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?

The correct answer is a mantle.

Explanation:

A mantle refers to an essential component of the body of molluscs. It refers to a delicate covering produced from the body wall that at certain occasions get strengthened by calcium carbonate.  

In various species of molluscs, the epidermis of the mantle discharges calcium carbonate and produces a shell over the entire body. The mantle encloses the stomach, heart, gonads, and intestines of the molluscs.  

"it built up in the atmosphere"

i think thats it, idk, i tried looking for this exact q& a for grad. and idk if i got this question right or sorry if im wrong