POLYPIFERA. 



21 



fluid ; and, to an ordinary observer, no fibres completely acrite ; nevertheless it is evidently 

 of any kind are distinguishable in any part of able to appreciate the presence or absence of 



light ; for if a number of these little animals 



Fig. 25. 



Hydras vlrides in different 'stages of extension and 

 contraction, reproducing gemmiparously, attached to 

 the roots of duck-weed. (From Roesel.') 



its body : nevertheless it is highly contractile, 

 shrinking, when disturbed, into an almost in- 

 visible jelly-like speck, and again slowly ex- 

 panding itself when left quiet. Incapable as 

 such a creature would appear to be of any 

 active exertion, this little gelatinous bag is 



soon found to be gifted with a capability of animals that crowd the water in its vicinity 

 locomotion, which is exercised in the fol- impinge against its outspread lines, when im- 



are confined in a glass vessel, one side of 

 which is exposed to light while the other is 

 kept in the shade, they are always found to 

 congregate on the illuminated side, and by 

 turning the glass round it will be found that 

 by changing their position, they will endea- 

 vour to regain a situation ex'posed to the 

 solar influences : seeing, therefore, that they 

 are absolutely destitute of eyes, it would seem 

 that they perceive light by the sense of touch 

 alone. 



It might naturally be supposed that a crea- 

 ture so low in the scale of organization would 

 be compelled to subsist upon the simplest pos- 

 sible aliment, yet, strange to say, this little 

 polype is carnivorous in its propensities, and is 

 moreover gifted with such terrible powers of 

 destruction, that animals far larger, stronger, 

 and more active than itself fall a prey to^its 

 voracity : the Entomostracous Crustaceans, 

 the larva? of insects, and minute Annelidans, 

 constitute its ordinary diet, and vainly endea- 

 vour to escape from its clutches. Whilst 

 watching for prey, the Hydra remains per- 

 fectly at rest, suspended by its tail, and keep- 

 ing its oral tentacula widely spread out in 

 different directions, nor has it generally to 

 wait long before some of the multitudinous 



is exercised in 



lowing manner : whilst attached to the side of 

 the glass by the sucker at its closed extre- 

 mity, which forms a minute foot, something 

 like that of a Gasteropod Mollusk, the little 

 polype gradually inflects its body until some 

 of the tentacula around its mouth are brought 

 in contact with the supporting surface, of 

 which they take a firm hold ; in this position 

 it detaches its posterior sucker, and, advanc- 

 ing it towards its head, again fixes it, and 

 thus progresses, after the manner of a leech, 

 by a repetition of the same manoeuvres. It 

 may, however, be easily imagined that, owing 

 to the minute size of the Hydra, and the ex- 

 treme slowness of its contraction, this mode 

 of progression is by no means remarkable for 

 its speed, and in fact a march of an inch will 

 occupy it many hours in its performance ; 

 accordingly the polype has been endowed with 

 another mode of transit, of which it can avail 

 itself at pleasure. Although its body is spe- 

 cifically heavier than water, so that when de- 

 tached from its hold it sinks helplessly to the 

 bottom, it is able, when occasion requires, to 

 row itself about in a very ingenious manner. 

 In order to accomplish this feat, it first creeps 

 to the top of the water, and protruding its 

 sucker to a little distance above the surface, 

 hollows it out into a saucer-like cavity, the 

 buoyancy of which is sufficient .to keep it 

 afloat ; and then, supported by this curiously- 

 contrived boat, the little Hydra rows itself 

 about by means of its tentacula in whatever 

 direction it chooses. No traces of nervous 

 matter are perceptible in the composition of 

 the Hydra, which, in its whole structure, is 



mediately, as if the wand of an enchanter had 

 been laid upon it, the career of its victim is 

 arrested; though apparently only touched, 

 not seized, it immediately sinks motionless as 

 though paralysed by the contact, and only 

 after some time recovers its former vivacity. 

 What is the benumbing power possessed by 

 the tentacula of the Hydra it is difficult to 

 conjecture; some writers attribute it to a tor- 

 pifying secretion ; others surmise the dis- 

 charge of an electric shock ; but whatever be 

 its nature, its effects are sufficiently potent 

 to prevent the escape of the animal subjected 

 to its influences. 



No sooner is the prey thus stricken motion- 

 less than the tentacle against which it im- 

 pinged begins slowly to contract and drag it 

 towards the orifice of the mouth. It would 

 seem that the slightest effort on the part of 

 the animal seized would be sufficient to tear 

 off the almost invisible gelatinous arm of the 

 polype, yet not more surely does the angler 

 land a trout by means of his silken line, than 

 the Hydra succeeds by its tenacious hold in 

 securing its victim ; tentacle after tentacle is 

 brought to bear upon it, and slowly it is ap- 

 proximated to the opening of the stomach of 

 the polype in which it is about to be engulphed. 

 When lodged in the stomach of its de- 

 vourer, so thin and diaphanous is the distended 

 bag of the Hydra's body, that the animal swal- 

 lowed is still distinctly visible, and the micro- 

 scopical observer would scarcely suspect that 

 the pellucid film which covers it was capab'e 

 of producing much effect upon its substance. 

 Gradually, however, the swallowed prey begins 



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