HYDROZOA ACRASPEDA. 783 



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 aper- 

 tures, 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 Rhizostomae, in the 

 Semostome Cyaneidae> and to a certain extent in Aurelia. Some Rhizostome 

 genera also possess * nettle-bulbs,' stalked processes with or without a ter- 

 minal 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 digitelli *. The angles of the manu- 

 brium 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. Ten- 

 tacles, 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 retained 2 ; 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 gan- 

 glion cells and muscle-cells, the latter aggregated along certain lines. The 

 principal muscular tracts are (i) a circular zone of striated muscles close to 

 the margin of the bell, often broken up into sixteen sub-divisions, corre- 

 sponding 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 



1 For an account of the structure of digitelli, nettle-bulbs, and whips, see Hamann, J. Z. xv. 

 1882 ; for the development of the first two, Glaus, ' 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, &c. end in ' terminal knobs,' at first perforate, later im- 

 perforate; 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. 



2 Claus states ('Untersuchungen,' &c. 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. 



