136 THE VOYAGE OF H.M.S. CHALLENGER. 



of the vestibule is formed by the cruciform gastrogenital membrane and by the four 

 interradial genital plates (fig. 4, ivi) which fill the interspace between its perradial 

 limbs. Looked at from above, the gastrogenital cross shines through the umbrella disk 

 as in many other Rhizostoma?. The delicate gastrogenital membrane (figs. 2, 4, gg) 

 formed by the cross, separates the vestibule from the underlying central stomach, of which 

 it at the same time represents the bottom or lower wall. It consists of a very thin, 

 flexible, and extensible fulcral plate, covered with gastral endoderm above and by 

 subumbral ectoderm below ; in the middle only it is strengthened by the gelatinous 

 ridge cross (fig. 4, gh), whose four perradial limbs separate the four horseshoe-shaped 

 genitalia from each other and pass at their distal end into the axial wall of the four oral 

 pillars. The interradial interspaces between the four perradial limbs of the gastro- 

 genital cross are filled by the four thick cartilage-like intergenital plates (fig. 4, wi) ; 

 these are strong, equilaterally triangular thickenings of the gelatinous umbrella, having 

 the interradial canals (ci) running in their middle line and the delicate gastrogenital 

 membrane inserted at their lateral margins. 



The four oral pillars (also termed arm pillars or floor pillars, " pilastri," figs. 2, 4, 7, 

 ap) form the only connection between the upper and the lower wall of the sub- 

 genital vestibule, and are separated from each other by the four wide subgenital 

 apertures. The oral pillars are four strong perradial gelatinous plates shaped like a 

 parallel trapezum (fig. 7, ap). They spring with a narrower base, 10 mm. broad, from the 

 distal end of the limb of the ffastrogenital cross from above and outside to below, and 

 inside expanding to a breadth of 15 mm. towards the arm disk which is formed by their 

 confluence. The thickness of the arm pillars amounts to from 5-7 mm., their 

 length to 15 mm. Their inner axial surface is curved concavely, their outer abaxial 

 surface curved convexly, the former is turned towards the porticus, the latter towards 

 the cavity of the umbrella corona. The broad perradial pillar canal (figs. 2, 4, 6, cd), 

 the distal process of the limb of the gastral cross, runs in the solid gelatinous mass of the 

 pillars, near their axial surface. 



The four subgenital apertures (figs. 1, 7, ig) represent, in some measure, the four 

 broad low doors, through which we reach from the outside (from the cavity of the 

 umbrella cavity outwards) in the central vestibule. They are rectangular in form (with 

 obtuse angles), are separated from each other laterally by the four perradial arm 

 pillars, and therefore lie interradially. They are limited above by the distal basal part of 

 the triangular intergenital plates, below by the lateral margin of the arm disk. The 

 breadth, or the largest horizontal diameter of the subgenital aperture in its middle 

 amounts to 25 mm., and is nearly three times as much as the smallest breadth of the 

 pillars separating it. 



The arm disk or oral disk (" stomodiscus, discus brachiferous," figs. 2, 6, 7, ah) 

 represents the bottom of the floor or the lower wall of the subgenital vestibule. It is only 



