510 RADIATES: ACALEPHS. 



their essential structure, in this respect, is the same as that 

 of other discophorae, differing only in degree. 



The Sub-Order Semaeostomse comprises discophorae 

 which have the so-called mouth plainly represented, 

 though surrounded by more or less extensive appendages, 

 and marginal tentacles more or less developed. 



AURELID^E, OR AURELIA FAMILY. This Family em- 

 braces discophorae which are characterized by the even 

 curve of the outer surface of the disk, while the lower 

 surface is excavated in its central portion by four large 

 genital pouches, between which hang four stout arms, 

 closing upon one another in the centre, so as to form a 

 rectilinear opening, prolonged in the undulating curves 

 or folds between the lower margins of the arms. The 

 margin of the disk has small tentacles, except where the 

 eight eyes occupy slight indentations. 



The Genus Aurclia contains the common " Sun-Fish," 

 A. flavidula, Per. & LeS., of the northeast coast of North 

 America. It attains eight or ten inches or more in di- 

 ameter, and lives but a single year. When first seen in 

 the spring, it is hardly a quarter of an inch in diameter; 

 and when the sky is clear and the sea smooth, it floats 

 in immense numbers near the surface of the water. They 

 grow rapidly, reaching their average size in early summer. 

 As they increase in size, they separate more and more, 

 but reassemble towards the close of summer, which is 

 the spawning season. Later, they are broken into frag- 

 ments, and destroyed by the autumnal winds ; but the 

 planulae as the newly-hatched jelly-fishes are called - 

 soon appear moving freely about by means of vibratile 

 cilia. After a little time, each becomes attached to the 

 rocks, sea-weed, or shells, and is then known as ScypJios- 

 toma, Fig. 494. Then the body begins to be divided into 

 rings by transverse constrictions, and the rings or seg- 

 ments become more and more numerous and more dis- 



