POLYPI. 121 



axis of tlie body, so as to form two tegumentary layers : the internal 

 cells are elongated transversely to the axis of the body, and form a 

 stratum of villi, projecting into the abdominal cavity : the thick 

 intermediate mass of nucleated 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 {^fig. 60, d\ situated at the 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 foot, 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 terminal foot, which it expands, and exposes to the 

 air : the disc soon dies, 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 foot, with the dependent oral tentacula spread abroad in quest 

 of prey : these are of great length and tenuity in the Hydra fusca. 



Should a small nais or entromostracan, or any of the larger infu- 

 sories, come within the reach of the little carnivorous polype, they are 

 immediately seized, pulled towards the mouth {fig. 60, h\ and swal- 

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

 diffusion of any characteristic colour of the animalcules swallowed 

 through the gelatinous parenchyme 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. The inner surface 

 of the digestive bag is lined by a ciliated epithelium, and has many 

 peculiar cells containing a clear fluid with brown pigment granules : 

 these are very conspicuous in the Hydra viridis ; the contents are 

 probably discharged into the stomach by the bursting of the cells, 

 which have been conjectured to perform a function akin to that of a 

 liver. A careful investigator, Corda *, afiirms the existence of an 

 anal outlet {fig. 60, c), and figures it of small size, close to the hind 

 sucker or foot ; and Baker f several times saw " the dung of the po- 

 lype in little round pellets discharged at this outlet or anus." Mr. 

 Hancock J, also, observing a Hydra viridis in a highly contracted 

 state and about to discharge an egg, saw a narrow channel passing 

 from the digestive cavity through the substance of the foot, appa- 

 rently about its centre. From this channel issued a long, linear 



* XCIX. t C. p. 27. X CIV. p. 288. 



