Llf

Figure 6.16

Evolution of the radially symmetric echinoderm body plan

(a) Echinoderm lineages have bilaterally symmetric larvae and radially symmetric adults. (b) Two hypotheses regarding the orientation of the ancestral bilaterian A/P axis in the adult echinoderm body plan: (left) multiplication of the bilaterian A/P axis may have created the radial nerve cords that extend down each echinoderm arm; (right) the ancestral bilaterian A/P axis may have evolved into the echinoderm oral—aboral axis.

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