Clava contains a common British species with a creeping hydrorhiza frequently attached to shells, and with adelocodonic gonophores. Cordylophora is the genus which has migrated into fresh water in certain European localities (see p. [269]). It forms well-developed branching colonies attached to wooden gates and piers or to the brickwork banks of canals. Several Anthomedusae, of which the hydrosome stage is not known, appear to be related to the Medusae of this family, but are sometimes separated as the family Tiaridae. Of these Tiara, a very brightly coloured jelly-fish sometimes attaining a height of 40 mm., is found on the British coasts, and Amphinema is found in considerable numbers at Plymouth in September. Turritopsis is a Medusa with a hydrosome stage like Dendroclava. For Stomatoca, see p. [415].
Fam. Corymorphidae.—This family contains the interesting British species Corymorpha nutans. The hydrosome stage consists of a solitary zooid of great size, 50-75 mm. in length, provided with two circlets of numerous long filiform tentacles. The free-swimming Medusae are produced in great numbers on the region between the two circlets of tentacles. These Medusae were formerly known by the name Steenstrupia, and are noteworthy in having only one long moniliform tentacle, opposite to one of the radial canals.
The gigantic Monocaulus imperator of Allman was obtained by the "Challenger" at the great depth of 2900 fathoms off the coast of Japan. It was nearly eight feet in length. More recently Miyajima[[309]] has described a specimen from 250 fathoms in the same seas which was 700 mm. (27.5 in.) in length. Miyajima's specimen resembles those described by Mark from 300 fathoms off the Pacific coast of North America as Branchiocerianthus urceolus in the remarkable feature of a distinct bilateral arrangement of the circlets of tentacles. Owing to the imperfect state of preservation of the only specimen of Allman's species it is difficult to determine whether it is also bilaterally symmetrical and belongs to the same species as the specimens described by Mark and Miyajima. These deep-sea giant species, however, appear to differ from Corymorpha in having adelocodonic gonophores.
Fam. Hydrolaridae.—This family contains the remarkable genus Lar, which was discovered by Gosse attached to the margin of the tubes of the marine Polychaete worm Sabella. The zooids have only two tentacles, and exhibit during life curious bowing and bending movements which have been compared with the exercises of a gymnast. The Medusae (Fig. 132, A and B) have been known for a long time by the name Willsia, but their life-history has only recently been worked out by Browne.[[310]]
Fam. Monobrachiidae.—Monobrachium, found in the White Sea by Mereschkowsky, forms a creeping stolon on the shells of Tellina. The zooids of the hydrosome have only one tentacle.
Fam. Myriothelidae.—This family contains the single genus Myriothela. The zooid of the hydrosome stage is solitary and is provided, as in the Corynidae, with numerous scattered capitate tentacles. The gonophores are borne by blastostyles situated above the region of the tentacles. In addition to these blastostyles producing gonophores there are, in M. phrygia, supplementary blastostyles which capture the eggs as they escape from the gonophores and hold them until the time when the larva is ready to escape. They were called "claspers" by Allman. In some of the Arctic species Frl. Bonnevie[[311]] has shown that they are absent. Each zooid of M. phrygia is hermaphrodite.
Fig. 134.—Pelagohydra mirabilis. Fl, The float; M, position of the mouth; Ten.Fl, filamentous tentacles of the float. (After Dendy.)
Fam. Pelagohydridae.—This family was constituted by Dendy[[312]] for the reception of Pelagohydra mirabilis, a remarkable new species discovered by him on the east coast of the South Island of New Zealand. The hydrosome is solitary and free-swimming, the proximal portion of the body being modified to form a float, the distal portion forming a flexible proboscis terminated by the mouth and a group of scattered manubrial tentacles. The tentacles are filiform and scattered over the surface of the float. Medusae are developed on stolons between the tentacles of the float. They have tentacles arranged in four radial groups of five each, at the margin of the umbrella.
As pointed out by Hartlaub,[[313]] Pelagohydra is not the only genus in which the hydrosome floats. Three species of the genus Margelopsis have been found that have pelagic habits, and two of them have been shown to produce numerous free-swimming Medusae by gemmation; but at present there is no reason to suppose that in these forms there is any extensive modification of the aboral extremity of the zooid to form such a highly specialised organ as the float of Pelagohydra.