326 



ELEMENTARY BIOLOGY 



377. Alternation of generations among animals. Among 

 some of the animals related to the sea anemone and hydra 

 there is found a fairly regular alternation between generations 

 that reproduce sexually that is, by means of gametes and 

 generations that reproduce asexually. Good examples of this 

 alternation are furnished by jellyfish found in the ocean off 



FIG. 154. The jellyfish aurelia 



The mature medusa, a, reproduces sexually, the gametes being thrown into the water, 

 where fertilization takes place. The egg develops into an individual having the general 

 form of a hydra, , and attaches itself to a rock. The animal elongates and breaks up 

 into a number of individuals by means of constrictions, so that it comes to resemble a 

 pile of bowls. Each individual, when separated, turns over and swims away, changing 



into a medusa, a 



our coasts (see Figs. 154 and 155). The complete life history 

 includes both kinds of individuals, male and female, and two 

 kinds of generations, sexual and asexual. 



Alternation of generations is also found in many parasitic animals, 

 especially parasites that inhabit two or more different hosts at differ- 

 ent stages in their development. Thus, the malarial parasite repro- 

 duces in the blood of human beings by sporulation ; that is, by the 

 formation of a large number of spores. But in the body of the mos- 

 quito there are produced tiny protoplasmic structures that unite in 

 pairs ; that is, they conjugate. There is thus present a sexual method 

 of reproduction and an asexual method, and these alternate regu- 

 larly so long as the organism has the opportunity to pass from one 

 host (man) to the other (mosquito) and back again (see pp. 403-407 

 and Fig. 209). 



