SUMMARY
In addition to symmetry (radial or bilateral) and type of body cavity (acoelomate, pseudocoelomate, or coelomate), taxonomists use developmental and molecular data to help classify animals. Systematists divide coelomate animals into two groups based on the early embryo cleavage pattern. However, after studying the sequence of rRNA and Hox genes, some biologists suggest that the acoelomate nemerteans (ribbon and proboscis worms) are closely related to some coelomates.

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