ADAPTATION 355 



nature of the integument. Animals living in a desert must be 

 adapted to an environment in which there is a scarcity of water. 

 Many of the smaller desert forms of mice and lizards never take 

 water in the free state and are able to derive sufficient amounts 

 from their food to meet their needs. The stomach of the camel is 

 provided with water reservoirs in the form of small flask-shaped 

 cavities, each with a constricted mouth. When the stomach is 

 filled with water, the muscles at the mouths of the cavities relax 

 automatically, allowing the cavities to be filled. Water is 

 absorbed from the stomach but not from the reservoirs. As the 

 demand for water arises it is released from the reservoirs into the 

 stomach. 



Pressure.- — Animals are adapted to conditions of atmospheric 

 or hydrostatic pressure under which they live. This is strikingly 

 brought out when the animal is subjected to a considerable 

 change in pressure conditions. Deep-sea fishes brought to the 

 surface from depths of 10,000 to 16,000 ft. are unable to survive 

 the transition for any length of time. At high altitudes bleed- 

 ing at the nose may occur in human beings owing to the 

 diminished air pressure. Since there is less oxygen, per unit 

 volume, in high altitudes than at lower ones, the reduced amount 

 of oxygen taken with each lungful of air also causes respiratory 

 distress. 



Adaptations for Race Preservation. — In many animals there is 

 no such thing as parental care of the young. Marine forms, such 

 as the starfish, sea urchin, and many fishes, deposit the eggs in 

 water, where they are fertilized, after which all parental responsi- 

 bility ceases. The development of starfish is so rapid that in 

 24 hours it has advanced to the stage where free-swimming 

 larvae are produced which are fully able to care for themselves. 

 In fishes developing outside of the body, the egg is provided with 

 a quantity of yolk which serves as a source of nutrition until the 

 young fish is able to shift for itself, but even here the rate of 

 development is very rapid. Among other fishes and among 

 amphibians the eggs are deposited with some attempt at conceal- 

 ment, so that they do not become too easy prey, but on the whole 

 the parental supervision and protection are rather scanty. Natu- 

 rally, an enormous number of eggs and embryos are destroyed, 

 but since the rate of reproduction is so great in such cases — a 

 cod is said to produce 6 million eggs — there is under ordinary 



