266 



Marvels of the Universe 



specimen with the long, thread-hke tentacles of all its members waving gentlv in the water affords 

 an extremely beautiful sight. Sooner or later, however, the buds become detached and commence 

 housekeeping on their own account. 



There is also a sexual method of reproduction, in which, instead of a bud being formed, an egg 

 takes its place. This, after fertilization, secretes round itself a hard coat, and drops off, and in 

 due time the shell splits and a young Hydra emerges. 



Like many other animals that reproduce their species ase.xually — that is, by budding, or some 

 similar process — Hydra is able to replace any lost parts ; but in Hydra this power is developed to 

 such a degree that even if cut up into pieces, each piece can replace all the rest and develop into 

 a complete individual. 



Though such an inconspicuous creature. Hydra presents us with a number of most interesting 

 phenomena. For instance, its very colour — green or brown, according to species — is due to a 



minute, single-celled plant that lives 

 A'ithin the cells of the digestive layer 

 of the body-wall. The plant is not a 

 parasite, as it causes no harm to its 

 host ; indeed, it is probably useful to 

 it, both by the fact of its colour 

 rendering the Hydra less conspicuous 

 and less easily found by its enemies, 

 and also by its vital processes yield- 

 ing products that are useful to the 

 Hydra. In return, the plant doubtless 

 receives considerable protection in 

 being enclosed within the body of the 

 Hydra, and probably obtains its food 

 from waste excretions of its host. 



In spite of its frail appearance, and 

 the benevolent part it plays as host, 

 the Hydra is a creature to be reckoned 

 with in pond-life. Its capacity for 

 nourishment is enormous and vastly dis- 

 proportionate to its size. Many are the 

 water-fleas and other small fry that the 

 Hydra can account for, and occasion- 

 ally even very minute fishes find their way down that capacious gullet. The victims are caught and 

 stung to death by the thread-like tentacles shown on page 264. Even its own kind are not outside 

 the limits of its voracious appetite. In the lower left-hand photograph on page 265 an instance 

 of this flagrant cannibahsm is shown. The vanquished is already becoming part of the victor. 



When we come to examine the affinities of the Hydra we find that it stands alone as an 

 inhabitant of fresh water, all its relatives being marine. It is closely allied to the jelly-fish, so 

 beloved of nervous bathers, and less closely to the sea-anemones ; indeed, it can hardly be doubted 

 that Hydra itself is descended from marine ancestors, but in adapting itself to fresh-water 

 conditions it has, as is usually found in such cases, become considerably modified, especially as 

 regards the development of the young. In Hydra this modification is evident in the suppression 

 of the free-swimming stage in the hfe history that is found in its marine cousins: in the 

 comparative rarity and simplicity of the sexual method of reproduction : and in the fact that the 

 buds sooner or later drop off and start life as new individuals, instead of remaining attached to 

 form a branching colony. 



Pholo fttf] 



in. nil- 



THE H^DRA AND ITS VICTIMS. 



In the upper figure the Hydra has captured and is 

 worm; in the lower it is a young fish that has fallen 

 distinction between captor and prey may be traced by 

 tentacles, which surround the mouth. 



swallowing a 

 I victim. The 

 means of the 



