The nematocysts of twenty species of Hydromedusae occurring at Plymouth are described. Of these, twelve were Anthomedusae and seven were Leptomedusae, and one was the Siphonophore, Muggiaea atlantica. The medusae fit very well into the generally accepted classification as regards the types of nematocysts they possess.Lar sabellarum, the hydroid of Willia stellata, has a type of nematocyst not found in the other species—a macrobasic mastigophore. This is a further justification for the formation of a separate family, the Williidae, to hold this species and its relations.
While the Anthomedusae are clearly split up into distinct groups as regards their types of nematocysts, the Leptomedusae form a single group.
It is suggested that the so-called acrophores and anacrophores recorded in siphonophores may be merely stages in the development of other nematocysts.
It is urged that much more research is required on variation in size and structure of nematocysts in individual species before they can be used as specific characters for separating doubtful species.
