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In my invertebrate bio class we are learning about the development of unidirectional movement (which occurred somewhere on the phylogenetic tree between the placozoan and acoelomate branches). My professor stressed that the three morphological developments associated with the evolution of unidirectional movement were cephalization, bilateral symmetry and triploblasty (i.e. the development of the mesoderm). I am pretty comfortable explaining how cephalization is associated with unidirectional movement- it makes sense that an organism would benefit from having its sensory organs concentrated at the anterior pole of its body, for this is the part of the organism that first comes in contact with the surrounding environment. However, I'm having trouble connecting the dots between how a third tissue layer (the mesoderm) and bilateral symmetry would be beneficial for unidirectional movement. Any thoughts?
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