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



low, opening wide below, whilst it is limited above by the subumbral gastral wall (conip. 

 figs. 1, 7, 9). 



The suburnbrella, with its annular muscular layer is divided, as in the previous 

 species, into two very different parts, limited by the circle of genitalia (fig. 9). The 

 central part of the suburnbrella, which corresponds in extent to the lower surface of 

 the umbrella lens, is formed by the lower, folded, very muscular gastral wall, and appears 

 pierced in the middle by the oesophagus (fig. 7). The peripheric part, on the other 

 hand, is composed of the circle of isolated muscular plates which line the inner concave 

 surface of the eighteen collar lobes. The velum completely fills the narrow interspaces 

 of these lobes, and, moreover, projects freely a little way further over the points of the 

 lobes like a connected circular edge (fig. 1, ve, right half v). Vertical sections of the 

 firm velum (fig. 12, left) show that the upper (subumbral or ventral) epithelium of the 

 velum (vw) is three times as high and as thick as the lower (exumbral or dorsal) epithe- 

 lium (mo). A strong circular muscular layer lies on the former (mv), and a thick elastic 

 supporting plate on the latter (zv). 



As in the other Peganthidse, the peculiar umbrella margin (characterised by the urticat- 

 ing ring and nerve ring) is deeply indented, and covers the selvage of the collar lobes 

 like a connected edge. In Pegantha pantheon it forms eighteen deep curves reaching 

 as far as the insertion of the tentacles (figs. 2, 3, 8, 12). In the radial transverse 

 section of the umbrella margin (PI. XII. fig. 12) the urticating ring (na) appears 

 covered by dense epithelium with long cilia. The dorsal nerve ring (re) is divided 

 from the ventral nerve ring (re") by the supporting lamella of the velum (zv) ; both lie 

 immediately outside (abaxially) the insertion of the velum (comp. the explanation of 

 fig. 12). The distal margin of the broad festoon canal (cf) touches the velum imme- 

 diately. Of tentacular organs, the umbrella margin bears eighteen tentacles and 

 numerous (over 400) free auditory clubs. 



The eighteen strong tentacles, which alternate with the eighteen collar lobes and are 

 inserted at their basis in the coronal furrow, were generally curved upwards in the 

 specimen examined, as often happens in the Narcomedusse (fig. 1). They are cylindrical, 

 somewhat thicker towards the base, thinner towards the point, and nearly as long as 

 the diameter of the umbrella. The endodermal axis is composed of a single row of coin- 

 shaped chordal cells (a millimeter broad), in which the nuclei form a central chain 

 (fig. 11). A visible layer of longitudinal muscular fibres (m) lies outside the strong- 

 supporting plate (fig. 10,2). The spheroidal nematocysts (n) in the exodermal epithelium 

 are chiefly accumulated in the abaxial side of the tentacles, most thickly at the point. 

 A thick, almost closed, urticating ring, which has only a break internally on the axial 

 side (figs. 2, 3, nb) is placed at the insertion of the tentacles. Inside this the tentacles 

 run into the pointed conical roots (tr) which pass centripetally into the gelatinous sub- 

 stance of the umbrella, 



