The atmosphere exerts a force on earth. Why does yhe atmosphere exert pressure on earth?

The atmospheric molecules have energy because they have a quantifiable temperature. It vibrates and circulates, and bumps into other gas molecules. Most of them are elastic, which means that the molecules bounce off more than they stick together. The "bump" is a strength. It becomes pressure when applied over an area, such as your skin or earth's surface.

Explanation:

How much is the pressure of the atmosphere?

Pressures are determined by height (mostly by the volume of water vapor), temperature, and weather. The average air pressure, however, is 14.7 pounds per square inch, 29.92, or 1.01 to 105 paschal in regular conditions at the marine level. At 5 km altitude, atmospheric pressure is just about 50%. (about 3.1 miles).

Why is the Earth's surface under pressure so much higher? It's because the weight of all the air at that location really is measured. There's not much air around you to press down while you are high in the atmosphere. The whole atmosphere is heaped above you on the surface of the Earth. While the gas molecules are extremely light and distant, they are many!

organisms that lack symmetry are called asymmetrical and belong to phylum porifera/sponges.

organisms with specialized stinging cells belong to phylum cnidaria. stinging cells are called cnidoblast cells.

cnidarians show two morphological forms : polyp (sessile) and medusa (mobile)

filter feeding is observed in porifera members.  

alternation of generation is observed in coelenterata or cnidaria. there is alternation of asexual polyp form and sexual medusoid form. it is also referred as metagenesis.

they deliver oxygen to the blood