Echinoderms: Metamorphosis

Abstract

An animal species, as Darwin reminded his readers, comprises all phases in development from egg to adult, and eggs, sperm, embryos, larvae, juveniles, and adults have all evolved. During evolution, each of these stages in development must have remained fully functional, for survival clearly implies the survival of them all, and as the various phases in development changed, the method of changing from one to another must also have been modified. If all modern species are solely the products of descent with modification from species to species, then the methods of metamorphosis from one phase to another must also have evolved in a series of gradual steps. I, however, dispute that this is the only method of evolution. There are undoubted similarities between the larvae of echinoderms and enteropneusts, but, as we shall see, metamorphosis in these two taxa is totally different. I question whether the two methods of metamorphosis could have evolved gradually from a common ancestral process, and I put forward an alternative in terms of larval transfer. This presupposes that the basic forms of all larvae have been transferred from foreign taxa and that each metamorphosis is a change from one taxon to another during development.