598 



MEDUS.E OF THE WORLD. 



umbrella margin is divided by 8 deep, adradial clefts into 8 principal lobes, which are about 

 twice as broad as they are long. Each of these lobes is in turn divided by a median cleft, and 

 there are also two slight notches upon the bell-niargin on either side of the median cleft; 

 the margin, therefore, displays 32 indentations, between which there are 32 lappets. The 

 margin of the bell is sharp-edged for the gelatinous substance, which is quite thick at the 

 center of the disk, becomes very thin as one approaches the periphery. The 8 marginal sense- 

 organs are found at the bottom of the median niches of the 8 principal lobes of the disk. Each 

 sense-organ is elongate and club-shaped, and protected above by a web which stretches between 

 the adjacent lappets; proximal half of club quite thick, with a well-developed swelling upon 

 its lower (subumbrella) side; this swelling is covered with wart-like elevations and provided 

 with one or two papillae. Distal to this swollen region the club extends outward as a cylindrical 

 tube which terminates in a swollen knob-like part containing an entodermal mass of crvstalline 

 concretions, but no ocelli. Two open pits project downward from the floor of the exumbrella 

 on either side of the base of the sensory-club. The structure of the sense-organ in Cyanca 

 has been studied by L. Agassiz, 1862; Eimer, 1878; and Fewkes, 1881. About 800 long ten- 

 tacles arise from 8 adradial, crescentic regions on the floor of the subumbrella, about midwav 

 between the periphery and the center. The horns of these crescentic areas point outward 

 and the tentacles are arranged in about 5 concentric rows in each crescent, the oldest and 

 longest tentacles being on the innermost row. The tentacles are hollow and highly contractile; 

 when fullv expanded they attain a length of about 25 times the bell-diameter; their surfaces 

 are thickl\- covered with nematocysts. Mouth 4-cornered and situated at center of subum- 

 brella; it is provided with 4 long perradial mouth-arms, the margins of which are greatl)' folded, 

 forming the curtain-like lips or oral fringes which hang downward in the water. Mouth-arms 

 about as long as bell-diameter, and with their fringes appear as a complexly folded, contractile 

 mass of curtain-like appendages hanging from the oral floor ot the bell. 



Fig. 582. — Cyt2nea postelsi, according to Brandt, after Vanhijffcn, in Nordischcs Plankton. 



Gonads occupy 4 complexly-folded pouches which project from subumbrella floor at the 

 4 interradial sides of the stomach. Numerous clusters of small gastral cirri project trom the 

 bases of the gonads into the stomach-cavity; these are far inore prominent in the young 

 medusa than in the adult, for in the mature medusa they become hidden away at the bases 

 of the pendant, pouch-like folds of the genital organs. There is a very powerful and con- 

 spicuous system of circular muscles in the subumbrella; these muscles occupy a zone about 

 one-eighth as broad as bell-radius and which lies adjacent to and centrifugal from the gonads. 

 This zone of muscles is composed of 16 trapezia, the 8 in the rhopalar radii being only halt 

 as wide as those in the adradii. 16 strands of radiating muscles extend from the outer side of the 

 zone of circular muscles and pass outward on either side of the sense-organs. 



The central stomach is a wide, lenticular space in the center ot the disk; peripherally it 

 gives rise to 16 radiating pouches, the outer edges gi\ing numerous branched canals which 

 ramify through the lappets without anastomosing. The 8 pouches in the radii ot the sense- 

 organs are less than half as wide as the 8 in the radii of the tentacles. The tentacles and the 

 stalks of the sense-organs are hollow and in direct connection with the gastrovascular space of 



