112 ELEMENTS OF BIOLOGY. • 



and the margins of the bell carry organs of sense and long 

 tentacles. The central polypite has a mouth and digestive 

 cavity, leading into a complex canal-system. At first of 

 small size, they feed eagerly, and increase largely in bulk, 

 in some cases attaining perfectly colossal dimensions (as 

 much in one species as twenty feet in circumference). After 

 awhile 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 organism 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 nourishing 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 

 v/ith which the series began. 



To the group of phenomena of which the above are ex- 

 amples, the name "alternation of generations" was applied 

 by Steenstrup ; 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 reproductive zooid, and the production of this from 

 the nutritive zooid is a process of gemmation or fission, and 

 nofa process 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 independent 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 ind^Dendent 

 organism. It must also be remembered that the nutritive 

 half of the '• individual " is usually, and the generative half 



