THE SEPARATION OF POST-BASICORONAL AREAS FROM THE BASICORONAL PLATES IN THE INTERAMBULACRA OF THE SAND DOLLAR, ECHINARACHNIUS PARMA (LAMARCK)

¹ Department of Zoology, University of New Hampshire, Durham, New Hampshire 03824

1. The general arrangement of plates on the oral surface of sand dollars is discussed.

2. Variations in this arrangement as they occur in Echinarachnius parma from several New England localities are indicated.

3. As this sand dollar increases in size, there is decreasing contact between the post-basicoronal interambulacral areas and the basicoronal plates.

4. The usual sequence in which this contact is lost among the areas is 5, 1, 4, 2 and 3, but all possible combinations of areas "out of contact" have been observed, except 2, 3 and 4.

5. The average numbers of areas "out of contact" for animals of comparable sizes vary among localities.

6. The asymmetry of loss of contact within the interambulacral areas has also been found to be highly variable.

7. The possibility that these variations may be related to differential environmental effects upon the rates at which different parts of the growth process occur is suggested.