A Monograph of the Echinoidea

Abstract

THIS monograph attracts a review, for it exhibits features of a distinctly novel nature and it approaches perfection as near as can be. The animals are the box-like sea-urchins, starfishes with radiating segments and brick-like walls (tests) decorated with spines and fine sculpturing. They walk by outpushings of fine tubes (tube-feet) with terminal sucking-disks arranged in five rays, such as cause starfish to be of unique personality. They belong to Mortensen more than to anyone else, for he has pursued the living forms in most parts of the world. Here, his study of the urchins' habits, food and ecology was antecedent to that of their classification, and he thus by analogy can tell us much of the life of the extinct forms. Of the latter there is a long series from the Palaeozoic, and it would seem probable that these animals have never been less numerous either in genera and species or in individuals than they are to-day. Thus the relatively deep-living Salenidae had eighty-five species in the Cretaceous as compared with only twelve species to-day, but all the extinct forms were not deep-living.