FAM. BOUGAINVILLIIDAE.—The zooids of the hydrosome have a single circlet of filiform tentacles at the base of the hypostome. In _Bougainvillia_ belonging to this family the gonophores are liberated in the form of free-swimming Medusae formerly known by the generic name _Hippocrene_. In the fully grown Medusa there are numerous tentacles arranged in clusters opposite the terminations of the four radial canals. There are usually in addition tentacular processes (labial tentacles) on the lips of the manubrium. _Bougainvillia_ is a common British zoophyte of branching habit, found in shallow water all round the coast. The medusome of _Bougainvillia ramosa_ is said to be the common little medusa _Margelis ramosa_.[305] Like most of the Hydroids it has a wide geographical distribution. Other genera are _Perigonimus_, which has a Medusa with only two tentacles; and {270}_Dicoryne_, which forms spreading colonies on Gasteropod shells and has free gonophores provided with two simple tentacles, while the other organs of the medusome are remarkably degenerate. In _Garveia_ and _Eudendrium_ the gonophores are adelocodonic, in the former genus arising from the body-wall of the axial zooids of the colony, and in the latter from the hydrorhiza. _Stylactis_ is sometimes epizoic (p. 268). Among the genera that are usually placed in this family, of which the medusome stage only is known, are _Lizzia_ (a very common British Medusa) and _Rathkea_. In _Margelopsis_ the hydrosome stage consists of a single free-swimming zooid which produces Medusae by gemmation. FAM. PODOCORYNIDAE.—The zooids have the same general features as those of the Bougainvilliidae, but the perisarc does not extend beyond the hydrorhiza. In _Podocoryne_ and _Hydractinia_ belonging to this family the hydrorhiza forms an encrusting stolon which is usually found on Gasteropod shells containing a living Hermit crab. In _Podocoryne_ the gonophores are free-swimming Medusae with a short manubrium provided with labial tentacles. _Hydractinia_ differs from _Podocoryne_ in having polymorphic zooids and adelocodonic gonophores. A fossil encrusting a _Nassa_ shell from the Pliocene deposit of Italy has been placed in the genus _Hydractinia_, and four species of the same genus have been described from the Miocene and Upper Greensand deposits of this country.[306] These are the only fossils known at present that can be regarded as Gymnoblastic Hydroids. The Medusa _Thamnostylus_, which has only two marginal tentacles and four very long and profusely ramified labial tentacles, is placed in this family. Its hydrosome stage is not known. FAM. CLAVATELLIDAE.—This family contains the genus _Clavatella_, in which the zooids of the hydrosome have a single circlet of capitate tentacles. The gonophore is a free Medusa provided with six bifurcated capitate tentacles.