Electron Micrographs of the Nematocysts of Hydra

Seventeen different kinds of nematocysts have been described and classified by Weill (1930) on the basis of the characteristics of the discharged tube. Only four of these are known to be present on hydras, namely, volvents (desmonemes), streptoline glutinants (holotrichous isorhizas), stereoline glutinants (atrichous isorhizes), and penetrants (stenoteles). The discharged penetrant (fig. 1) consists of the capsule a, butt, which is composed of the shaft b, stylet c, spinneret d, and tube e. In the glutinants the tube is attached directly to the tube, and is either spiny (streptoline glutinant) or smooth (stereoline glutinant). In the resting nematocyst all the component parts are inverted and contained within the capsule