20 MECHANICAL FUNCTIONS. 



complete animal. If the head of the hydra and a por- 

 tion of the body be divided by a longitudinal section, 

 the animal is thereby the gainer, for the divided parts 

 form t^'o b-ods instead of one. with complete sets of 

 tentacula for each mouth, and thus he can enjoy the sat- 

 isfaction of eating with two mouths at the same time. 

 If the head be split into half a dozen parts, each part 

 will form a new head with mouth and tentacula to match, 

 the whole being united to one body. Fig n 



Fig. 11 represents a seven-headed mon- 

 ster, the result of several mutilations 

 and divisions of one of these protean 

 creatures. 



39. Sometimes of its own accord a hy- 

 dra will split in two parts lengthwise, each 



division becoming independent of the other, and growing 

 to the same size, and attaining the same organs as the 

 original animal. 



40. Mr. Trembley also found that any portion of one 

 hydra might be engrafted on another, in the same man- 

 ner that pieces of India-rubber may be joined, that is, by 

 cutting their surfaces and pressing them together. By this 

 means they would unite and become a compound animal. 

 Thus many heads may be united to one body, or many 

 bodies to one head ; and so, on the contrary, when one 

 hydra is introduced into the mouth of another, so that 

 their heads are kept in contact, for a time, they unite and 

 become one individual animal. 



41. Even the figures of other animals, as quadrupeds, or 

 man, might be constructed in this manner, though every- 

 where covered with moving tentacula. 



PENNATUL.E. 



42. Another form under which polypi exist, is that call- 

 ed pennatidce. This is called sea-pen, from its resem- 



How many parts of these animals may be engrafted upon each other* 

 Give some account of the pennatulae. 



