A defensive zooid in a hydroid or siphohonophore, bearing many stinging cells: The nematozooids have organs for protection which are found in the siphohonophores which belong to the "Siphonophora", an order of marine hydrozoan coelentrates, both medusoid and polypoid individuals joining to form free-swimming colonies, often large and brightly colored, with highly toxic stings, including the Portuguese man-of-war.