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



largest horizontal diameter near the umbrella margin amounted to 30 mm., double the 

 vertical height of the umbrella (15 mm.). As the umbrella margin in the specimen 

 examined was strongly contracted, the height in the living animal must be proportion- 

 ately greater (20 mm. or more). The umbrella, seen from the upper or lower surface, 

 appears distinctly octagonal, as the eight principal radia (with peronia and tentacles) 

 project more strongly outwards than the eight interlying side walls (fig. 2), so that the 

 umbrella of the dead Medusa has really the shape of a short, regularly octagonal prism. 

 The gelatinous substance of the umbrella is tolerably soft (as in all true iEginidse), not 

 so firm as in the Cunanthidae and Peganthidse, though there, as here, it is traversed by 

 numerous elastic fibres. The gelatinous umbrella is very thick throughout the flattened 

 apical surface (equal to one-third of the height of the umbrella) but very thin (and 

 decreasing proportionately below) on the thin lateral walls (fig. 11, ug). 



The exumbrella is flat, without any special characteristic, and only traversed by eight 

 shallow peronial furrows (fig. 7, es) ; these run vertically from the insertion of the tentacles 

 to the umbrella margin, and are connected by thin " peronial plates " with the peronia or 

 "umbrella clasps" lying beneath them (em). The eight peronial plates ("laminae peroni- 

 ales," figs. 7, em; 12, em) consist of a double layer of the exodermal flat epithelium of the 

 exumbrella, and originate from the two gelatinous walls of the umbrella, which limit the 

 open peronial groove laterally in the Cunanthidae (PI. IX. fig. 5, es), but lie above the 

 grooves with their edges fused together in the iEginidae. The peronium in the ZEginidae 

 is therefore completely enclosed by the gelatinous substance of the umbrella on the 

 abaxial side, and by the subumbrella on the axial side, whilst in the Cunanthidae the 

 abaxial side of the peronium lies free at the bottom of the open peronial groove (PL IX. 

 fig. 5, en). In JEginara the distal end of the peronium joins that of the peronial plate 

 under the umbrella margin ; both pass continuously into the marginal urticating ring 

 (PI. XIII. figs. 1, 2, 4, nc ; PL XIV. fig. 11, nc). 



The umbrella cavity and the subumbrella lining it do not present in JEginura, any 

 more than in the other iEginidae, any of the striking peculiarities which distinguish the 

 two families of the Narcornedusse, the Cunanthidae, and Peganthidae ; the conditions do not 

 differ essentially from those usual in the Craspedotse. Hence it comes that the umbrella 

 collar is not divided into separate lobes by deep peronial incisions, and the margin of the 

 umbrella is therefore almost entire. The peculiar lobe cavities of the Cunanthidae (PL IX. 

 fig. 6, nl) and the Peganthidse (PL XII. fig. 7, nl) are consecpiently wanting. In 

 JEginura the umbrella cavity is more a simple cylindrical, or almost octagonal, hollow 

 space, with the oesophagus hanging in its axis, whose horizontal roof is formed by the 

 subumbral bottom of the stomach (PL XIV. fig. 11, gw), whilst it opens wide below, and 

 is limited laterally by the vertical side walls of the subumbrella. The latter has an 

 unbroken broad layer of circular muscular fibres, which is divided by the eight peronia 

 into eight quadrangular plates, but not cut through by it (comp. figs. 7, 11, 12, mw). 



