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 Carybdea marsupialis contains four types of cells (types 1-4) bearing specialized cilia. Epitheliomuscular cells (type 1) are characterized by motile cilia with dynein-decorated axonemes. 200 nm long extramembranous filaments of unknown function are restricted to a belt-like region distal 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 short microvilli. The axonemes of these cilia are composed of incomplete microtubules and lack dynein. Microvilli and cilia are linked by intermembrane connectors. Microtubule-doublets and ciliary membrane are interconnected by microtubule-associated cross-bridges only within this contact 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 axonemal modifications of the cilium, its interconnections with the surrounding stereovilli and the linkages between ciliary axoneme and ciliary membrane are similar to those known from the cnidocil-complexes of hydrozoons and other epithelial mechanosensitive cells of the collar-receptor type. Our data indicate that besides the nematocyte two other types of mechanosensory cells (types 2 and 4) are integrated in the ectodermal cell layer of Carybdea which possibly affect the triggering mechanism of nematocyst discharge.