HYDROZOA 



865 



The latter inference is founded upon the oft-repeated observation 

 that if the living prey seized by the tentacles have a body destitute 

 of hard integument, as is the case with the minute aquatic worms 

 which constitute a large part of its aliment, this speedily dies, 

 even though, instead of being swallowed, it escapes from their grasp ; 

 whilst, on the other hand, minute Entomostraca, insects, and other 

 animals or ova, with hard envelopes, may escape without injury, even 

 after having been detained for some time in the polype's embrace. 

 The contractility of the 

 tentacles (the interior 

 of which is traversed by 

 a canal that communi- 

 cates with the cavity 

 of the stomach) is very 

 remarkable, especially 

 in the Hydra fttsca, 

 whose arms, when ex- 

 tended in search of 

 prey, are not less than 

 seven or eight inches in 

 length ; whilst they are 

 sometimes so contract- 

 ed, when the stomach 

 is filled with food, as 

 to appear only like little 

 tubercles around its en- 

 trance. By means of 

 these instruments the 

 Hydra is enabled to 

 draw its support from 

 animals whose activity, 



as compared with its 



i i .L f 



own slight powers ot 



locomotion, might have 

 been supposed to re- 

 move them altogether 

 from its reach ; for 

 when, in its movements 

 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. 80011, 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 dis- 

 solved, and the harder indigestible portions are rejected through the 

 mouth. A second orifice has been observed at the lower extremity 



3 K 



FIG. 659. Ga/nvpanulafia gelatinosit. 



