Cnidocil apparatus: sensory receptor of Physalia nematocytes

Abstract

The cnidocil apparatus, a cluster of subcellular structures at the external surface of the nematocytes of Physalia physalis , was examined by scanning and transmission electron microscopy. The cnidocil apparatus consists of a modified cilium or cnidocil surrounded by 15 to 21 stereocilia. The stereocilia contain closely packed, longitudinally arranged microfilaments. The cnidocil contains numerous singlet and doublet microtubules that lack the classical 9 + 2 microtubule pattern characteristic of most cilia and flagella. The basal body of the cnidocil does, however, maintain the usual circle of nine triplets of microtubules. It is proposed that the cnidocil of the nematocyte is truly the sensory receptor for mechanical and chemical stimuli that elicit nematocyst discharge.