Hydra. Z. HYDROIDA. 99 



The Hydrae are found in fresh and, perhaps, also in salt waters, but 

 the former species only have been examined with care, and are the 

 objects of the following remarks. They prefer slowly running- or al- 

 most still water, and fasten to the leaves and stalks of submerged 

 plants by their base, which seems to act as a sucker. 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 time than at another, the tentacula suifering changes in 

 their length and diameter equal to those of the body. " It can lengthen 

 out or shorten its arms, without extending or contracting its body ; 

 and can do the same by the body, without altering the length of its 

 arms : both, however, are usually moved together, at the same time 

 and in the same direction." — The whole creature is apparently homo- 

 geneous, composed of minute pellucid grains cohering by means of a 

 transparent jelly, for even with a high magnifier no defined organiza- 

 tion of vessels and fibres can be detected. On the point opposite the 

 base, and in the centre of the tentacula, we observe an aperture or 

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

 midst of the jelly,* and from which a narrow canal is continued down 

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

 body appears smooth and even, but " in its middle degree of exten- 

 sion," the sides seem to be minutely crenulated, an efi'ect probably of 

 a wrinkling of the surface, although from this appearance Baker has 

 concluded that the Hydra is annulose, or made up of a numl)er of 

 rings capable of being folded together or evolved, and hence, in some 

 measure, its extraordinary ability of extending and contracting its 

 parts.-j- That this view of the Hydra's structure is erroneous, Trem- 

 bley has proved ;J and the explanation it afforded of the animal's con- 

 tractility was obviously unsatisfactory, for it was never pretended that 



Pallas denies this. " Ab alimento recepto cavata, inquam, baud enim Hy- 

 dra corpus naturaliter intestini instar cavum crediderim. Totum solidum et 

 medullare, pro admoto alimento, cerae instar, digitum admittentis, cavari concipio 

 parenchyma et alimentis insinuatis sese circumfundere. Qui alias per longi- 

 tiidinem dissecta Hydra, illico qualibet portione deglutire, et cavo clauso alimen- 

 ta condere posset ? quod tamen observai'e rarum non est." Elench. Zooph. 27, 

 28. — For a view of the Hydra's stomach see Tremb. Mem. pi. 4, fig. 7, copied 

 by Roget in his Bridgew. Treat, ii. 74, fig. 241. 



-|- " The outward coat is white like the arms, and made up of minute annuli 

 or ringlets, that double in the midst, and can, occasionally, be folded close to- 

 gether, in the manner of a paper lanthorn." — Hist, of the Polype, 25. 



\ Mem. 27. 



