HYDRA, OR FRESH-WATER POLYPE. 535 



to the organ a great prehensile power ; the minute filaments form- 

 ing a rough surface adapted to prevent the object from readily 

 slipping out of the grasp of the arm, whilst the central spicule or 

 1 dart ' is projected into its substance, probably conveying into it 

 a poisonous fluid secreted by a vesicle at its base. The latter infer- 

 ence is founded upon the oft-repeated observation, that if the living 

 prey seized by the Tentacles have a body destitute of hard integu- 

 ment, as is the case with the minute Aquatic Worms which constitute 

 a large part of its aliment, this speedily dies, even if, instead of 

 being swallowed, it escapes from their grasp ; on the other hand, 

 minute Entomostracous Crustacea, Insects, and other animals with 

 hard envelopes, may escape without injury, even after having 

 been detained for some time in the Polype's embrace. The contrac- 

 tility of the Tentacles (the interior of which is traversed by a canal 

 which communicates with the cavity of the Stomach) is very re- 

 markable, especially in the Hydra fusca ; whose Arms, when ex- 

 tended in search of prey, are not less than seven or eight inches in 

 length ; whilst they are sometimes so contracted, when the stomach 

 is filled with food, as to appear only like little tubercles around its 

 entrance. By means of these instruments the Hydra is enabled to 

 derive its subsistence from animals whose activity, as compared 

 with its own slight powers of locomotion, might have been supposed 

 to remove them altogether from its reach ; for when, in its move- 

 ments through the water, a minute Worm or a Water- Flea happens 

 to touch one of the tentacles of the Polype, spread-out as these are 

 in readiness for prey, it is immediately seized by this, other arms 

 are soon coiled around it, and the unfortunate victim is speedily 

 conveyed to the Stomach, within which it may frequently be seen 

 to continue moving for some little time. Soon, however, its 

 struggles cease, and its outline is obscured by a turbid film, which 

 gradually thickens, so that at last its form is wholly lost. The soft 

 parts are soon completely dissolved, and the harder indigestible 

 portions are rejected through the Mouth. A second orifice has 

 been observed at the lower extremity of the Stomach ; but this 

 would not seem to be properly regarded as Anal, since it is not used 

 for the discharge of such exuviae ; it is probably rather to be con- 

 sidered as representing, in the Hydra, the entrance to that rami- 

 fying cavity, which, in the Compound Hydrozoa, brings into con- 

 nection the lower extremities of the stomachs of all the individual 

 Polypes (Plate xx.). A striking proof of the simplicity of the 

 structure of the Hydra, is the fact that it may be turned inside 

 out like a glove ; that which was before its external Tegument 

 becoming the lining of its Stomach, and vice versa. 



411. The ordinary mode of Multiplication in this animal is by a 

 Gemmation resembling that of Plants. Little bud-like processes 

 (Fig. 274, 6, c) are developed from its external surface, which are 

 soon observed to resemble the parent in character, possessing a 

 digestive sac, mouth, and tentacles ; for a long time, however, 



