442 



MEDUSA OF THE WORLD. 



Pegantha pantheon Haeckel. 



Pegantha pantheon, Haeckel, 1879, Syst. der Medusen, p. 332; 1881, Report Deep Sea Medusae Challenger Expedition, Zool., 

 vol. 4, p. 37, plates II, 12, figs. I to 12. 



Bell about 20 mm. wide and 10 mm. high. A deep coronal furrow separates the central 

 apical disk from the marginal zone of gelatinous lappets. Gelatinous substance of central 

 disk thick and about one-half to two-thirds as wide as entire medusa. The exumbrella exhibits 

 numerous branched, strongly projecting ribs, between which there are deep radial furrows. 

 18 of these ribs are more prominent than the others and extend outward over the middle of each 

 of the 18 lappets. These main ribs increase in thickness from the center outward and give 

 rise to side branches; thus a main central rib flanked on either side by several less prominent 

 ribs extends outward over each of the lappets. 



Fig. 295.— Pegantha martagon (young medusa), after Bigelow, in Bull. Mus.Comp. Zool. at Harvard College. Upper 



half gives appearance of contracted bell, while lower half shows expansion. 

 Fig. 296.— Pegantha martagon (young medusa), after Bigelow, in Bull. Mus. Comp. Zool. at Harvard College. 

 Fig. 297. — Pegantha triloba, after Haeckel. 

 Fig. 298.— Pegantha dactyletra, after Maas, in Ergeb. Plankton Expedition. 



The 18 oval lappets are about half as broad as long and alternate in position with 18 ten- 

 tacles which project stiffly from the sides of the bell, their roots being inserted in the zone of 

 the deep ring-furrow which separates the central, apical, disk-like part of the bell from the 

 more marginal zone of the 18 lappets. These tentacles are of uniform length and taper gradu- 

 ally from base to tip. They are about as long as the bell-diameter; their entoderm consists of a 

 single row of chordate cells, placed coin-like one after another in an axial row. The roots of the 

 tentacles taper inward to the floor of the stomach with which their entoderm is continuous. 

 A broken ring of nematocysts partially embraces each tentacle at its base, leaving its upper 

 side free. 23 to 25 marginal sense-clubs arise from the aboral side of each of the 18 lappets. 

 Thus there are 400 to 450 of these clubs. Each club is elongate, narrow at its base but 

 widest at its free end, and contains an axial row of 3 to 5 crystals inclosed in large entodermal 

 cells. The club arises from a sensory cushion provided with long bristles, and above this 

 cushion there is a well-defined, swollen tract of cells over the exumbrella side of the lappet. 

 These tracts are oval in outline and about as long as the clubs themselves and they converge 

 slightly toward the axial line of the lappet. There is a thick nerve and nematocyst-ring around 

 the exumbrella margin of the lappets above the velum. The velum is well developed and 

 spans the spaces between the edges of adjacent lappets, partially closing the bell-opening in the 

 manner of a diaphragm. Its circular muscles are well developed. 



