of 



339 



water, where numbers are often seen clus- 

 tering upon aquatic plants, &c. These ani- 

 mals present us with the simplest kind of 

 structure which has yet been ascertained. 

 The Hydra consists simply of a fleshy tube, 

 open at both extremities, and the aperture 

 of the tube serving as a mouth, which is 

 situate in the more dilated end, and this 

 mouth is provided at its margin with a 

 single row of tentacula, or loug flexible 

 arms, which diverge from each other like 

 the spokes of a wheel. Looking to this ani- 

 mal, we may suppose that nature has formed 

 it to prove that animal life may be carried 



HYDRA FUSCA. 



on without the aid of the complicated ma- 

 chinery which she has given to the higher 

 orders of creation. The Hydra viridis, or 

 Green Polype, has the power of fixing itself 

 in an erect position by the foot, and if it 

 wishes to change place, it slowly bends till 

 its head touches the plane on which it is 

 moving, and adheres to it by the mouth, or 

 one or two of its tentacula ; the foot is then 

 detached, and by a curve of the body placed 

 close to the head, where it is again fixed, 

 preparatory to a new step, which it performs 

 by a repetition of the same movements. 

 When in search of prey, the Hydra permits 

 its arms to float loosely through the water ; 

 by which means it succeeds in obtaining a 

 supply of food ; for if, in their active course, 

 any of the minute Crustacea and aquatic 

 worms should but touch one of the tentacula, 

 it is immediately seized, other arms are soon 

 coiled round it, and the unfortunate victim 

 is speedily conveyed to the mouth. 



With regard to the powers of reproduction 

 possessed by these simple animals, it is to be 

 observed that, when mature and well sup- 

 plied with food, minute gemmules or buds 

 are seen to become developed from the com- 

 mon substance of the body : these gemmaB 

 appear at first like delicate gelatinous tu- 

 bercles upon the exterior of the parent 

 polype ; but, as they increase in size, they 

 gradually assume a similar form, and be- 

 come perforated at their unattached ex- 

 tremity. During the first period of the for- 

 mation of these sprouts, they are evidently 



continuous with the general substance from 

 which they arise ; but, at length, when the 

 young is fully formed and ripe for indepen- 

 dent existence, the point of union between 

 the two becomes more and more slender, 

 until a slight effort on the part of either is 

 sufficient to detach them, and the process is 

 completed. 



But among the many remarkable features 

 in the history of the Hydra, that which ap- 

 pears the most so, is its capability of repro- 

 ducing the whole structure from separate 

 portions of it. New tentacula will replace 

 any which have been accidentally lost or 

 removed. If the body is divided transversely, 

 each segment will become a new animal ; 

 the upper one closing the aperture at its 

 base, and the lower one speedily developing 

 tentacula around the newly-formed mouth. 

 If divided longitudinally, each half will, in 

 a very short space of time, begin to ply its 

 tentacula ; nay, if cut transversely into 

 several segments, each will in time become 

 a perfect animal. Trembly was the fir=t 

 man who gave a detailed account of this 

 curious polype. [See POLYPI.] 



HYDRACHNA. A genus of aquatic in- 

 sects closley allied to the Acaridce [which 

 see]. One of the largest and by far the most 

 elegant of the genus is the Hydrachna geo- 

 graphica, so named from the fancied map- 

 like distribution of its variegations. Its 

 shape is globular, and its colour a polished 

 black, ornamented with red spots, which in 

 a certain light have a kind of gilded lustre. 

 The legs of this insect, as in the rest of the 

 genus, are hairy ; it swims with great ce- 

 lerity, and appears in almost constant mo- 

 tion. The eggs of the Hydrachna are small 

 and round : the young, when first excluded, 

 are furnished with six legs only ; but they 

 acquire two more legs after the first or 

 second change of their skin. 



IIYDROBRANCHIATA. The first sec- 

 tion of the order Gasteropoda, containing 

 Mollusca which breathe water only. 



HYDROCANTHARI, or WATER 

 BEETLES. The name of a great group of 

 Coleoptera, containing Dytiscus, C'olymbetes, 

 and numerous other genera. 



HYDROCHCERUS. [See CAPYBARA.] 

 HYDROMETRID^E. A family of in- 

 sects, some species of which may be met 

 with in almost every pond or stream, skim- 

 ming along the surface, and turning about 

 With the greatest rapidity. The body is 

 boat-shaped, the hind feet servjng as a 

 rudder, while the two middle feet brush 

 along the surface of the water, and give the 

 required motion : the under side of the body 

 is clothed with a thick coating of fine hairs, 

 evidently intended to prevent the insect 

 from coming in contact with the water. 



HYDROPHILUS. A remarkable genus 

 of aquatic insects, differing from that of Dy- 

 tiscus only in the structure of the antennae, 

 which, instead of being setaceous, are short, 

 and furnished with a clavated and perfo- 

 liated tip or knob. One large species, com- 

 mon in our ponds and ditches, is an inch 



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