228 REPRODUCTION IN PLANTS AND ANIMALS 



amoeba-like oi^anisms produce asexual spores ; the malarial para» 

 site is a good example. Sponges, the hydra, hydroids, and some 

 other animals may produce new forms by budding. In the life 

 of the hydroid there is a regular alternation of a period in which 

 asexual reproduction takes place (by means of budding off new in- 

 dividuals of the colony) with a period in which sexual reproduction 

 takes place. In the latter stage eggs and sperms are produced by 

 free-swimming jellyfish. This is known as alternation of genera- 

 tions. This alternation of a sexual with an asexual generation is 

 also plainly seen in many plants, notably mosses and ferns. 



Life History and Metamorphosis. — The period from the time 

 the egg is fertilized to the death of the organism is called the life 



history. In many ani- 

 mals this life history is 

 simple and easy to follow. 

 In others it is very com- 

 pHcated and frequently 

 involves great changes in 

 form. For example, a tiny 

 flatworm, known as the 

 liver fluke of sheep, repro- 

 duces within the liver of 

 the sheep, on which it is a. 

 parasite. The eggs pass 

 out with the faeces oi 

 the sheep and develop in 

 w^ater. If the young fluke 

 is so fortunate as to find 

 a snail of a certain kind in which it can live at this stage of its 

 existence, it enters the snail's body, goes through several changes 

 of form there, and eventually gets out of the snail's body, forms 

 a hard protective covering or cyst, and attaches itseK to a blade 

 of grass. If a sheep happens to eat the grass containing the 

 cyst, the parasitic fluke begins a life of activity again, the cyst is 

 dissolved by the digestive juices of the sheep, the fluke soon begins' 

 laying eggs, and the life cycle is completed. In this life cycle the 

 fluke goes through several changes of body form. Such a develop- 

 ment is called ^ metamorphosis. 



Stages in the life history of the grasshopper. 

 Note the absence of wings in 1 and 2. The adult 

 female 4 is lajdng eggs in a hole she has made in 

 the ground. 



