It was of the nature of a contract, entered into by mutual promise, the clasping of hands, and exchange of an agreement in writing (tabula hospitalis) or of a token (tessera or symbolum), and was rendered hereditary by the division of the tessera.
Taking as a starting-point the wide archenteric cavity which the medusa inherits primitively from the antecedent actinula-stage (see article Medusa), we find, in such a form as Tessera, four interradial areas of concrescence between the exumbral and subumbral layers of endoderm, four so-called septal nodes or " cathammata," subdividing the stomach into four wide, radially situated pouches which communicate with each other beyond the septal nodes by wide apertures constituting what is termed by courtesy a ring-canal.