Answer:
The heart of a fish is known as a branchial heart
Function and structure;it's main structure is to pump Venous blood to ventral aorta into gills(i.e branchial) and then to somatic vasculature.Thus,branchial and systematic vascular beds are arranged in series with heart.
Apart from heart,heart like organs are present in Agnatha(Myxine and petromyzon).The heart of fishes consist of four Chambers a sinous venosus, an atrium,a ventricle and a conus or a bulbus Arteriosus.