ULTRASTRUCTURAL EVIDENCE FOR THE OCCURRENCE OF 3 TYPES OF MECHANOSENSITIVE CELLS IN THE TENTACLES OF THE CUBOZOAN POLYP CARYBDEA-MARSUPIALIS

The ectodermal cell layer in the tentacles of the cubozoan polyp Caryb dea marsupialis contains four types of cells (types 1-4) bearing speci alized cilia. Epitheliomuscular cells (type 1) are characterized by mo tile cilia with dynein-decorated axonemes. 200 nm long extramembranous filaments of unknown function are restricted to a belt-like region di stal to the transition zone. Up to 40 mum long rigid cilia formed by a slender epithelial cell type (type 2) are surrounded by rings of shor t microvilli. The axonemes of these cilia are composed of incomplete m icrotubules and lack dynein. Microvilli and cilia are linked by interm embrane connectors. Microtubule-doublets and ciliary membrane are inte rconnected by microtubule-associated cross-bridges only within this co ntact region. At the tip of each tentacle a single nematocyte (type 3) is surrounded by 7-10 accessory cells (type 4). These both cell types are equipped with similar cilium-stereovilli-complexes consisting of a cone-like arrangement of stereovilli and a modified cilium. The axon emal modifications of the cilium, its interconnections with the surrou nding stereovilli and the linkages between ciliary axoneme and ciliary membrane are similar to those known from the cnidocil-complexes of hy drozoons and other epithelial mechanosensitive cells of the collar-rec eptor type. Our data indicate that besides the nematocyte two other ty pes of mechanosensory cells (types 2 and 4) are integrated in the ecto dermal cell layer of Carybdea which possibly affect the triggering mec hanism of nematocyst discharge.