HYDRA. 



[ 398 ]■ 



HYDEA. 



of leaves. The colour of Hydra has been 

 ditferently accounted for. Laurent states 

 tliat he succeeded in colouring them blue, 

 ■white, and red, by feeding them with in- 

 digo, chalk, and carmine, — whilst Hancock 

 has shown that the colouring is much 

 affected by exposure to light — those not 

 exposed to light, from living under stones, 

 &c., having the natural colour, whilst those 

 exposed to the light became bleached. It 

 is generally admitted, however, that the 

 colour depends upon or is modified by the 

 nature of the food ; but exact experiments 

 are wanting to decide the question. Towards 

 the inner surface of the body, the granules 

 are brownish or blackish. 



Imbedded in the superficial portions of the 

 substance of the Hydra are certain curious 

 bodies, termed the stinging organs or nema- 

 tocysts (PI. 41. fig. 23 rt). These are best 

 seen upon the tentacles ; they consist of 

 an oval, truncate, firm capsule (PI. 41. 

 fig. 22 h) of comparatively considerable 

 thickness, as indicated by its marked double 

 outline. Within the capsule is contained 

 a very long and slender filament, at the 

 base of which are four minute spines. In 

 the undisturbed state of the Hydra, the 

 filament with the spines is coiled up in the 

 capsule (fig. '22 a); but when the animal is 

 touched, pressed, or heated, the filament 

 with the spines flies out with extraordinary 

 rapidity, so that we have not been able to 

 determine exactly how the spines are ar- 

 ranged within the capsule. Most probably 

 the spines, while within the capsule, are 

 directed forwards and in close contact, and 

 then, in assuming their recurved position, 

 they are the means of projecting the filament 

 forwards. A capsule, containing an unex- 

 panded filament and spines enclosed within 

 a detached globule of sarcode, is represented 

 in fig. 22 (/. When these capsules are heated 

 with a solution of nitrate of silver, a portion 

 of the silver is reduced to the metallic state. 

 This action is a property of formic acid ; 

 hence,when it isconsidered that these organs 

 closely resemble in structure those of the 

 Acalephre,which possess an urticatiug power 

 like stinging-nettles,- arising from the pre- 

 sence of formic acid, and that in Hydra 

 these filaments are driven into and wound 

 the prey, it may appear probable that tlu'V 

 secrete and contain formic acid. IJut, as 

 many other su1)stances reduce salts of silver, 

 and as the sarcode, from which it is perhaps 

 impossible to separate these bodies, may 

 produce this eflect, the point must be con- 



sidered doubtful and requiring further in- 

 vestigation. In addition to these stinging 

 organs, we have found other very minute 

 capsules (fig. 22 c), containing a filament, 

 curved even when emitted, the nature of 

 which is obscure. 



A third kind of organ is said to have been 

 met with also in the sm-face of the body, 

 consisting of ovate capsules or bodies, from 

 which a stout and short filament projects; 

 these appear to resemble the organs of ad- 

 hesion of the Acalephpe. 



The body and tentacles of Hydra are hol- 

 low. The prey, which consists of Entomos- 

 traca, small Annulata &c., when caught by 

 one or more of the tentacles extended for 

 the purpose, is slowly brought to the mouth, 

 and forced into the cavity of the body, in 

 which it is digested, certain portions he'mg 

 taken up by the amoeboid bodies ; the un- 

 digested portions are evacuated through the 

 mouth. The posterior part of the body is 

 more or less dilated into a flattened disk, 

 which, by its suctorial power, enables the 

 animal to attach itself to various bodies ; 

 Hancock has seen excrementitious matter 

 passing through the body at this part. The 

 cavities of the tentacles sometimes exhibit 

 a kind of circulation ; and those of both the 

 tentacles and the body are lined with cilia. 



The extraordinary power which Hydra 

 possesses of reproducing lost parts is truly 

 wonderful. Thus, if the body be cut into 

 two or more, even into forty parts, each 

 portion continues to live, and develops a 

 perfect new animal. If the section be made 

 lengthwise, so as to divide the body all but 

 the end, the two portions become resoldered 

 and form a perfect being ; if the pieces be 

 kept asunder, each becomes a Hydra, the 

 two possessing but one posterior end ; if the 

 section be made from the tail towards the 

 head, the two bodies will be perfected and 

 remain attached to the one head. If a 

 tentacle be cut off, a new animal is formed 

 from it. When one end of the body of a 

 Hydra is introduced into the body of 

 another, the two unite and form one. The 

 head cut off one may be engrafted upon 

 the body of another which wants one. And 

 when tiie body is turned inside out, the 

 outer surface, which has thus become the 

 inner, will perform the ordinary digestive 

 functions, and the auiiual will continue to live, 



Tlie ordinary mode of reproduction of 

 Hydra is by gemmation : a minute swelling 

 forms upon some part of the surface of the 

 body ; this enlarges, and gi-aduaUy assumes 



