Eccentricity of the Apical System and Peristome of Sand Dollars (Echinodermata: Echinoidea: Clypeasteroida: Scutellina)

Eccentricity, location of structures away from a central position, is associated with directional movement. Although sand dollars have directional movement, only eccentricity of the anus is apparent. Eccentricity of the apical system and peristome is less apparent. We have found the apical system and the peristome are statistically significantly slightly anterior in Mellita tenuis, Mellita quinquiesperforata, Mellita isometra, and Encope aberrans. The apical system of Leodia sexiesperforata is central and that of Echinarachnius parma is anterior, whereas the peristome of both is statistically significantly slightly posterior. The usual selective pressure for pronounced anterior location of the mouth in animals with directional movement may be countered by the mode of feeding in sand dollars that utilizes the oral surface. The basis for the eccentricity of the apical system is not known