By a series of elaborate experiments, Mr G. J. Romanes has shown that the contractions of the swimming-bell of the Sea-blubbers obey the same laws as the contractions of muscular tissue generally, being excitable by the same stimuli, and similarly affected by chemical reagents. The removal of the margin of the umbrella causes a more or less total paralysis, and the same effect is produced by the excision of the marginal bodies, showing that these parts are the seat of the energy by which the movements are effected. Moreover, the severed margin of the swimming-bell continues to contract rhythmically for a considerable time after its complete separation from the organism itself. We are therefore justified in concluding that the margin of the swimming-bell contains what is functionally, if not structurally, a nervous system, though we have at present no direct anatomical evidence to demonstrate this.
In the Rhizostomidae the reproductive zooids (figs. 60, 61), differ from those we have just described as occuring in the first section of the Pelagidae, in not possessing tentacles on the margin of the umbrella, and in having the simple central polypite replaced by a composite dendriform process, which bears numerous polypites, projects far below the umbrella, and is thus described by Professor Huxley: " In the Rhi-zostomidae (figs. 60, 61), a complex, tree-like mass, whose branches, the 'stomatodendra,' end in, and are covered by, minute polypites, interspersed with clavate tentacula, is suspended from the middle of the umbrella in a very singular way. The main trunks of the dependent polypiferous tree, in fact, unite above into a thick, flat, quadrate disc, the ' synden-drium/ which is suspended by four stout pillars, the 'dendro-styles,' one springing from each angle, to four corresponding points on the under surface of the umbrella, equidistant from its centre. Under the middle of the umbrella, therefore, is a chamber, whose floor is formed by the quadrate disc, whilst its of this genital cavity." According to other authorities, how-ever, the apparent polypites of the "stomatodendra" are really mouth-like apertures formed by the extraordinarily complex manner in which the oral lobes are folded, and there is then in reality but one single central polypite, hanging from the under surface of the umbrella.
Roof is constituted by the under wall of the central cavity of the umbrella, and its sides are open. The reproductive elements are developed within radiating folded diverticula of the root.
Fig. 60. - Rhizostomidae. Generative zooid of Rhizostoma (after Owen). a Umbrella; bb" Stomatodendra," covered with clavate tentacles and minute polypites; c c Anastomosing network of canals.
Fig. 61. - Generative zooid of Rhizostoma pulmo, reduced in size. (After Gosse.)
It appears, finally, that amongst the old Pulmograde Aca-lephae, or amongst what would commonly be called Jelly-fishes, we have the following distinct sets of beings, which resemble each other more or less closely in appearance, but differ in their true nature:
2. True Medusidae, entirely resembling the former in anatomical structure, but differing in the fact that their ova do not give rise to a fixed zooid, but to free-swimming organisms exactly like the parent hydrosoma (Trachynejmidae, Geryonidae, and AEginidae).
3. Hydrozoa, which are provided with an "umbrella" (with all the peculiarities belonging to this structure), but which reproduce themselves without the intervention of free generative zooids produced by fission (Pelagia).
4. The free generative zooids of most of the Pelagidae, with an umbrella and a single polypite, the primitive hydrosoma being fixed and sexless (Aurelia, Cyanea, etc.)
5. The free generative zooids of the Rhizostomidae, with an umbrella and a complex central tree bearing many polypites (Rhizostoma, Cephea, etc.)
Of these five classes of organisms, Nos. 1 and 2 constitute the Gymnophthalmate Medusae of Professor E. Forbes, whilst Nos. 3, 4, and 5 are the Steganophthalmate Medusae of the same naturalist.