The differentiation of nematocysts and associated structures in the autozooid tentacles of the anthozoon R. reniformis are described by EM. The stages include transformation of the interstitial cell; development of an external tube and a capsular region; elongation of the capsular region to include the external tube; and, differentiation of the internal thread from the capsular matrix. The nematocyst of R. reniformis, an atrichous isorhiza in Hyman''s terminology, is redescribed as having an internal tube with spines.