Figure 23.33: Echinoderm Diversity (A) The flexible arms of this feather star are clearly visible. (B) Sea urchins are important grazers on algae in the intertidal zones of the world’s oceans. (C) Sea cucumbers are unique among echinderms in having an anterior–posterior rather than an oral–aboral orientation of the mouth and anus. (D) Sea stars are important predators on bivalve mollusks such as mussels and clams. Suction tips on its tube feet allow a sea star to grasp both shells of the bivalve and pull them open. (E) The arms of the brittle star are composed of hard but jointed plates.