APLUTEAL DEVELOPMENT OF THE SEA-URCHIN HOLOPNEUSTES PURPURESCENS AGASSIZ (ECHINODERMATA, ECHINOIDEA, EUECHINOIDEA)

The external features of a shortened, apluteal development (lacking a pluteus larva) are described. Some features are unusual for echinoids. The large egg is distinctively marked by dark and pale coloured yolk. The sperm entry point is marked by a dark yolk spot and the first cle avage plane in most embryos is through the meridian on which the sperm entry point lies. Dark yolk in the animal hemisphere segregates large ly to one blastomere in the two-cell embryo and pale yolk segregates t o the other as a result of yolk movements during the first cell cycle. Progeny of the pale-yolk blastomere form adult oral structures and pr ogeny of the dark-yolk blastomere form adult aboral structures. There is no feeding planktonic pluteus larva. The gastrula develops into a d emersal vestibula larva with bilateral symmetry. The plane of symmetry is coincident with the Carpenter axis that defines a plane of symmetr y through the madreporite in adult echinoderms. The coincidence shows that the anterior ambulacrum is vegetal with respect to egg polarity a nd the interradius originating at the madreporite is animal. The bilat eral symmetry of the vestibula offers insight into the origin of radia l symmetry in echinoderms and the body plan of an echinoderm ancestor. (C) 1995 The Linnean Society of London