XVIII.—Abnormal Echinoids in the Collection of the Royal Scottish Museum

Abstract

“Echini are a particularly good group in which to study questions of variation, because here variations can usually be expressed in very definite terms of numerical or other equally positive character.” On this account, and because, in spite of much description, the variants liable to occur in sea urchins have not yet been exhausted, the three specimens described below are recorded. Each of these exhibits a pronounced abnormality in the major symmetries. Two of them resemble another abnormal echinoid in the same collections, already discussed inProc. Zool. Soc., in lacking part of a definite ambulacrum; but the means by which the tests have accommodated themselves to changed conditions of growth differ markedly in each of the three cases. The third specimen exhibits, in place of the normal five-radiate arrangement, almost perfect hexamery—a type of abnormality very different from that of the first two specimens. For in these the distortion is due to incomplete development caused, by interference with the processes of growth, while there the hexamery is a fundamental change in symmetry, is congenital in origin, and probably represents the type of variation known as duplication of parts.