ALTERNATION OF GENERATIONS. 2Q 



obtain the power of generating the essential elements of 

 reproduction. Under certain circumstances, however, the 

 hydra-tuba enlarges, and, after a series of preliminary changes, 

 divides by transverse fission into a number of segments, each 

 of which becomes detached and swims away. These liberated 

 segments of the little hydra-tuba (it is about half an inch in 

 height) now live as entirely independent beings, which were 

 described by naturalists as distinct animals, and were called 

 Ephyrae. They are provided with a swimming - bell, or 

 "umbrella," by means of which they propel themselves through 

 the water, and with a mouth and digestive cavity. They 

 now lead an active life, feeding eagerly, and attaining in some 

 instances a perfectly astonishing size (the Medusoids of some 

 species are several feet in circumference). After a while 

 they develop the essential elements of reproduction, and 

 after the fecundation and liberation of their ova they die. 

 The ova, however, are not developed into the free-swimming 

 and comparatively gigantic jelly-fish by which they were 

 immediately produced, but into the minute, fixed, sexless 

 hydra-tuba. 



We thus see that a small, sexless zooid, which is capable of 

 multiplying itself by gemmation, produces by fission several 

 independent locomotive beings, which are capable of nourish- 

 ing themselves and of performing all the functions of life. In 

 these are produced generative elements, which give rise by 

 their development to the little fixed creature with which the 

 series began. 



To the group of phenomena of which the above are examples, 

 the name "alternation of generations " was applied by Steen- 

 strup ; but the name is not an appropriate one, since the 

 process is truly an alternation of generation with gemmation 

 or fission. The only generative act takes place in the repro- 

 ductive zooid, and the production of this from the nutritive 

 zooid is a process of gemmation or fission, and not a pro- 

 cess of generation. The "individual," in fact, in all these 

 cases, must be looked upon as a double being composed of two 

 factors, both of which lead more or less completely inde- 

 pendent lives, the one being devoted to nutrition, the other to 

 reproduction. The generative being, however, is in many 

 cases not at first able to mature the sexual elements, and is, 

 therefore, provided with the means necessary for its growth 

 and nourishment as an independent organism. It must also 

 be remembered that the nutritive half of the " individual " is 

 usually, and the generative half sometimes, compound; that is 

 to say, composed of a number of zooids produced by con- 



