The hierarchical system moves upward into larger and larger groups of organisms. Thirty-two of the world's 33 animal phyla are found in the oceans, while only 12 are found on land. Twenty-one of these phyla exist exclusively in the oceans, while only 1 is exclusive to land. Seawater is pumped into these tube feet, hydrostatically extending the feet. Some animals may have more than 2,000 tube feet.
Sponges lack a nervous system and have no true musculature.
Thus, they do not move and must feed on food particles suspended in the water. Life on earth and in the oceans depends on organisms that capture small percentages of the sun's energy that reaches earth's surface. This energy is then passed through food webs from one organism to the next. On land, plants capture the sun's energy, are the base of food webs, and constitute the largest physical bulk of the living world. Cell for cell, planktonic algae fix more solar energy and manufacture more protoplasm than plants on land.