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NAZ NAZ
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13 years ago
 1) It is easy to see that starfish exhibit radial pentamerous symmetry. Sea urchins also exhibit the same type of symmetry. Given that sea urchins do not have arms, how can you tell they are radially pentamerous?

2)how do see urchins repire?
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13 years ago
2. Most sea urchins possess five pairs of external gills, located around the mouth. These are thin-walled projections of the body cavity, and are the main organs of respiration in those urchins that possess them. Fluid can be pumped through the gills' interior by muscles associated with the lantern, but this is not continuous, and occurs only when the animal is low on oxygen. Tube feet can also act as respiratory organs, and are the primary sites of gas exchange in heart urchins and sand dollars, both of which lack gills.
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