CHAPTER XIX 



CHANGES IN THE LIFE-CYCLE AND CHANGES IN THE 

 ENVIRONMENT 



The Life Histories of Some Animals 



In the life histories of many animals and plants a series of 

 stages succeed each other until, with the completion of the cycle, 

 the starting point is reached again. In some species the series 

 of forms that make up the hfe-cycle seem to change without any 

 change taking place in the environment, that remains the same 

 throughout, but in other cases the change in the cycle is asso- 

 ciated with a change in the environment. It is this latter class 

 that we may now examine, although it will also be profitable not 

 to neglect the other. 



The life-cycle of most animals is made up of a single adult 

 individual and a series of embryonic stages through which the 

 embryo passes to become an adult. The cycle consists, therefore, 

 of adult, egg (and sperm), embryo, larva, adult. In other cases 

 the Ufe-cycle is more complex, as it may contain more than 

 one adult form and include several modes of reproduction. 



In some species the cycle seems to complete itself under con- 

 stant external conditions, as stated above, while in closely related 

 groups the change appears to be connected with a change in the 

 environment. For instance, many of the marine hydroids show 

 an alternation of generations that does not appear to be directly 

 connected with changes in their surroundings. The egg develops 

 into a swimming embryo that settles down and becomes a polyp. 

 The polyp produces buds, all of which are also polyps. This 

 process continues until the colony has reached a certain stage, 

 when a new kind of bud arises that becomes a jellyfish. The 

 jellyfish detaches itself, and comes to lead an independent exist- 



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