The exumbrellar and subumbrellar walls of the peripheral part of the gastric cavity fuse from place to place with the formation of a gastral lamella, leaving pouches, simple or branched vessels, or retia, and there is very generally a circumferential canal2. The gastric filaments are motile cylindrical processes disposed in groups (phacelli) on the subumbrellar wall of the gastric cavity centrally to the genital organs. Four single filaments only are present in the genus Ephyra. The endoderm covering them consists of ciliated cells, gland cells, muscle cells, and cnidoblasts. The genital organs are either ridge-like or lamellate, and project into the gastric cavity. They are situated on the inner aspect of the thin gastro-genital membranes (p. 783); and the genital products are derived from the endodermal cells of the peripheral or lower face of the ridges or lamellae. The original shape of the ridge is that of a horseshoe, with the concavity turned to the margin of the bell, but in the majority of Semostomae and Rhizostomae the curvature is reversed. The ridge is rarely divided into two parts, as in a few Cannostomae, but it is very generally converted into a lamella which may become lobed l.
The gastrogenital membrane may increase in size, become folded and hang down into the cavity of the bell in some Semostomae, e. g. Pelagia, Cyanea; or be invaginated into the gastric cavity, e. g. in Aurelia, most Rhizostomae. The walls of the subumbrella surrounding the four membranes may be much thickened, and consequently the membranes come to lie at the bottom of subgenital cavities or lemnia, as in Aitreliadae and most Rhizostomae. The aperture into the cavities may, in the last-named, be greatly reduced, or covered by an axial and abaxial process of the subumbrellar wall, so that each of them acquires the appearance of a double cavity. In the Rhizostome Versuridae and Crambessidae the four gastrogenital membranes meet in the centre of the bell. The basal portion of the manubrium is consequently resolved into four hollow pillars which correspond to its four angles. The cavity thus formed is the 'sub-genital portico' or syndemnium2. The Semostome Chrysaora is hermaphrodite. Sperm sacs occur upon its genital lamellae, which produce ova in a normal manner, as well as upon the subumbrellar wall of the gastric cavity, in the gastric pouches, and on the arms 3.
1In Aurelia and Cyanea Annaskala the base of the marginal bodies is surrounded by sensory epithelium, nerve-fibrils and ganglion cells. There is also a similarly constituted elongated patch extending centripetally from near the base of the marginal body on the surface of the subumbrella. C. Annaskala and C. capillata possess sensory patches also upon the inner and outer faces of the two lateral protective lobes of the rhopalia. A layer of nerve-fibrils underlies the olfactory epithelium which in C. Annaskala at least is scarcely distinguishable from that of the sensory bodies themselves.
The number of rhopalia is more than 8 in a few instances; 19-22 occur in Atolla; 32 in Collaspis (Cannostomae); 12 are found in the Semostome genera Phacellophora, Patera and Medusina, and the Rhizostome genus Polyclonia; 16 in the Rhizostome Cassiopeia. Von Lendenfeld has observed that the young Ephyra of the Rhizostome Stylorhiza punctata has 24; reduced in an older Ephyra to 16, and finally to 8; see Z. A. vii. pp. 430-31. He does not mention that its Scyphostoma shows any peculiarity: cf. Proc. Lin. Soc. N. S. Wales, ix. p. 297, under Phyllorhiza punctata; and for figures of the bell-margin of the Ephyra, ibid. Pl. V.
2The gastral lamella formed by the cohesion of the exumbrellar and subumbrellar walls of the gastric cavity consists of a single layer of cells in the fully formed central region of the bell, and sometimes, if not always, of a double in the peripheral growing parts. It appears to extend at its edges as the bell grows in size, and vessels are excavated between its two layers. See von Lendenfeld, Z. W. Z. xxxvii. p. 490, and Haeckel, 'Deep-sea Medusae,' Challenger Reports, iv. Pl. XXV. Figs. 8 and 10. Indications of its double nature are always to be discerned in the Cubomedusan Charyddaea, accoiding to Claus.
Variations in the number of the radial segments sometimes occur. Aurelia aurita is particularly liable to such malformations, and in it the number may be increased from eight to twelve, or diminished to three or four. The bell may be affected alone or the manubrium, and the genitalia as well. Sexual individuals of this species occur in autumn half to a quarter the normal size, i. e. four inches.
1The primitive ridge is converted into a lamella by the growth of endoderm cells from its margin into the mesoglaea of the gastrogenital membrane. Connecting pillars of cells, or of mesoglaea covered by cells, are often left between the genital lamella and the gastrogenital membrane. See von Lendenfeld, Z. W. Z. xxxvii. p. 536 et seqq.; and Claus, Untersuchungen, p. 33 et seqq. and p. 38.
2Von Lendenfeld appears to have observed the formation of this chamber in Stylorhiza punctata. He states that it occurs as Haeckel supposed, i. e. by the encroachment of the gastrogenital membranes on the base of the manubrium until they meet and fuse. See Proc. Lin. Soc. New South Wales, ix. p. 307, or Z. A. vii. p. 431. In the first-named paper the author describes the Medusa in question under the generic name of Phyllorhiza. This is an error, because Phyllorhiza is a Pilemid genus, and there is no subgenital portico in Pilemidae.
3 The subumbrellar saccules of certain Linergidae (Cannostomae) are perhaps testes. See Haeckel, System, p. 493.