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



coronal furrow of the exumbrella (figs. 1-3, 6). The twenty-five tentacles of our Pohj- 

 colpa forskalii are l| to 2 times as long as the diameter of the umbrella, and are some- 

 times curved upwards like a crown (as in Pegantha pantheon, PI. XL fig. 1), sometimes 

 turned downwards under the umbrella (PL X. fig. 2) ; they are cylindrical, cartilage- 

 like filaments, somewhat thickened at the bases, becoming gradually pointed finely 

 towards the end, and combining a high amount of stiffness and firmness with considerable 

 elasticity. The solid chordal axis resembles the chorda dorsalis of the vertebrata, and 

 consists of large, clear, thick-walled endoderm cells, which have a firm elastic membrane, 

 transparent contents, and a large nucleus. The chordal axis of each filament forms a 

 single row or column of such coin-shaped chordal cells (comp. PL XII. figs. 10, 11). 

 Its exodermal epithelium contains numerous spheroid nematocysts, especially in the 

 abaxial side of the filament. The clear conical tentacle root is also composed of thicker 

 chordal cells, it penetrates radially (centripetally) some way from the insertion of the 

 tentacles into the gelatinous substance of the disk, and often lies, bent like a hook, with 

 its lower oral side on the periphery of the stomach (PL X. fig. 3, tr ; fig. 7). A net of 

 branched protoplasmic filaments radiating from the layer surrounding the nucleus is 

 visible in each chordal cell of the root (fig. 7). The endodermal supporting plate, 

 which encloses the chordal axis of the tentacles like a tube, also surrounds the root up to 

 the point, which, on the other hand, the layer of longitudinal muscular filaments (lying 

 outside the endodermal supporting plate) does not. " Umbrella clasps " or peronia, 

 which appear so strongly developed in Cunarcha and JEginura, are only rudimentary 

 in Polycolpa and Pegantha as in most Peganthidse. As the deep indentations of the 

 umbrella margin extend between each two lobes almost to the base of the tentacle, the 

 peronia are naturally so much shortened that they almost disappear. By their retrograde 

 formation the insertion of the tentacle remains in continuous direct connection with the 

 urticating ring as it passes immediately into the former at the base of the lobes. The 

 interlobar points of the velum, therefore, also extend to the tentacle root between 

 each two lobes (fig. 6). 



Pobjcolpa forskalii has 130 to 170 auditory clubs, 5 to 7 on each of the twenty-five 

 lobes (fig. 6). One of them is placed on the point of the lobe, the others (in pairs 

 opposite each other) on its lower lateral margin. Their structure is the same as those 

 previously described in Cunarcha. Here, however, each of the 3 to 4 endodermal 

 axial cells usually contains an otolite (fig. 8, ol). The otoporpse or " auditory clasps " at 

 their bases (fig. 8, oo) are club-shaped urticating streaks of the exumbrella covered with 

 ciliated sense-epithelium with larger and smaller nematocysts (fig. 8, n). Their other 

 functions are the same as in Pegantha pantheon (comp. PL XL fig. 4). 



The gastrovascular system (figs. 1, 3, 6, 8) has the special formation, peculiar to all 

 Peganthidse, which distinguishes this family of the Medusae from all the rest. It con- 

 sists of two principal sections, the central stomach and the peripheric festoon canal (fig. 



