Echinoderms: Sea-Urchins and Brittle-Stars

Abstract

Adult sea-urchins, heart-urchins and sand-dollars, most of which have rounded, rigid hodies and spacious coeloms, look very different from adult brittle-stars, with their snake-like arms and restricted coeloms, but the majority of species in both classes have a unique form of larva, the pluteus. Its uniqueness lies in its internal skeleton of slender calcareous rods that support its ciliated arms. Hyman (1955, The Echinodermata, p. 700) thought it impossible to account for the occurrence of pluteus larvae with similar skeletal rods in both echinomorphs and ophiuromorphs “except on the basis of some community of ancestry”, and a similar view was expressed by Jägersten (1972) in his book, Evolution of the Metazoan Life Cycle. MacBride (1914, p. 511), in his Text-Book of Embryology, went so far as to say that the differences between echinoplutei and ophioplutei were “of minor taxonomic importance, and would be such as one would expect to find separating the larvae of two families.” These comments illustrate that some specialists in echinoderms and their larvae have been very impressed by the similarities between echinopluteus and ophiopluteus larvae.