150 BIOLOGY 



principle is found in a less developed extent in some of the organ- 

 isms with a higher structure, though nowhere do we find it so 

 highly developed as among the hydroids. Where polymorphism 

 is developed the whole colony acts as a unit, and the colony, 

 therefore, may be compared to a more highly complex organ- 

 ism with its various organs. Polymorphism always arises as 

 the result of asexual growth and not by sexual reproduction, 

 and when it occurs the members of the colony always show a 

 differentiation in function as well as in shape and structure. 



Regeneration of Lost Parts. Hydra has a wonderful power 

 of reproducing lost parts. If it is cut into two pieces, each 

 part will develop the part that it has lost and becomes a 

 new Hydra. Indeed, it may be cut into a large number of frag- 

 ments, and every fragment is capable of growing and developing 

 into a new form like that of which it was originally a part. 

 If the small conical projection containing the tentacles is cut 

 off from the rest of the Hydra, each piece will develop the part 

 that it has lost. The animal may be split lengthwise into two 

 or four parts and each will become a perfect animal. If a head is 

 split in two and the parts slightly separated, each will develop 

 its crown of tentacles and a two-headed animal will result. If 

 an animal is turned wrong side out, it will adjust itself to new 

 conditions and a perfect animal will soon be produced. This 

 power of regenerating lost parts is found in many of the lower 

 animals, but in no place is it better developed than in Hydra. 

 In the higher animal the power of regenerating lost parts eventu- 

 ally disappears entirely. It is very evident that this power must 

 be of considerable advantage to the animal in the struggle for 

 existence. In Hydra the power is so extraordinarily developed 

 that a piece of the animal not more than one-hundredth of an 

 inch in length is capable of reproducing all of the parts that are 

 lacking and developing into a new animal. In some cases the 

 new animal is produced by a multiplication of cells from these 

 pieces, so that a fair-sized animal is developed; while in other 

 cases the cells and fragments are remolded into new individuals 



