In a Tide Pool, a Student Encounters an Organism with a Hard

In a tide pool, a student encounters an organism with a hard outer covering that contains a large amount 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 definitive identification of the organism?

A) a mantle
B) a heart
C) a body cavity
D) a filter-feeding apparatus