A STUDY OF LOWER LIFE. 



451 



from a quiet pool, which has become stagnant 

 and overgrown with lower plant-life and water- 

 weeds flourishing apace under the kindly inllu- 

 ences of the summer sun, and place this water 

 along with a small quantity of the weeds in a clear 

 glass vessel exposed to the light, we may be al- 

 most sure to find that in due time certain small 

 bodies of greenish color have attached themselves 

 to the sides of the vessel. These bodies will con- 

 gregate chiefly on the side of the vessel next the 

 light, and, as regards their size, the beings referred 

 to are seen to be by no means large. A length 

 of about a quarter of an inch may be regarded as 

 a fair statement of their average dimensions ; al- 

 though occasionally a specimen may greatly ex- 

 ceed the proportions of its neighbors, and exhibit 

 a length of half an inch or more. 



Examined by the aid of a hand-lens, each of 

 these little organisms or hydras is seen to pos- 

 sess a tubular or cylindrical body, which is at- 

 tached by one extremity to the glass or duck- 

 weed, and which exhibits at the opposite and 

 free extremity a mouth-opening, surrounded by a 

 circle of arms or tentacles. These latter are deli- 

 cate, thread-like organs, which in the undisturbed 

 and natural state of the animal remain out- 

 stretched in the water. In the common or green 

 hydra, the tentacles are not disproportionately 

 developed as regards the body, but in certain 

 other forms or species, in which the body is col- 

 ored brown {Hydra fused), the tentacles are very 

 long, and the animal obtains in consequence the 

 distinctive name of the " Long-armed Hydra." 



The observation of the common incidents of 

 the hydra's life reveals certain interesting feat- 

 ures, which assist us in some degree in the appre- 

 ciation of the nature and structure of these or- 

 ganisms. 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 impressions, a feat- 

 ure in its history which is of high interest when 

 we endeavor to understand the nature and rela- 

 tions of the nervous system of higher animals ; 

 and that these animals are also sensitive to more 

 delicate impressions is proved by their clustering 

 in numbers on that side of the glass vessel which 

 is next the light. If the hydra is left in an un- 

 disturbed condition after being irritated, the 

 body and tentacles will become elongated and ex- 

 panded, and will once more resume their normal 

 condition. 



That the polyps are not permanently rooted 

 or attached to the weeds in which thev are com- 



monly found, may be proved by the simple ob- 

 servation of their habits. They may be seen to 

 detach themselves from fixed objects, and to 

 move slowly about in leech-like fashion, or like 

 the looping caterpillars, by alternately fixing and 

 extending the mouth and root-extremity of the 

 body; while occasionally they may be seen to 

 float listlessly, with extended tentacles, amid their 

 native waters. 



When any minute animal, such as a water- 

 flea, or some similar organism, comes in contact 

 with the tentacles of the hydra, an interesting 

 series of acts is witnessed. The tentacles are 

 then observed to act as organs for the capture of 

 prey, the victim being seized and conveyed by 

 their contraction toward the mouth of the animal, 

 within which cavity it finally disappears from 

 view. That the hydra, therefore, possesses in- 

 stincts common to all forms of animal life, high 

 and low alike, and which lead it to supply the 

 wants of its frame, cannot be doubted ; and 

 Schiller's maxim that hunger is one of the pow- 

 ers that rule the universe, may thus be aptly il- 

 lustrated within the small domain and in the sim- 

 ple life-history of the hydra. 



As might be expected, the prey at first strug- 

 gles violently to escape from the clutches of its 

 captor, but after a short period the efforts become 

 less and less marked, and the captive may be 

 noted to become somewhat suddenly helpless and 

 paralyzed. The observation of these details 

 leads us to expect that the hydra possesses some 

 offensive apparatus, through the action of which 

 the capture of prey is facilitated. And an ex- 

 amination, by aid of the microscope, of the tenta- 

 cles of the polyp, and in fact of its body-sub- 

 stance as well, would reveal the presence of nu- 

 merous minute capsules, named " thread-cells," 

 which are developed in the tissues of the body. 

 Each of these curious little cells consists of a 

 tough outer membrane, within which a delicate 

 thread or filament lies coiled up amid fluid. 

 When one of these structures is irritated, as by 

 pressure, the cell is observed to rupture, the 

 thread being thrown out or everted, while the 

 fluid at the same time escapes. A thread-cell of 

 the hydra, in its ruptured condition, appears as 

 an oval capsule having attached to one extremity 

 the thread, which is provided at its base with 

 three little spines or hooks. The consideration 

 of the structure and functions of these thread- 

 cells clearly indicates their offensive nature. 

 Each may in fact be regarded as representing a 

 miniature poison-apparatus; the "thread" being 

 the dart or sting, and the fluid constituting the 



