1. Eucopidae (figs. 55, 59).—Trophosome with stalked hydrothecae; gonosome, free medusae with otocysts and four, rarely six or eight, unbranched radial canals. Two of the commonest British hydroids belong to this family, Obelia and Clytia. Obelia forms numerous polyserial stems of the characteristic zigzag pattern growing up from a creeping basal stolon, and buds the medusa of the same name. In Clytia the polyps arise singly from the stolon, and the medusa is known as Phialidium (fig. 59).
2. Aequoridae.—Trophosome only known in one genus (Polycanna), and similar to the preceding; gonosome, free medusae with otocysts and with at least eight radial canals, often a hundred or more, simple or branched. Aequorea is a common medusa.
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.
| 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.)
| 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. |