In a tide pool, a student encounters an organism with a hard outer covering that contains 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 heart
B. a mantle
C. ten legs
D. eyes
Answer: B