Final answer:
Bivalvia uses an open circulatory system with a heart that pumps hemolymph directly onto the organs. Gas exchange is carried out through ctenidia, with waste expelled by nephridia. This system suits their filter-feeding and sedentary lifestyle in aquatic environments.
Step-by-step explanation:
The class Bivalvia, within the Phylum Mollusca, manages circulation primarily through an open circulatory system. Bivalves, such as clams and mussels, have a heart that pumps hemolymph (blood-like fluid) into the body cavity where organs are directly bathed in it. The hemolymph is moved through the animal's body by the muscular contractions of movement and feeding behavior. Gases are exchanged and nutrients are distributed without the need for a closed network of vessels.
Oxygen and carbon dioxide exchange occurs across the thin and highly vascularized surfaces of the gills or ctenidia. These structures are involved in both respiration and feeding. Bivalves also have a pair of nephridia for excretion and osmoregulation, straining waste materials from hemolymph and expelling them from the body.
In a way similar to other Mollusca classes, the structure of the bivalve circulatory system reflects its lifestyle. For instance, filter feeding and minimal movement require less energy; thus, an open circulatory system suffices. These organisms take advantage of the continuous water flow through their bodies, both for obtaining oxygen and for feeding, through their specialized mantle cavity and siphons.