564 



MEDUSvE OF THE WORLD. 



The center of the subumbrella is occupied by the shallow proboscis, the 4 lips of which 

 are simple and cruciform. Surrounding the base of the proboscis is a zone in which are 

 situated the 8 adradial gonads. The gonads vary considerably in size and shape, but normally 

 appear to be composed of bean-shaped sectors, each genital ridge being constricted radially 

 by one or more deep furrows (see Vanhoffen, 1892, taf. 4, fig. 4, g). 8 groups of radial-muscles 

 lie in the floor of the subumbrella between the 8 gonads. Immediately centrifugal to the 

 zone of the gonads lies a zone of circular muscle-fibers which is divided into separate parts 

 by furrows which lie in the radii of the sense-organs. Beyond this zone lies the outermost 

 ring of circular muscles developed so remarkably that it projects outward from the floor of 

 the subumbrella as a thick annular mass, which is entire, not divided, as are the inner, circular 

 muscles. The cavity of the central stomach communicates with a broad ring-sinus in the 

 subumbrella by means of 4 perradially situated ostia. This ring-sinus corresponds in position 

 to the zone of the gonads upon the subumbrella. Centrifugally it gives rise to a radiating vessel 

 in each tentacular radius and also to an equal number of vessels to the sense-clubs. The ten- 

 tacular vessels each give off a pair of side-branches which lead into the rhopalar vessels near 

 the bases of the sense-clubs. At the point of origin of each tentacular vessel there is a region 

 of fusion of the subumbrella and exumbrella walls of the gastrovascular cavity thus forming 

 a small cathammal plate in the middle of each tentacular vessel (see fig. 359 C). 



FIG. 358. Aiolla valdivia, after Vanhoffen, in FalJivia Expedition. View of subumbrella. 



The excretory system of the exumbrella has been described under the genus Atolla. 



In the corners of the central stomach are 4 interradial rows of gastric cirri forming a 

 cross with perradial axes. Altogether the gastrovascular system, the pedalia of the exumbrella, 

 and the structure of the muscular system, with the exception of the remarkable unitary ring- 

 muscle of Atolla, are quite alike in both Atolla and Periphylla and indicate a genetic rela- 

 tionship between the two forms. 



The gelatinous substance of the bell of Atolla bairJii is translucent and milky-blue in 

 color. The central disk of the exumbrella is sprinkled over with rust-red colored patches 

 which become more numerous in the region of the annular furrow, which is of a decided red- 

 brown color. The powerful centrifugal circular-muscle band of the subumbrella is dark-red 

 and a radial red streak extends inward on the subumbrella side from the base of each sense- 

 organ to the band of circular muscles. Gonads and entoderm of central stomach red. The 8 

 adradial excretory pores near the 4 angles of the central stomach are dark-red of the hue 

 so commonly seen in deep-sea medusae of all ports. 



