Most animals, including vertebrates (fishes, frogs, birds, humans), arthropods (insects, spiders, crustaceans), and molluscs (snails, mussels, squid), are part of the group Bilateria. This name is derived from the fact that representatives generally show bilateral symmetry. A single median axis divides the body into equivalent halves that are mirror images of one another. These halves run from the front end, which is usually differentiated into a head, to the back end, often in the form of a tail. This body plan contrasts with that of radially symmetrical animals like jellyfish and sea anemones which have multiple similar parts arranged around a central axis, so their bodies lack a front and back end.
Some bilaterians, such as sea stars and sea urchins, actually have radially symmetrical bodies. However, it is clear that they evolved from bilaterally symmetrical ancestors, and their larvae still display bilateral symmetry.
The alternative name Triploblastica is sometimes applied to this group. This name refers to the fact that the early embryo of bilaterians (the blastula stage) is generally composed of three primary germ layers: the ectoderm (outer layer), the endoderm (inner layer), and the mesoderm (middle layer) between them. The blastula of diploblastic animals, like corals and comb jellies, lacks a mesoderm and therefore has only two layers.
Evolution and Systematics
Discussion of Phylogenetic Relationships
This 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.
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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