Bilaterally symmetrical animals can be divided into two major groups that separated during the Cambrian. These two lineages differ fundamentally in their

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The two major groups of bilaterally symmetrical animals that separated during the Cambrian differ fundamentally in their early embryological development. One group, the protostomes, undergoes spiral cleavage during cell division, while the other group, the deuterostomes, undergoes radial cleavage. This fundamental difference in embryonic development has profound implications for the development of the gut and other organs in these animals. The other choices are not necessarily true for all bilaterally symmetrical animals and are not fundamental differences between the two groups that separated during the Cambrian.

Bilaterally Symmetrical Animals Can Be Divided into Two Major Groups