HYDNOGLCEA. 



[ 397 ] 



HYDRA. 



Eyes absent, 



"No teeth Eiiferoplea. 



Teeth present. 



Jaws with numerous teeth Hudafina. 



„ with a single tooth Pleuroirocha. 



Eyes present, 

 Eye single, 



Eye frontal Furcularia. 



„ cervieal. 



Foot styliforra Monocerca. 



„ forked. 



Frontal cilia, no hooks nor 



styles Notommata. 



Frontal cilia, styles present... St/ncha-ta. 



„ hooks Smiridium. 



Foot absent ; with cirrhi or fins. Fulijurthra. 

 Eyes two. 

 Eyes frontal. 



Foot forkeil Diglena. 



„ styliform. 



With cirrhi Triarthra, 



Without cirrhi Eiiffidu.^. 



Eyes cervical Dislemiiia. 



Eyes three. 

 Eyes not stalked, 



Eyes cervical TriopMhalmus. 



Two eyes frontal, one cervical . Eosphora. 

 Two frontal eyes stalked, one cer- 

 vical not stalked Otoglena. 



Eyes more than three in a sinp;le 



group Cycloglena. 



Eyes more than three in two groups Theorus. 



BiBL. Ehrenberg-, Infus. p. 410. 



HYDNOCtLCE'A, Berk. & Ourr.— A 

 genus of Tremelloid Fungi. H. c/elatinosa 

 occurs in this country occasionally on very 

 rotten pine wood. (Berk. & Br. Ann. N, 

 H. 1871, vol. vii. 429.) 



HY'BRA, Linn. (Freshwater Polype). — • 

 A genus of Hydroid Zoophytes, of the family 

 Hydridte. 



Char. Locomotive, single, naked, gelati- 

 nous, subcylindrical, but very contractile 

 and variable in form ; the mouth surrounded 

 by a single row of filiform tentacles. Pro- 

 pagation by the formation of gemmce, and 

 ova upon or within the substance of the 

 body of the animal. 



Hydra viridis (PL 41. fig. 21, adhering to 

 the radicles of duck-weed {Lemna)). Body 

 leaf-green, cylindrical or insen.-ibly narrowed 

 towards the base ; tentacles 6 to 10, shorter 

 than the body, naiTowest at their origin. 



Common in ponds and still waters. 



H. vidyaris. Body orange-brown, yel- 

 lowish or red, cylindrical ; tentacles 7 to 12, 

 as long as or longer than the body, tapering 

 to the free ends. 



In weedy ponds and slowly running 

 waters ; common. 



H. attenuata. Body pale olive-green, 

 attenuated below ; tentacles pale, consider- 

 ably longer than the body. In ponds ; rare. 



H. fusca (oligaetis). Body brown or 

 greyish, lower half suddenly attenuated ; 

 tentacles 6 to 8, 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 wlien the animals are 

 touched, shaken, or in any way disturbed, 

 the body assumes very variable forms, 

 becoming 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 structure, however, has been pointed out 

 by Ecker. This author regards the animal 

 as consisting of sarcode or protoplasm, and 

 neither furnished with an outer nor an inner 

 coat. The transparent, gelatinous, sarcodic 

 substance forms the entire mass of the body 

 and tentacles ; on the surface it is frequently 

 irregularly rounded or nodular, or exhibits 

 spiral or other raised lines (PI. 41. fig 23 6) ; 

 and it contains numerous vacuoles within. 

 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 Amoeha. Two of them are 

 represented in PI. 41. fig. 29 ; 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, how- 

 ever, it does not correspond to a cell, the 

 entire substance representing cell-contents 

 aroimd which a cell-wall has never been 

 formed. A number of these vacuoles exist 

 naturally, diffused throughout the substance 

 of the body. The protoplasts of the inner 

 surfiice are ciliated, and solid particles are 

 ingfestedbv them as in Amoeba. The inter- 

 mediate stratum, whicli 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 intervacuolar substance, thus giving 

 the tissue an elegantly reticular appearance. 

 They appear to consist of chlorophyll ; they 

 are insoluble in potash ; they become 

 coloured purplish red-brown by iodine and 

 sulphuric acid, after ti'eatment with potash ; 

 and the green-granules of Hydra vulyaris 

 are rendered bluish green by sulpliuric 

 acid, in the same manner as the chlorophyll 



