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THE POPULAR SCIENCE MONTHLY. 



growth the segments become more and more marked, more and more 

 separated, and at length the uppermost one drops off, then the next 

 one drops, then the next, and so each in its turn breaks away from the 

 parent stalk ; and as each breaks away it assumes the natural attitude, 

 mouth downward, and floats away to lead its independent life as a 

 genuine sunfish. Here we have a good illustration of that strange 

 mode of reproduction called " partheno-genesis " or " alternations of 

 generations " — tliat is, the o.^'g hatches into the i^lanida^ which soon 

 becomes a scyphistoyna ; that little plant-like body becomes a stro- 

 bila, and this breaks into segments, each of which becomes a perfect 

 jelly-fish, which produces the eggs. 



The species of jelly-fishes of the disk, or partially hemispherical 

 form, are very numerous and varied in details of structure, in form, 

 and in size. Some have the appendages around the mouth and the 

 margin greatly prolonged as in Fig. 20. A few kinds attain a diam- 



FiG. 20.— Jelly-fish (Pehigia cyanella, 

 Agassiz). 



Fig. 21.— Pledrobkachia (P. rhododactyla, 

 Agassiz). 



meter of two or three feet ; and these largest kinds have in some cases 

 tentacles a hundred feet long. 



One of the most beautiful of all the jelly-fishes is the rose-colored 

 idyia. It is often seen near the shore, and is so transparent tliat it 

 reveals almost its whole structure as it floats in the water. It is 



