Reproduction 61 



its environment this worm has been forced, according to Harms dC 

 Dragendorff (1933) who have investigated it thoroughly, to leave 

 the ocean and live in moist situations or beaches where oxygen can 

 be obtained from the atmosphere. It cannot hve without sea water 

 and therefore is confined to the intertidal zone. 



Enough has been said to show that the conditions of life in water 

 are such that respiration at times becomes difficult or impossible. 

 Animals thus tend to go to the surface where aeration is best or 

 come to depend on the air above the surface for respiratory needs. 

 Some animals, having thus become airbreathers, may wander out 

 of aquatic habitats and finally become terrestrial animals. The res- 

 piratory difficulties in water and the advantages in the atmosphere 

 thus contribute to the trend of life toward land. 



Reproduction 



An animal may live in a habitat where it has an abundance of 

 food, where many other environmental conditions of life are favor- 

 able, and where it has few or no enemies, but if proper conditions 

 for reproduction are lacking, the species will become extinct. Many 

 animals leave situations were conditions for feeding and resting are 

 highly desirable and make long migrations to localities where con- 

 ditions for rearing young are more favorable. Many birds and 

 mammals make annual journeys to particular breeding grounds 

 and spend the remainder of the year where conditions are more 

 desirable for the adults of their species. 



An eel spends five to twenty years in fresh water, where it feeds 

 and grows. Then it dons its silvery spawning dress and hastens 

 back to the depths of the ocean to lay its eggs. The young eels 

 spend one to three years in floating back to fresh-water streams and 

 ponds. Salmon, on the other hand, hatch on the pebbly bottoms 

 of cool, clear streams and lakes. After about a year in fresh water 

 they make their way to the ocean, where they feed and grow. When 

 mature salmon migrate up streams to seek out appropriate spawn- 



