POLYPI. 83 



cells are condensed^ and elongated in the axis of the body, so as to 

 foi'm two tegumentary layers : the internal cells are elongated trans- 

 versely to the axis of the body, and form a stratum of villi, projecting 

 into the abdominal cavity : the thick intermediate mass of nucleatt>d 

 cells seems to fulfil the ordinary functions of muscular or contractile 

 tissue. 



The hydra commonly adheres by a small prehensile disc or rudi- 

 mentary foot (yiff.4;5. d), situated at tlie extremity of the stem or 

 body opposite to the mouth. When the little animal would change 

 its position it slowly bends its body, and, fixing one or more of its 

 tentacula to the supporting surface, detaches the posterior sucker 

 approximates it to the head, and advances by a succession of these 

 leech-like motions. The hydra can make progress in water, as well as 

 on a solid plane ; when it would swim it suspends itself to the surface 

 of the water by its foot or terminal sucker, which it expands, and ex- 

 poses to the air : the disc soon dries, and in this state, repelling 

 the surrounding water, it serves as a float, from which the hydra 

 hangs with its mouth downwards, and can row itself along by means 

 of its tentacula. Its ordinary position is one of rest, adhering to 

 an aquatic plant by its terminal sucker, with the dependent oral 

 tentacula spread abroad in quest of prey. 



Should a small Niiis or Entomostracan, or any of the larger 

 Infusories, come within the reach of the little carnivorous polype, they 

 are immediately seized, pulled towards the mouth (Jiff. 45. b), and 

 swallowed. The rapidity of the digestive process is manifested by the 

 diffusion of any characteristic colour of the animalcules swallowed, 

 through the gelatinous parenchyma of the devourer ; and when this 

 process is completed, the indigestible debris of the prey are rejected 

 by the same aperture which had just gorged it. Although the in- 

 digestible parts of the food are palpably rejected by the mouth, yet 

 a careful investigator, Coi'da*, affirms the existence of an anal outlet 

 (Jig. 45. c), and figures it of small size, close to the hind sucker or foot. 

 It may give passage to certain excretions of the villous lining membrane 

 of the alimentary cavity. 



Each tentaculum in the Hydra grisea, acording to this observer, 

 is a slender membranaceous tube, filled with a fluid albuminous 

 substance mixed with oil-like particles. This substance swells out 

 at certain definite places into denser nodules, which are arranged 

 in a spiral line (Jig. 45. a, a). Each nodule is furnished with 

 an organ of touch, and another singularly constructed one for 

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



* Nova Acta Physico-Medica, &c. Bonn, vol. xviii, 18:36, tab. xvi. 

 G 2 



