149 



HYALITE. 



HYDRA. 



150 





aperture shut by a diaphragm which is convex externally but not 

 terminal, b-jing surpassed by the walls of the cylinder. 



M. Rang remarks that he established this curious genus on a 

 species equally common in the Indian Sea, tha ocean, and the 

 South Sea. 



Etiribia (Rang). Animal furnished with two horizontal fins, at 

 the base of whicli is the mouth ; the intermediate lobe is very small 

 and of a triangular form ; body globular, short ; gills and organs of 

 generation unknown. 



Shell cartilagino-inembranous, delicate, transparent, regular, and in 

 form of a reversed cap (calotte). 



M. Rang states that there is only a single species which he has not 

 been able to observe sufficiently, but which presented well-defined 

 generic characters. 



Piyi'he (Rang). Shell absent. Animal enveloped in a membranous 

 mantle, furnished with two rather long fins, but which do not appear 

 united on the ventral side by an intermediate lobe ; branchue over- 

 spreading the fins. 



M. Rang established this genus on a species from the seas of New- 

 foundland (Terre-Neuve) ; he adds that Mr. R-jyoaud brought back 

 from his Indian voyage some drawings of Pteropods which appear to 

 be rrferrible to it. 



M. Deahayes says that he is led to reject many genera proposed 

 norne years since by M. Rang, in the ' Annales des Sciences Naturelles,' 

 it* well 11.1 in his ' Manuel,' under the name of Grai and Cuvieria. 

 M. Deshayeg observes that M. Rang has comprehended under his sub- 

 genua Crait a living shell named tiadiis by Montagu, and some other 

 fossils placed by Lamarck in the genus Dentalium. [DfiNTAUCM.] 

 Although M. Deshayes is as yet uncertain as to these species, he 

 a loptg tha opinion of Lamarck as preferable, because, he says, M. 

 Rang has contested it only on the supposition that they had been 

 defined after the mutilation of their posterior extremity, which, being 

 naturally short, only presented itself as open by accident. This view 

 taken by M. Rang, he continues, is not founded on any good observa- 

 tion, and he says that he has seen a sufficiently Urge number of indi- 

 viduals perfectly preserved, to be able to affirm that their posterior 

 extremity was open when the animal was alive. These shells then, he 

 states in conclusion, do not belong to the Pteropods, and are more pro- 

 lubly Dentalia. This opinion is a very strong one, and, coming from the 

 quarter it does, is deserving of all respect: but as M. Rang has justly 

 tlie reputation of a good observer in this department of natural 

 history, we have thought it right to lay before the reader the descrip- 

 tions and figures given by him. 



f'oiM Jlyalaidas. 



M. Ran?, a? we have seen, mentions one fossil species of Hyalcca, 

 and Mr. O. B. Sowerby states that the genus occurs in a fossil state in 

 hicily. M. Rang notices the fossil analogue of Cleodora from Pied- 

 mont ; if Vayinclla and Gadtix are to be considered as belonging to 

 this family, they must be added. The last-mentioned author says 

 that he has detected a fossil species of Cuvieria in the shell sand of 

 Piedmont, where it had been collected by the elder De Luc. M. 

 JJeshayes, in his tables, enumerates two fossil (tertiary) of Hyalcea 

 and three of Cleodora,' tertiary also ; of the latter he records Cleodora, 

 laneeolala as a species found both living and fossil (tertiary). 



HYALITE, a Mineral, a variety of Opal. [SlUCA.] 



HYAL03IDERITE, a synonym of the Mineral Chrysolite. 

 [CHRYSOLITE.] 



H YAS, a genus of Brachyurous Crustaceans belonging to the Mala 

 family. [MAIAD.B.] 



H Y 'BOD US, a genus of Fossil Fishes placed in the order of Placoi- 

 dians by M. Agassiz (' Recherches sur les Poissous Fossiles," voL iii., 

 tab. 8, 9, 10). The information which M. Agassiz has collected con- 

 cerning this extinct group of fishes appears to be considerable, yet 

 only in a few instances (from the Lias of Lyme Regis and Bristol) has 

 he been able to reconstruct the whole skeleton. In consequence, the 

 spinoiH r.iys and the teeth of one species may be, and probably are, 

 described under different specific names. The species of jfjybodits are 

 supposed to amount to 22, and extend from the New Red-Sandstone 

 (Gre Bigarre) to the Chalk inclusive. They present analogies to the 

 genu.i Xrju'tlut of Linnaeus, in the teeth and spinous rays ; it appears 

 that there were two dorsal fins, each having spinous rays, not differing 

 more tliau in recent species of fishes with two spinous dorsal fins. 



HYBRID. This term is generally applied in natural history to the 

 I >p i luce of two organic beings supposed to belong to different species. 

 It i not in all cases that animals or plants of different species will 

 breed together, and where offspring occurs it is generally regarded as 

 indicative of a closer relationship or affinity between the species, 

 jciiliarity of all hybrids is their inability to continue the charac- 

 ters of both parents. Although they are known amongst birds and 

 higher animals to produce fertile offspring, yet in the course of time 

 the tendency to the character of the original species of one or other 

 of the first parents is so strong, that the hybrid character is lost. 

 Amongst plants the same law holds good. The knowledge of the 

 fact that the pollen-cells of one species of plant will produce embryoes 

 Witt the ovules of another in often used ia horticulture for the purpose 



* Including, probably, C'rttii and rayinula. 



of producing new forms of plants. [HYBRIDISATION, in ARTS AND 

 Sc. Div. ; REPRODUCTION IN PLANTS ; REPRODUCTION IN ANIMALS.] 



