A STUDY OF LOWER LIFE. 47 



be regarded as a fair statement of their average dimensions ; 

 although occasionally a specimen may greatly exceed the 

 proportions of its neighbours, and exhibit a length of half an 

 inch, or more. Examined by aid of a hand-lens, each of 

 these little organisms or hydrae is seen to possess a tubular 

 or cylindrical body, which is attached by one extremity 

 to the glass or duckweed, and which exhibits at the oppo- 

 site and free extremity, a mouth-opening, surrounded by 

 a circle of arms and tentacles. These latter are delicate 

 thread-like organs, which, in the undisturbed and natural 

 state of the animal, remain outstretched in the water. In 

 the common or green hydra, the tentacles are not dispro- 

 portionately developed as regards the body ; but in certain 

 other forms or species, in which the body is coloured 

 brown (Hydra fusca\ the tentacles are very long, and the 

 animal obtains, in consequence, the distinctive name of the 

 " Long-armed Hydra " (Fig. i). 



The observation of the common incidents of the hydra's 

 life reveals certain interesting features, which assist us in 

 some degree in the appreciation of the nature and structure 

 of these organisms. When the tentacles are touched, they at 

 once contract and shorten, and the body also shrinks or 

 shrivels up into a somewhat rounded mass. This simple 

 fact proves to us that the hydra is sensitive to outward im- 

 pressions a feature in its history which is of high interest 

 when we endeavour to understand the nature and relations 

 of the nervous system of higher animals. That the hydrae 

 are also sensitive to more delicate impressions, is proved by 

 their clustering most thickly on that side of the glass vessel 

 which is next the light. If the hydra is left in an undisturbed 

 condition after being thus irritated, the body and tentacles will 

 become elongated and expanded, and will once more resume 

 their normal condition. That the polypes are not perma- 

 nently rooted or attached to the weeds on which they are 

 commonly found, may be proved by the simple observation 

 of their habits. They may be seen to detach themselves 

 from fixed objects, and to move slowly about in leech-like 



