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



thread cells are scattered freely over it. The gelatinous substance of the umbrella shows 

 a considerable degree of firmness, in spite of its being very thin and without any cellular 

 elements. The gelatinous substance varies in thickness in different places, according to 

 the different longitudinal furrows of the exumbrella and the subumbrella, being thinnest 

 along the interradial furrows (in the middle of the corner pillars) and thickest at the two 

 sides of the pillars, and above in the cap-shaped apical cover of the umbrella (figs. 1-3, ag). 



The subumbrella or nectocalyx is nearly cubical. The four corners of the cube are 

 interradial and formed by the narrow septa of the broad gastral pouches, or by the 

 "fused streaks" by which the subumbrella is connected with the umbrella. The 

 muscular layer of the subumbrella is thus divided into four rectangular muscular plates, 

 which are placed nearly vertically to each other in the interradial " fused streaks "; they 

 correspond to the four lateral surfaces of the cube, and form the axial wall of the four 

 radial pouches (fig. 3, mw). The circular fibres of each muscular plate are, however, 

 interrupted in its perradial middle line by a band-shaped, longitudinal muscle, which 

 extends from the ocular niche, upwards to the mesogonia and downwards to the 

 frenulum (fig. 3, mp). The broad coronal muscle is therefore actually divided here into 

 eight quadrangular coronal areae as in Pericolpa (System, taf. xxiii.). Whilst, however 

 in Pericolpa, these areae lie in the principal radii (four perradial and four interradial), in 

 Charybdea they are placed adradially. 



The umbrella margin (figs. 1,5, 8), in a wider sense, bears four perradial sense clubs 

 and four interradial tentacles. These marginal organs are connected by a remarkable 

 nerve ring of peculiar structure. Below this nerve ring, however, the umbrella margin 

 passes into a broad velarium, a thin marginal membrane resembling the velum of the 

 Craspedotae, but, however, essentially different. As regards the eight marginal organs 

 they are undeniably derived phylogenetically from the eight principal tentacles of 

 Tessera and Tesserantha (PI. XV.) ; the four sense-clubs from the four perradial principal 

 tentacles, and the four tentacles from the four interradial. In this respect the condition 

 is exactly inverted in the Cubornedusae, as in the Peromedusae (specially in the 

 Pericolpidae). In the Discomedusae all the eight principal tentacles are transformed into 

 rhopalia. 



The velarium or marginal membrane (figs. 2, 5, 8, va), represents a membranous, 

 annular distal process of the umbrella margin. It has hitherto been simply termed 

 velum, and placed beside the similarly termed velum of the Craspedotae. These two 

 formations are, however, only analogous, not homologous ; they have originated independ- 

 ently of each other, and their structure though similar is in no way identical, that is, 

 their relation to the nerve ring is essentially different. As in all Charybdea belonging to 

 the sub-family of the Tarnoyidae, the velarium is traversed by special canals, and is 

 fastened in a very peculiar fashion to the subumbrella by the four perradial frenula 

 (suspensors or supporting folds, figs. 2, 8, vf). These frenula are muscular, vertical, 



