The proboscis ciliation of the echiuran Bonellia viridis

Abstract

Special club-shaped cilia with an expanded tip are seen by both light and electron microscopy on the proboscis of Bonellia viridis in addition to the usual filamentous cilia. Using transmission electron microscopy the club tip is seen to consist of a distally curled axoneme enclosed within a sac-like expansion of the ciliary membrane. The axoneme may describe up to two complete loops inside the membrane sac or may occasionally follow an S-shaped course.

A quantitative analysis of the distribution on the proboscis of both filamentous and club cilia is made. No discrete ciliary tracts are present. However, there is a definite pattern of ciliary density distribution. The cilia are densest on both the dorsal and ventral surfaces of the distal fringe of the proboscis. The dorsal fringe cilia are concerned with locomotion and the ventral cilia with food particle uptake. The club cilia are confined to the terminal lobes and neck region of the proboscis. The dorsal surface of the proboscis posterior to the fringe is virtually non-ciliated. The functional significance of the club cilia is discussed.