Ciliary Bands in Echinoderm Larvae: Evidence for Structural Homologies and a Common Plan

A series of laterally projecting ridges develop along the ciliary band of late stage auricularia larvae. These are similar in position to the larval arms of bipinnaria larvae and the epaulettes and vibratile lobes of echinoid pluteus larvae, all of which structures are potentially homologous. When the auricularia is converted to a doliolaria with a series of circumferential ciliary bands, the ridges of the former are retained as basic elements from which the circumferential bands of the latter then develop. There is a simple repeating pattern in the arrangement of these elements in which bands composed of two elements alternate with bands composed of four. The available evidence does not resolve the question of which of the above four larval types, whether feeding or non‐feeding, is more primitive. The common plan apparent among them suggests, however, that this plan, whatever its origin, predates the diversification of larval types among eleutherozoan echinoderms.