ALTERNATION OF GENERATIONS. 3 I 



in size. The substance of the disc is penetrated by a complex 

 system of canals, and from its margin hangs a series of tentacu- 

 lar 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 generated, 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 to a locomotive ciliated body, which ulti- 

 mately 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 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 fecunda- 

 tion 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, 