HYBRID PLANTS. [SEXES OF PLANTS.] 



HYDATICA, a genus of Fossil Plants (probably aquatic) from the 

 Coal Formation. (Artis.) 



HYDATID3. [ENTOZOA.] 



HYDNOCARPUS. [FLACOURTIACE*.] 



HYDNO'PORA (Fischer), a genus of Polypiaria, nearly synonymous 

 with Monticularia of Lamarck. Goldfuss rauks some of the species 

 under his somewhat indefinite group of Astrcea,. 



HYDNORA. [CYTINACE.E.] 



HYDRA (Linuicus), a genus of Polypiferous Animals, including 

 the Fresh- Water Hydra, or Polype. It has the following technical 

 definition : -Polypes locomotive, single, naked, gelatinous, subcylin- 

 drical, but very contractile aud mutable in form ; the mouth encircled 

 with a single series of granulous filiform teutacula. 



As of all the forms of polypiferous animals the Hydra is the most 

 interesting, we give an abstract of their history, from Dr. Johnston's 

 ' British Zoophytes :' 



Leeuweuhoek discovered the Hydra in 1 703, and the uncommon 

 way its young are produced ; and an anonymous correspondent of the 

 Royal Society made the same discovery in England about the same 

 time ; but it excited no particular notice until Trembley made known 

 its wonderful properties about the year 1744. These were so con- 

 trary to established experience, and so foreign to every preconceived 

 notion of animal life, that by many they were regarded as impossible 

 fancies. Leading meu of our learned societies were daily experiment- 

 ing on the creature, and transporting it by careful posts from one to 

 another, while even ambassadors were forwarding to their respective 

 courts early intelligence of the engrossing theme. The Hydrce are 

 found in fresh waters only. They prefer slowly-running or almost 

 still water, aud adhere to the leaves aud stalks of submerged plants. 

 The body is exceedingly contractile, and hence liable to many changes 

 of form ; when contracted it is like a tubercle, a minute top, or button, 

 and when extended it becomes a narrow cylinder, being ten or twelve 

 times longer at one period than another, the tentacula changing iu 

 size and form with the body. On the point opposite the base, and 

 in the centre of the teutacula, we observe an aperture, or mouth, 

 which leads into a wider cavity, excavated as it were in the middle 

 of its body, aud from which a narrow canal is continued down to the 

 sucker. When contracted, and also when fully extended, the surface 

 appears smooth and even; but in ' its middle degree of extension' 

 the sides seem to be minutely creuulated, an effect probably of a 

 wrinkling of the skin. The tentacula encircle the mouth and radiate 

 in a star-like fashion ; but they seem to originate a little under the 

 lip, for the mouth is often protruded like a kind of small snout ; they 

 are cylindrical, linear, or very slightly tapered, hollow, and roughened, 

 at short and regular intervals, with whorls of tubercles, which under 

 the microscope form a very beautiful and interesting object. 



Each teutaculum forms a slender membranaceous tube, filled with 

 an albuminous nearly fluid substance, intermixed with some oleaginous 

 particles; aud at certain defiuite places this substance swells out into 

 tubercles or denser wartlike nodules, which are arranged in a spiral 

 line. Every nodule is furnished with several spinigerous vesicles, 

 used as organs of touch, and with a very singularly constructed organ 

 for catching the prey. The organ of touch consists of a fine sac, 

 inclosing another with thicker parietes, and within this there is a 

 small cavity. From the point where the two sacs coalesce above there 

 projects a long ciliutn, or capillary spine, which is non-retractile and 

 apparently immovable. Surrounded by these cilia, aud in the centre 

 of the nodule, is placed the captor organ, called the ' hasta ;' this 

 consists of an obovate transparent sac, immersed in the nodule, with 

 a small nperture even with the surface. At the bottom of the sac, and 

 within it, there is a saucer-like vesicle, on whose upper depressed 

 surface is seated a solid ovate corpuscle, that gives origin to, and termi- 

 nates iu, a calcareous sharp sagitta, or arrow, that can be pushed out at 

 pleasure, or withdrawn, till its point is brought within the sac. When 

 the Hydra wishes to seize an animal, the sagittaj are protruded, by 

 which means the surface of the teutacula is roughened, and the prey 

 more easily retained ; and Corda believes that a poison is at the same 

 time injected a conjecture offered to explain the remarkable fact of 

 the almost instant death of the prey. The nodules of the tentacula 

 are connected together by means of four muscular fibres, or bauds, 

 which run up, forming lozenge-shaped spaces by their intersections. 

 These are the extensor muscles of the tentaculum. They are again 

 joined together by transverse fibres, which Corda believes to be 

 adductor muscles, aud to have also the power of shortening the ten- 

 tacula. But it may be doubted whether this muscular apparatus is 

 of itself sufficient to effect the wonderful extensibility of these organs 

 from a line, or, as iu 2J. fusca, to upwards of eight inches ; and to 

 produce this degree of elongation, it seems necessary to have super- 

 added the propulsive agency of a fluid. Water flows, let us sav by 

 suction, into the stomach through the oral aperturo, whence it is 

 forced by the vis-a-tergo, or drawn by capillary attraction, into tho 

 canals of the teutacula, aud its current outwards is sufficient to push 

 before it the soft yielding material of which they are composed, until 

 at last the resistance of tho living parts suffices to arrest the tiny 

 11 j.jd, or the tube has become too hue iu its bore for the admission of 



