Echinoderms: Adults and Larvae

Abstract

The Echinodermata are a major phylum of marine animals, and they provide many striking examples of inconsistencies between the apparent relationships of adults and larvae. These inconsistencies occur in trying to assess the relatedness, first, of the echinoderms to other phyla, second, of classes within the echinoderms, and, finally, of several pairs of species within the same class. I hope to show that these inconsistencies are resolved by the suggestion that some larval forms were acquired from animals in distinct evolutionary lineages. Before this, however, let us consider what the animals are like, both as adults and larvae.

Adult echinoderms described—Adult echinoderms radially symmetrical, larvae bilaterally symmetrical—Conventional explanation assumes ancestral echinoderms bilaterally symmetrical throughout life but adults secondarily adopted radial symmetry—This view criticised on theoretical grounds and for lack of evidence—Partial bilateral symmetry of holothurians not inherited from ancestors of all echinoderms but probably a recent case of neoteny—Alternative view: original echinoderms radially symmetrical throughout life, without planktonic larvae; bilateral larvae transferred from another phylum; further transfers within echinoderms