22 NATURAL HISTORY. 



numerous, but their pale light is there lost in the more brilliant phos- 

 phorescence of the other forms. 



A few lines above mention was made of the fact that there are repro- 

 ductive organs in hydroid jelly-fishes, and this leads to a series of very 

 interesting facts, common not only to these animals, but to many others. 

 The reproductive organs of a single hydroid colony will give rise to a 

 large number of jelly-fishes. These in turn produce eggs, which do not 

 develop into other jelly-fishes, but into colonies of hydroids. In short, 

 among these forms any individual does not in the least resemble its own 

 parents, but is just like its grandfather and grandmother. It is a case of 

 what is called alternation of generations. When the egg from a jelly-fish 

 hatches, and the young settles down on some submerged object, it begins 

 to grow in two directions. The upward growth soon produces a hydroid 

 fitted for eating, while laterally from the base grows out the root-like 

 portion of the colony. This at intervals sends up branches, which develop 

 into other feeding hydroids, or into those which are to serve only repro- 

 ductive purposes. 



In many hydroids there are but the merest traces of a nervous system. 

 The naturalist finds scattered among the superficial cells of the jelly-fishes 

 cells, which, from their general appearance, he knows to be nervous ; but 

 there is nothing like a brain, no nervous cords. Still the jelly-fish has its 

 organs of sense, and these pretty well developed. At the base of certain 

 of the tentacles (see the figure of Obelia, p. 23) there are eyes, differing 

 in structure in the different species, while in some forms some of the ten- 

 tacles are modified to form ears ; in others the ears are open pits. In 

 other hvdroids a nervous ring exists. 



One of these hydroid jelly-fishes possesses considerable interest, not 

 only from the fact that it is the only fresh-water jelly-fish known (it has 

 been reported that there is a jelly-fish in the lakes of Central Africa), but 

 from the great uncertainty which surrounds its origin. In Regent's Park, 

 London, there is a tank containing specimens of the enormous water-lily, 

 Victoria regia. A few years ago some pickerel-weed (Pontedera) was 

 placed in the tank, and the next year the jelly-fish made its appearance, 

 causing no little excitement among naturalists, for nothing of the kind had 

 ever been known before. Whether it was introduced with the pickerel- 

 weed, or in some other way, is also uncertain, and until 1885 it was not 

 known that it had a hydroid condition ; but in that year hydroids, far 

 simpler even than hydra, were found attached to the plants in the tank, 

 which, presumably, were connected with this jelly-fish (Limnocodium) in 

 the same way as in the case described above. 



In our seas we have many of these hydroids which produce jelly-fishes, 



