In fishes, blood enters a chamber called the sinus venosus before entering the atrium and moving into the ventricle. From the ventricle, blood enters the chamber called the .

Bulbus arteriosus

Explanation:

The heart of a fish is a two-chambered organ which works in a single cyclic way but the heart also has the entrance and exit organs.

The deoxygenated blood from the body of the fish is first collected in an entrance structure called sinus venosus from where the blood enters the atrium. The atrium passes its blood to the ventricle and before out of the heart, the blood is collected in a chamber called the Bulbus arteriosus.

The Bulbus arteriosus is a pear-shaped organ which maintains the flow of the blood to the gill in the fish.

Thus, Bulbus arteriosus is correct.