90 



THE RISE OF ANIMAL LIFE 



Active Staqe 



Fig. 5-2. Some Protozoa such as the human intestinal 

 parasitic amoeba {Endomoeba histolytica) withstand 

 periods outside the digestive tract where there is very 

 little moisture. This it does by secreting a resistant 

 cyst wall that prevents desiccation. It is in this stage 

 that the parasite is transmitted from person to person. 



tuned to such a narrow temperature band, 

 try to imagine what would happen if the 

 earth's orbit shifted ever so sHghtly, just 

 enough to change the average mean tem- 

 perature on the earth by a paltry 100° C. 

 up or down. All life would stop abruptly; 

 every living thing would congeal either 

 from freezing or cooking. Think how pre- 

 carious our existence is, reckoned in astro- 

 nomical terms. A slight celestial slip would 

 mean the end of life as we know it. For- 

 tunately that slip has not occurred in the 

 past one or two billion years, and probably 

 will not for a few billion more. 



Moisture 



^ We are already familiar with the impor- 

 tance of water in relation to life processes 

 (p. 40); here we need to consider it as an 

 essential part of the environment. Getting 

 the proper amount of water at the right 

 time is one of the basic problems that con- 

 fronts animals. This is sometimes very diffi- 

 cult and, as a result, animals have devised 

 various means for maintaining their water 

 supply at a constant level. Although too 

 much water is as detrimental to some 

 animals as too little is to others, probably 

 the greatest problem for most animals is 

 the conservation of water. 



Even Protozoa have provided themselves 

 with a method of withstanding desiccation. 

 Amoeba, for example, forms a resistant cyst 

 which is impervious to water loss (Fig. 



5-2). While beautifully housed in this 

 tiny container the amoeba can withstand 

 long periods of drouth without ill effects. 

 The eggs of many metazoan animals such 

 as Crustacea and rotifers are provided with 

 a thick shell which resists drying. The eggs 

 of many parasitic roundworms are likewise 

 resistant to moisture loss. Some can be 

 blown around in the dust for months and 

 still become viable when picked up by the 

 proper host. In fact, some even rely on this 

 period of desiccation to disseminate the 

 species. 



Some larger animals, desert turtles and 

 lizards for example, never require water in 

 the liquid state; they manage very well on 

 that which is taken in with their food. 

 Camels are notorious for their ability to 

 work long periods without water. They can 

 exist a week or more on dry food and if 

 green plants are available it is not uncom- 

 mon for them to go without water for a 

 month. Jack rabbits, mountain goats, jump- 

 ing mice, and other mammals living in 

 arid regions are very well fitted to conserve 

 their water intake, which is usually only 

 that provided in the food. Most mammals, 

 however, require a great abundance of 

 water, especially those that perspire, such 

 as man and the horse. 



Excessive moisture is fatal for some 

 animals. The earthworm, for example, is 

 driven from its burrows after heavy rains 

 because it cannot get enough oxygen from 

 the water. Even frogs may drown in spite 

 of the fact that they are usually near water 

 and require it in large quantities. High hu- 

 midity often creates a favorable environ- 

 ment for certain types of parasites which 

 under normal amounts of moisture could 

 not gain a foothold. 



Light 



Wave lengths extend from a fraction of 

 a submicron (cosmic rays) to more than 

 a thousand meters (Hertzian or radio 

 waves), yet most animals are sensitive to 

 ethereal vibrations that range only from 



