The venom apparatus of the globiferous pedicellariae of the toxopneustid Sphaerechinus granularis (Echinodermata,Echinoida): Fine structure and mechanism of venom discharge

Summary Globiferous pedicellariae of Sphaerechinus granularis are venomous defensive appendages consisting of a stalk bearing a head made of three movable jaws. Each jaw is supported by a calcareous valve ending with a terminal grooved tooth. A venom apparatus is located in each jaw and consists of a venom gland surrounded by a muscular envelope and terminating in a duct which completely encircles the terminal tooth of the valve. Contrary to previous statements, the duct does not lie inside the groove of the terminal tooth. In mature pedicellariae, the venom is stored in intracellular vacuoles of highly differentiated cells which are no longer active. The cells fill the whole space of the venom gland which is without a lumen; they are segregated into two types that occur in distinct regions and differ from each other by morphological and staining properties of their secretions. Upon contraction of the muscular envelope, the venom is released via a holocrine mechanism and infiltrates the predator's tissues through the wound inflicted by the three calcareous teeth of the valves. In no case is the venom emitted through the tooth groove.