The anatomy, terminology, and homology of acrorhagi and pseudoacrorhagi in sea anemones

The morphology of the column margin is an especially important feature in the taxonomy and biology of members of the sea anemone family Actiniidae. Despite the importance of marginal structures, their anatomy and the terminology describing them is poorly delimited. Consequently, homologies of the components of the marginal structures are poorly understood and taxonomies that depend on the presence, absence, or particular manifestation of marginal structures are confused. I review and synthesize anatomical descriptions of the marginal structures and find that they consist of two parts: a projection of the column and a holotrich-dense pad (= acrorhagus) that is histologically differentiated from the surrounding tissues. Although acrorhagi are functionally similar to the catch tentacles of some Actiniaria and to the capitate tentacles of Corallimorpharia and Scleractinia, as structures of the column, acrorhagi are not homologous to them.