THE CYCLE OF LIFE 445 



life-history, and produce ova and spermatozoa. The 

 fertihzed ova develop into free-swimming embryos, which 

 soon settle down and become polyps. Each polyp is the 

 beginning of a hydroid colony which is formed by repeated 

 budding. Thus there is a remarkable alternation between 

 a fixed, plant-hke, vegetative, asexual hydroid colony or 

 zoophyte, and a free, active, sexual medusoid or swimming 

 bell. A similar separation of the Hfe-history into two very 

 markedly contrasted chapters is common among Ccelentera 

 or Stinging Animals ; we find it again in many Trematodes 

 like the hver-fluke ; in some insects, Uke the gall- wasps ; 

 and in remarkable expression in the free-swimming Tuni- 

 cates known as Salps. It is also characteristic of ferns 

 and mosses and the hke, and it occurs in disguised form 

 in flowering plants. It may be defined as the alternate 

 occurrence in one hfe-history of two or more different 

 forms difierently produced. 



The Common Jelly-fish. — ^Every one who knows the 

 sea at all is famiUar with swimming or drifting shoals 

 of the common jelly-fish, Aurelia aurita, one of the most 

 cosmopohtan of animals. The glassy disc, with a shimmer 

 of hght violet, is usually about four inches in diameter ; 

 it is surrounded by minute circumference tentacles, and 

 eight sense-organs symmetrically arranged in niches ; four 

 frilled Kps hang down from the central mouth onthe under 

 side ; eight branched and eight unbranched canals radiate 

 out from the central stomach to a peripheral canal ; and 

 there are four conspicuously coloured male or female 

 reproductive organs. The fertihzed eggs develop into 

 minute free-swimming oval larvae, which after a short 

 period of activity settle down on a stone or seaweed. 

 They develop into little polyp-hke forms, known as ' Hydra 



