REPRODUCTION. 37 



is able to increase considerably in size. The substance of the 

 disc is penetrated by a complex system of canals, and from its 

 margin hangs 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 gen- 

 erated, 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 col- 

 onies in this way ; but it does not obtain the power of gener- 

 ating the essential elements of reproduction. Under certain 

 circumstances, however, the hydra-tuba enlarges, and, after a 

 series of preliminary changes, divides by tranverse 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 inde- 

 pendent beings, which were described by naturalists as distinct 

 animals, and were called Ephyrse. They are provided with a 

 swiming-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 reproduc- 

 tion, and after the fecundation and liberation of their ova they 

 die. The ova, however, are not developed into the free-swim- 

 ming 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, 



