ULTRASTRUCTURAL-STUDY OF SENSORY CELLS OF THE PROBOSCIDIAL GLANDULAR EPITHELIUM OF RISERIELLUS-OCCULTUS (NEMERTEA, HETERONEMERTEA)

Only one sensory cell type has been observed within the glandular epit helium of the proboscis in the heteronemertine Riseriellus occultus. T hese bipolar cells are abundant and scattered singly throughout the pr oboscis length. The apical surface of each dendrite bears a single cil ium enclosed by a ring of six to eight prominent microvilli. The ciliu m has the typical 9 x 2 + 2 axoneme arrangement and is equipped with a cross-striated vertical rootlet extending from the basal body. No acc essory centriole or horizontal rootlet was observed. Large, modified m icrovilli (stereovilli) surrounding the cilium are joined together by a system of fine filaments derived from the glycocalyx. Each microvill us contains a bundle of actin-like filaments which anchor on the inden ted inner surface of a dense, apical ring situated beneath the level o f the ciliary basal body. The tip of the cilium is expanded and modifi ed to form a bulb-like structure which lies above the level where the surrounding microvilli terminate. In the region where the cilium emerg es from the microvillar cone, the membrane of the microvillar apices m akes contact with a corresponding portion of the ciliary membrane. At this level microvilli and cilium are apparently firmly linked by junct ional systems resembling adherens junctions. The results suggest that these sensory cells may be mechanoreceptors. (C) 1996 Wiley-Liss, Inc.