift within the last decade. The new hypotheses shown in the tree above are now widely accepted, but there are also many sceptics who emphasize the pitfalls and inconsistencies associated with the new data. One of the most prominent alternative views based on morphological evidence is championed by Nielsen (2001):
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Title Illustrations
| Location | Sacha Lodge, Oriente, Ecuador |
|---|---|
| Specimen Condition | Live Specimen |
| Source | Hoatzin |
| Source Collection | Flickr |
| Copyright |
© 2007 Jim Scarff
|
| Scientific Name | Pisaster ochraceus |
|---|---|
| Location | Protection Island, British Columbia |
| Specimen Condition | Live Specimen |
| Source | Purple Starfish |
| Source Collection | Flickr |
| Image Use |
 This media file is licensed under the Creative Commons Attribution-NonCommercial-NoDerivs License - Version 2.0.
|
| Copyright | © 2006 Alanna@VanIsle |
| Scientific Name | Argiope |
|---|---|
| Location | Dallas, Texas |
| Comments | Argiope spider (Arthropoda) on web with stabilimentum |
| Copyright |
© Greg and Marybeth Dimijian
|
| Scientific Name | Cyphoma gibbosum |
|---|---|
| Location | Curacao, Netherland Antilles |
| Specimen Condition | Live Specimen |
| Source | Flamingo Tongue Snail in Profile |
| Source Collection | Flickr |
| Image Use |
This media file is licensed under the Creative Commons Attribution License - Version 2.0.
|
| Copyright | © 2006 LASZLO ILYES |
| Scientific Name | Bipalium |
|---|---|
| Location | Dallas, Texas |
| Comments | A planarian flatworm (Platyhelminthes) after emerging from wet soil after rain |
| Copyright |
© Greg and Marybeth Dimijian
|
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Bilateria. Triploblasts, Bilaterally symmetrical animals with three germ layers.
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