Chaetae

Abstract

Annelid chaetae have long been valued by systematists for their diversity of form and intricate structural detail, especially in the class Polychaeta. In this group these epidermal products occur in the most widely varying form, ranging from the elongate, flexible spirals of the maldanid genus Nicomache (Kennedy and Kryvi 1978) to short, dentate uncini in sessile, tube-dwelling forms such as the spirorbids (Knight-Jones and Fordy 1979, Fig. 1B). Although most annelid chaetae are sculptured from a single piece, some polychaete chaetae are compound, composed of a distal article joined to an elongate, more proximal shaft by a relatively complex articulation (Fig. 1C). Fauchald’s (1977) key to the polychaete genera illustrates many chaetal types with simple line drawings. In contrast to their abundance and diversity among the polychaetes, chaetae are known to occur in only a single primitive genus of leeches (Acanthobdella); these characteristic structures are otherwise absent from these specialized annelids. Among the oligochaetes, chaetae are fewer and less diverse (Fig. 1 A) but also demonstrate a variety of taxonomically useful patterns (Brinkhurst 1982). Annelid chaetae have been discussed recently by Richards (1978) and Jamieson (1981).