4. _Clavatellidae._--Trophosome, polyps with a single whorl of capitate tentacles; gonosome, free medusae, with tentacles branched, solid. _Clavatella_ (fig. 21), with a peculiar ambulatory medusa is a British form. 5. _Pennariidae_.--Trophosome, polyps with an upper circlet of numerous capitate tentacles, and a lower circlet of filiform tentacles. _Pennaria_, with a free medusa known as _Globiceps_, is a common Mediterranean form. _Stauridium_ (fig. 2) is a British hydroid. 6. _Tubulariidae._--Trophosome, polyps with two whorls of tentacles, both filiform. _Tubularia_ (fig. 4), a well-known British hydroid, bears gonophores. 7. _Corymorphidae_ (including the medusa-family _Hybocodonidae_).--Trophosome solitary polyps, with two whorls of tentacles; gonosome, free medusae or gonophores. _Corymorpha_ (fig. 3), a well-known British genus, sets free a medusa known as _Steenstrupia_ (fig. 22). Here belong the deep-sea genera _Monocaulus_ and _Branchiocerianthus_, including the largest hydroid polyps known, both genera producing sessile gonophores. [Illustration: After Haeckel, _System der Medusen_, by permission of Gustav Fischer. FIG. 53.--_Pteronema darwinii_. The apex of the stomach is prolonged into a brood pouch containing embryos.] 8. _Dendroclavidae._--Trophosome, polyp with filiform tentacles in three or four whorls. _Dendroclava_, a hydroid, produces the medusa known as _Turritopsis_. 9. _Clavidae_ (including the medusa-family _Tiaridae_ (figs. 27 and 51). Trophosome, polyps with scattered filiform tentacles; gonosome, medusae or gonophores, the medusae with hollow tentacles. _Clava_ (fig. 5), a common British hydroid, produces gonophores; so also does _Cordylophora_, a form inhabiting fresh or brackish water. _Turris_ produces free medusae. _Amphinema_ is a medusan genus of unknown hydroid. 10. _Bythotiaridae._--Trophosome unknown; gonosome, free medusae, with deep, bell-shaped umbrella, with interradial gonads on the base of the stomach, with branched radial canals, and correspondingly numerous hollow tentacles. _Bythotiara_, _Sibogita_. 11. _Corynidae_ (= hydroid families _Corynidae_, _Syncorynidae_ and _Cladocorynidae_ + medusan family _Sarsiidae_).--Trophosome polyps with capitate tentacles, simple or branched, scattered or verticillate; gonosome, free medusae or gonophores. _Coryne_, a common British hydroid, produces gonophores; _Syncoryne_, indistinguishable from it, produces medusae known as _Sarsia_ (fig. 51). _Cladocoryne_ is another hydroid genus; _Codonium_ and _Dipurena_ (fig. 50) are medusan genera. 12. _Myriothelidae._--The genus _Myriothela_ is a solitary polyp with scattered capitate tentacles, producing sporosacs.