EEPORT ON THE DEEP-SEA MEDUSAE. 129 



the radius of the umbrella. It is divided from the broad tentacle zone by the velar 

 furrow (or the marginal coronal furrow) of the subumbrella, on which the eight sense 

 clubs lie. As the firm gelatinous substance of the umbrella suddenly becomes very much 

 thinned away at the velar furrow, the lobe zone is very flexible ; it is more or less bent 

 round, and in most of the specimens before me, turned back towards the tentacle zone of 

 the subumbrella. Its subumbral surface is quite smooth, covered by a layer of circular 

 muscular fibres, bears no tentacles, and is only traversed by very shallow, almost 

 imperceptible, radial furrows, which correspond to the deeper radial furrows between the 

 marginal lobes of the exumbrella. The eight rhopalia or sense clubs (fig. 1, below) lie in 

 the subumbrella, at the proximal margin of the velar zone, immediately outside the velar 

 coronal furrow. 



The eight sense clubs (4 perradial and 4 interradial) are remarkable from their 

 completely subumbral position ; their distance from the umbrella margin amounts to 

 nearly the half of their distance from the umbrella centre, therefore, to nearly one-third 

 of the whole radius of the umbrella. The sense clubs lie entirely hidden in eight deep 

 subumbral sense niches, at the distal end of the eight principal radial furrows. Each 

 sense niche (" antrum rhopalare," fig. 2, on) is broadly lanceolate in shape and is enclosed 

 by a pair of thick, narrow bean-shaped gelatinous swellings, like a pair of fleshy lips ; 

 these clearly correspond to the two sense folds of other Discomedusae, or to the rolled- 

 in medial inner margins, the original ephyra lobes of the umbrella margin ; they are 

 here thickened, and have their concave medial margins turned to each other in such a 

 way that both their distal and their proximal ends touch, and the sense club only 

 remains open between them below (fig. 2). Their upper covering (corresponding to the 

 protective scale or protective covering of the other Discomedusse) is formed by the 

 gelatinous substance of the umbrella, The sense club lies almost in the middle of this 

 deep lanceolate sense niche (rather nearer the proximal margin), and is fastened to the 

 under side of its covering in such a way that its radial longitudinal axis appears directed 

 from the inside and above, towards the outside and below, therefore towards the distal 

 entrance of the niche. The rhopalia themselves are comparatively small, almost acorn- 

 shaped, and were sufficiently well preserved in the spirit specimens before me to admit 

 of closer examination with the aid of fine transverse and longitudinal sections (figs. 2-7). 

 Their form and structure on the whole do not differ essentially from that of Cyanea. 

 Each rhopalium consists of a thicker proximal and a thinner distal part ; separated by a 

 slight circular constriction ; the base of insertion of the proximal part is also strongly 

 constricted (fig. 5, longitudinal section). The sense canal (co), which is very much 

 narrowed at this basal stricture, immediately becomes enlarged again and is not limited 

 here to the proximal half (as is usual in the Discomedusse), but also passes over into the 

 distal half containing the otolite (fig. 6). The acoustic ectodermal epithelium of the sense 

 club is single-layered, and consists of flagellate, cylindrical cells in the basal half, and 



(ZOOL. CHALL. EXP. — PART XII. — 1881.) M 17 



