62 THE VITAL FUNCTIONS. 



as to the correctness of the observations. But the researches 

 of l^oiinet and of Spallanzani, who repeated the experiments 

 of Trembley, have borne aniple testimony to their accuracy, 

 which those of every subsequent observer have farther con- 

 tributed to confirm. 



The experiments of Trembley have also proved that eve- 

 ry portion of the hydra possesses a wonderful power of re- 

 pairing all sorts of injuries, and of restoring parts which 

 have been removed. These animals are found to bear with 

 impunity all sorts of mutilations. If the tentacula be cut 

 off, they grow again in a very short time: the whole of the 

 fore part of the body is, in like manner, reproduced, if the 

 animal be cut asunder; and from the head which has been 

 removed there soon sprouts forth a new tail. If the head 

 of the hydra be divided by a longitudinal section, extending 

 only half way down the body, the cut portions will unite at 

 their edges, so as to form two heads, each having its sepa- 

 rate mouth, and set of tentacula. If it be split into six or 

 seven parts, it will become a monster with six or seven 

 heads; if each of these be again divided, another will be 

 formed v/ith double that number. If any of the parts of this 

 compound polypus be cut off, as many new ones will spring 

 up to replace them; the mutilated heads at the same time 

 acquiring fresh bodies, and becoming as many entire poly- 

 pi. Fig. 245 represents a hydra with seven heads, the re- 

 :sult of several operations of this kind. The hydra will 

 sometimes of its own accord split into two; each division 

 becoming independent of the other, and growing to the 

 same size as the original hydra. Trembley found that dif- 

 ferent portions of one polype might be ingrafted on ano- 

 ther, by cutting their surfaces, and pressing tlicm together; 

 for by this means they quickly unite, and become a com- 

 pound animal. When the body of one hydra is introduced 

 into the mouth of another, so that their heads are kept in 

 contact for a sufficient length of time, they unite and form 

 but one individual. A number of heads and bodies may 

 thus be joined together artificially, so as to compose living 



