3. _Thaumantidae._--Trophosome only known in one genus (_Thaumantias_), similar to that of the _Eucopidae_; gonosome, free medusae with otocysts inconspicuous or absent, with usually four, sometimes eight, rarely more than eight, radial canals, simple and unbranched, along which the gonads are developed, with numerous tentacles bearing ocelli and with marginal sense-clubs. _Laodice_ and _Thaumantias_ are representative genera. 4. _Berenicidae._--Trophosome unknown; gonosome, free medusae, with four or six radial canals, bearing the gonads, with numerous tentacles, between which occur sense-clubs, without otocysts. _Berenice_, _Staurodiscus_, &c. [Illustration: After Haeckel, _System der Medusen_, by permission of Gustav Fischer. FIG. 58.--_Octorchandra canariensis_, from life.] 5. _Polyorchidae._--Trophosome unknown; gonosome, free medusae of deep form, with radial canals branched in a feathery manner, and bearing gonads on the main canal, but not on the branches, with numerous hollow tentacles bearing ocelli, and without otocysts. _Polyorchis_, _Spirocodon_. 6. _Campanularidae.-_-Trophosome as in _Eucopidae_; gonosome, sessile gonophores. Many common or well-known genera belong here, such as _Halecium_, _Campanularia_, _Gonothyraea_, &c. 7. _Lafoëidae._--Trophosome as in the preceding; gonosome, free medusae or gonophores, the medusae with large open otocysts. The hydroid genus _Lafoëa_ is remarkable for producing gonothecae on the hydrorhiza, each containing a blastostyle which bears a single gonophore; this portion of the colony was formerly regarded as an independent parasitic hydroid, and was named _Coppinia_. Medusan genera are _Mitrocoma_, _Halopsis_, _Tiaropsis_ (fig. 29, &c.). (So far as the characters of the trophosome are concerned, the seven preceding families are scarcely distinguishable, and they form a section apart, contrasting sharply with the families next to be mentioned, in none of which are free medusae liberated from the colony, so that only the characters of the trophosome need be considered.) [Illustration: After E. T. Browne, _Proc. Zool. Soc. of London_, 1896. FIG. 59.--Three stages in the development of _Phialidium temporarium_. a, The youngest stage, is magnified about 22 diam.; b, older, is magnified about 8 diam.; c, the adult medusa, is magnified.] 8. _Sertularidae._--Hydrothecae sessile, biserial, alternating or opposite on the stem. _Sertularia_ and _Sertularella_ are two very common genera of this family. 9. _Plumularidae._--Hydrothecae sessile, biserial on the main stem, uniserial on the lateral branches or pinnules, which give the colony its characteristic feathery form; with nematophores. A very abundant and prolific family; well-known British genera are _Plumularia_, _Antennularia_ and _Aglaophenia_.