ALTERNATION OF GENERATIONS. 27 



margin hang a series of tentacular processes. After a period 

 of independent locomotive existence, the Medusa attains its 

 full growth, when it develops ova and spermatozoa. By the 

 contact of these embryos are produced ; but these, instead of 

 resembling the jelly-fish by which they were immediately gene- 

 rated, proceed to develop themselves into the fixed Hydroid 

 colony by which the Medusa was originally produced. 



Still more extraordinary phenomena have been discovered 

 in other Hydrozoa, as in many of the Lucernarida. In 

 these the ovum gives rise (as in Campanularia) to a locomotive 

 ciliated body, which ultimately fixes itself, becomes trumpet- 

 shaped, and develops a mouth and tentacles at its expanded 

 extremity, when it is known as the ' hydra- tuba,' from its 

 resemblance to the fresh-water polype, or Hydra. The hydra- 

 tuba has the power of multiplying itself by gemmation, and 

 it can produce large colonies in this way ; but it does not 

 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 

 Ephyrse. 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 



