72 



ECOLOGY 



Part II 



ORAL 



lone or section 



ABORAL 

 RADIAL 



BILATERAL 

 Fig. 5.6. Axes, planes and regions in animal bodies. 



Environments 



Rhythms of Sun and Moon. The lives of all plants and animals are inter- 

 woven with the rhythms that originate outside the earth, their income of 

 energy from the sun, the changes of the tides, and shifts of climate. Patterns 

 of living change from hour to hour as the earth rotates on its axis in its journey 

 round the sun. Evening with its own ways comes to a countryside as it is 

 turned from the sun. If it is New England and early June, the wood thrushes 

 sing through the sunset and afterglow; the whippoorwills begin calling when 

 the hedges are black; the mosquitoes are enlivened by the subdued light and 

 the dampness. From moment to moment animals as well as plants respond 

 punctually and precisely to changes in light and atmosphere. 



The gravitational attraction between the sun and the earth and the moon 

 and the earth constantly pulls upon these bodies, its strength varying with 

 their respective positions in their orbits. On land its effect is relatively slight 

 but upon the sea it is the basic cause of tides. Sun and moon both take part in 

 the changes of the tides, but the moon, being much nearer the earth, has the 

 stronger influence upon them. With many variations there are in general four 

 tides on every seashore, two high and two low ones in each period of 24 

 hours. The tide rises and water that has swept the ocean bottom floods over 

 the tide pools bringing additions to the already crowded communities of ani- 

 mals, some of them to eat, others to be eaten. Each little group is continually 

 changed by flooding and ebbing water. Everything that belongs to the sea 

 waits on the tides. Fishermen in harbors put out their seines for the fishes that 

 follow the rising tide. Great ocean steamers wait at their docks until the tide 

 rises. 



The Sun, a Great Provider. The sun sustains life upon the earth, providing 

 living organisms with heat, light, the energy stored in food, and indirectly with 

 water. The sun is a great furnace of transmuting atoms, extraordinarily differ- 

 ent from the earth yet with a similar chemical content. According to certain 

 theories the earth originated from a torn-out piece of it. It is the source of 



