HYDRA. 



[ 328 ] 



HYDRA. 



lower half suddenly attenuated; tentacles 

 several times longer than the body. 



Still waters ; rare. 



The characteristic forms of the body can 

 only be judged of when fully extended in 

 search of prey ; for when the animals are 

 touched, shaken, or in any way disturbed, 

 the body assumes very variable forms, be- 

 coming rounded, ovoid, &c. 



The structure of the body of Hydra has 

 been much investigated and discussed. By 

 some it has been regarded as consisting of 

 three layers — an internal and external coat, 

 and an intermediate muscular layer. The 

 true stmcture, however, has been pointed out 

 by Ecker. This author correctly regards the 

 animal as consisting of the substance deno- 

 minated sarcode by Dujardin, and neither 

 furnished with an outer nor an inner coat. 

 The transparent, gelatinous sarcodic sub- 

 stance forms the entne mass of the body and 

 tentacles ; on the surface it is frequently 

 irregularly rounded or nodular, or exhibits 

 spiral or other raised lines (PL 33. fig. 23 b)i 

 and within it contains numerous vacuoles. 

 If a Hydra be crushed between glasses, 

 portions of the sarcode will be separated, 

 and assume a globular form, closely resem- 

 bling that of cells ; the vacuoles will also 

 become greatly distended, just as occurs in 

 the substance of the Infusoria, and these 

 separated portions will often continue con- 

 tracting like an Amaha. Two of them are 

 represented in PI. 33. fig. 2.9 ; in a, a rather 

 small vacuole is present, whilst in h, this is 

 very large. Now in the latter instance, the 

 globule, as regards structure, forms a true 

 cell, consisting of a closed sac, with liquid 

 contents. Physiologically speaking, however, 

 it does not correspond to a cell, the entire 

 substance representing cell-contents, around 

 which a cell-wall has never been formed. A 

 number of these vacuoles exist naturally dif- 

 fused throughout the substance of the body. 

 The intermediate stratum, which is not 

 organically distinct, contains imbedded in it 

 a number of very minute green or otherwise 

 coloured granules ; these are of a rounded 

 form, and present a double outline, as if 

 composed of cells. In the uninjured Hydra, 

 they exist in the inter-vacuolar substance, 

 thus giving the tissue an elegantly reticular 

 appearance. We believe that these granides 

 consist of chlorophyll ; they are insoluble in 

 potash ; they become coloured purplish red- 

 brown by iodine and sulphuric acid, after 

 treatment with potash ; and the green 

 granules of Hydra vidgaris are rendered 



bluish-green by sulphuric acid, in the same 

 manner as the chlorophyll of leaves. The 

 colour of Hydra has been differently 

 accounted for. Laurent states that he suc- 

 ceeded in colouring them blue, white and 

 red, by feeding them with indigo, 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 

 coloui", 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 this 

 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 cu- 

 rious bodies, termed the stinging organs (PI. 

 33. fig. 23 «). These are best seen upon the 

 tentacles ; they consist of an oval, truncate, 

 firm capsule (PI. 33. fig. 22 6) of compara- 

 tively considerable thickness, as indicated by 

 its mai-ked double outline. Within the cap- 

 sule 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 extra- 

 ordinary rapidity, so that we have not been 

 able to determine exactly how the spines are 

 arranged within the capsule. Most probably 

 the spines, while within the capsule, are 

 directed forwards and in close contact, and 

 then in assuming their recui'ved 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 d. 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 is considered that these organs 

 closely resemble in structure those of the 

 Acalephse, which possess an urticating 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 they 

 secrete and contain formic acid. But as 

 many other substances reduce salts of silver, 

 and as the sarcode, from which it is perhaps 

 impossible to separate these bodies, may 

 produce this effect, the point must be con- 



