Fine Structure of the Archiannelid Cuticle and Remarks on the Evolution of the Cuticle Within the Spiralia

Abstract

The fine structure of the cuticle in the archiannelids Polygordius, Protodrilus, Trilobodrilus, and Diurodrilus is described. Polygordius and Protodrilus are unique with respect to the filiform microvilli penetrating through the cuticle. Basically all forms conform to the most common type of cuticular structure in the Annelida: a fibrous matrix of varying complexity penetrated by micro-villar-like cell processes. Following the ideas expressed by Lyons (1970), a hypothesis on the evolution of the cuticle within the vermiform Spiralia (e.g. Turbellaria, Nemertea, Annelida) is proposed. New data on interstitial Turbellaria and other new forms of interstitial worms suggest a morphological and functional sequence from the multiciliated epidermis of Turbellaria to the cuticularized hypodermis of Annelida.