The same eight bifid lobes and the rhopalia appear in each of the remaining segments of the Strobila. The basal pyriform portion of the Scyphostoma developes, sooner or later, a set of sixteen tentacles, and after the detachment of the last Ephyra grows in size. In spring it has been observed to pass again into the Strobila condition.
The Ephyra grows in size after its detachment. It may retain its external appearance (certain Cannostomae). But with few exceptions it undergoes changes of shape. The eight adradial intervals between the eight marginal lobes increase much in extent, and either remain simple, or are divided into a number of lobes, simple or bifid. The lobes in question are either independent outgrowths of the bell-margin, which may or may not divide, or are produced by fission (I) from the sides of the Ephyra lobes, and (2) of the lobes thus derived. The bell itself is somewhat flattened, of greater breadth than depth. It is sometimes divided into a central and peripheral portion by a circular furrow in the exumbrella, the fossa coronaris. Its margin never becomes inflected inwards: when it is thin and velum-like, as e. g. in Aurelia, it is termed by Haeckel 'velarium.' The mouth retains its square outline in Cannostomae, or its four angles are prolonged outwards into four oral arms, with a deep adoral furrow lined by endoderm and folded edges in Semostomae: and in Rhizostomae these four arms become bifid at their apex during growth; the edges of their furrows become much folded and produced into processes; the surfaces of the folds and processes concresce from place to place, leaving only funnel-shaped openings, which lead into the adoral furrows now converted into canals.
At the same time the mouth is closed by the concrescence of the folds bordering it and by the fusion of the bases of the arms. Crustaceans, and even fish of fair size have been found within the funnel-shaped apertures, and a Rhizostoma (Pilemd) has been captured with a semi-digested animal embraced by the arms. The ectoderm of the edges of the arm-grooves is raised into papillae with cnidoblasts in Semostomae. These papillae develope into short mobile processes or digitelli in all RAizostomae, in the Semostome Cyaneidae, and to a certain extent in Aurelia. Some Rhizostome genera also possess 'nettle-bulbs,' stalked processes with or without a terminal opening, ending in a knob covered with cnidoblasts, and 'nettle-whips,' which are elongated funnel-shaped openings, sometimes closed except at their apices, and beset with digitellil. The angles of the manubrium become thickened and the thickenings extend on to the subumbrella, giving rise to four pillars. The subumbrella itself is also more or less thickened, except at the base of the four sides of the manubrium, where it remains thin. These thin spots are the gastro-genital membranes.
Tentacles, absent in all Rhizostomae, are, when present, either solid or hollow; situated one between each of the eight marginal lobes when the Ephyra-form is retained2; usually numerous, and then placed above the margin of the bell, e. g. in Aurelia between the lobes of the margin, or in groups towards the periphery of the bell on the surface of the subumbrella, as in Cyanea. The ectoderm of the exumbrella may contain pigment cells and groups of cnidoblasts. The ectoderm of the subumbrella developes ganglion cells and muscle-cells, the latter aggregated along certain lines. The principal muscular tracts are (1) a circular zone of striated muscles close to the margin of the bell, often broken up into sixteen sub-divisions, corresponding to the perradii, interradii, and adradii; (2) radial striated muscles corresponding to the pillars of the manubrium. There are also muscles in connection with the bases of the tentacles and the marginal lobes. The sense-bodies, tentaculocysts or rhopalia, are cylindrical, straight or curved organs, with the following structure.
The endoderm at their apex forms a mass of nucleated cells or protoplasm, imbedding a variable number of calcareous otoliths, which do not however contain Lime Carbonate. The ectoderm cells of the apex are flattened (? ciliated), those of the sides are more or less columnar (1) supporting cells, (2) sense-cells, furnished with sense-hairs, and prolonged basally into filaments in connection with ganglion cells which lie among the sub-epithelial nerve-fibres (Claus, von Lendenfeld). A patch of visual pigment is commonly present on the dorsal aspect of the organ near its base, but in Nausithoe and its congeners there is a ventral eye provided with a lens, with sense-cells, pigmented supporting cells and ganglion cells (?). The sense-bodies are usually protected by a dorsal, i. e. exumbrellar covering-piece or hood, sometimes of large size, and by two lobes which curve round it below, and are developed from the inner edges of the bifid Ephyra lobes. There is often a depression in the exumbrella above the base of each sense-body. It is lined by ciliated cells, and is supposed to test the character of the sea-water, i. e. to exercise a gustatory or olfactory function1. The sense-bodies appear to be so many nerve-centres, and they are connected to a sub-ectodermic plexus of ganglion cells in the subumbrella, and control the movements of the animal.
1 Haeckel in his System and elsewhere appears always to count the two divisions of these lobes as separate lobes. He terms them subradial, the tentacles intervening between successive pairs being adradial.
1For an account of the structure of digitelli, nettle-bulbs, and whips, see Hamann, J. Z. xv. 1882; for the development of the first two, Claus, 'Untersuchungen,' pp. 44 and 55; and of the Rhizostome arms, the last-named, op. cit. pp. 43 et seqq. The arms of Pilema, e.g. P. pulmo (Rhizostoma Cuvieri), of Mastigias papua, etc. end in 'terminal knobs,' at first perforate, later imperforate; for their origin, cf. Claus, op. cit. p. 51, under 'Arm-kolben.' Two of the terminal arm-folds in the Rhizostome, Pilemidae and Crambessidae, pass during growth to the aboral aspect of the arms, which consequently appear in transverse section triangular, each angle carrying a folded ridge. The majority of Pilemidae possess 'scapulets,' eight in number, springing from the sides of the manubrium above the bases of the arms. Haeckel and Hamann regard them as dissociated arm-folds; Claus, however, has proved that they originate as eight hollow outgrowths which become perforate. The edges of their apertures become folded, concresce from place to place, and are beset with digitelli.
See in his work, cited above, the account of the metamorphosis of Rhizostoma.
3 Claus states ('Untersuchungen,' etc. p. 21) that in the young Ephyrae of Chrysaora and Aurelia, as well as of Cyanea (on Fewke's authority), there is a stage with only four tentacles, which he distinguishes as Metephyra. If the Ephyra be regarded from the dorsal, i. e. exumbrellar aspect, the four tentacles in question were in all specimens examined by him outgrowths of the adradius to the right of the four perradial marginal lobes when the perradius was directed forwards.