Seven different types of nematocyst are detailed and illustrated from the Zoanthidea. We studied the size population structure of nematocyst capsules: how they are affected by preservation, and how and what should be measured. Populations of two types, large holotrichs and p-mastigophores, from the mesenterial filaments of Protopalythoa heliodiscus and Pr. mutuki have been compared using length, width, and (for p-mastigophores) shaft length. The analysis employed bivariate scattergrams and discriminant analysis. The two species are strongly differentiated by their holotrichs, less well by their p-mastigophores. The problem of bimodal size distributions is considered and resolved. It is recommended that nematocyst capsule population descriptors should always be included in species’ descriptions, in addition to or instead of the customary minimum–maximum range.