138 



MARINE INVERTEBRATES 



Strobila of Aurelia flavidula. 



the end of July they are fully developed, and begin to discharge their eggs, 

 which go into the folds around the mouth and remain there until they at- 

 tain the planula stage. After the spawning 

 period the medusae, reduced in strength, are 

 unable to resist the storms of the autumn, 

 and many of them are cast ashore ; many 

 others, in a more or less wasted condition, 

 float near the surface, but the body is less 

 transparent, its tissues are thickened, its 

 tentacles gone, and general dissolution has 

 commenced. In this condition the medusae 

 are frequently capsized by the air which ac- 

 cumulates in the empty egg-cavities, and, 

 floating helplessly on the surface, are at- 

 tacked and destroyed by swarms of small 

 crustaceans ; thus their cycle of life is ter- 

 minated. It has been suggested that the 

 destruction of the mothers, by being cast 



upon the beaches in the autumnal gales, is a provision to set free the 

 planulae in a position favorable to their existence ; for when liberated 



they fasten upon the rocks and sea- 

 weeds of the shore, where, during the 

 winter months, they develop into stro- 

 bilae, which in turn free their saucer- 

 like disks early in April. 



GENUS Cyanea 



C. arctica, the sun-jelly or sea- 

 blubber. This is the largest jellyfish 

 known. Some individuals measure 

 seven and a half feet across the disk 

 and have tentacles more than one 

 hundred feet long. Usually they are 

 three to five feet in diameter, with 

 tentacles thirty to forty feet long. The 

 disk is red, the margin white and scal- 

 loped. The tentacles, of different 

 colors, are covered with lasso- or sting- 

 ing-cells, and are arranged in eight 

 distinct, thick clusters on the mai'gin. 

 From the mouth hang four long and 

 very broad, thin curtains, much folded 

 and ruffled, whose edges at times look 

 as if they were embroidered, because 

 great numbers of discharged eggs are 

 attached to them. Four egg-sacs hang 

 from the disk near the manubrium, and 

 eight sense-organs (tentaculocysts). in 

 hardened coverings, lie in some of the 



Cyanea aretica, greatly reduced in size. deep incisions of the margin. 



