In a tide pool, a student encounters an organism with a hard outer covering that contains much calcium carbonate, an open circulatory system, and gills. The organism could potentially be a crab, a shrimp, a barnacle, or a bivalve. The presence of which of the following structures would allow for the most certain identification of the organism?
A) a mantle
B) a heart
C) a body cavity
D) a filter-feeding apparatus
E) eyes
Answer: A
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