Fishes -

1. Heart is 2 chambered 1 auricle & 1 ventricle present. Sinus venosus, truncus or conus arteriusus present.
2. Only impure blood come in heart so heart is venous heart or brachial heart.
3. Single circulation present i.e. blood comes in heart only once in one complete circulation.
4. An advantage of single ciruclation is that entire body receives pure blood. A disadvantage is that the narrow gill capillaries slow down the flow of blood and body receives blood at the low pressure.

Note :- In lung fishes heart is 3 chambered.