Invertebrates possess no heart, as this organ first made its appearance in fishes. A fish's heart contains a single ventricle and auricle, making it a primordial version of the human heart (which has two ventricles and two auricles). As a consequence, using fish for basic research is very helpful in areas such as drug screening or studying mechanisms of action.
以心臟為例，無脊椎動物沒有心臟，生物真正有心臟是從魚開始，魚的心臟是一心房、一心室，可以說是人類心臟（二心房、二心室）的雛形，因此，用魚來做基礎實驗，無論在藥物篩選或研究作用機制上都有很大的助益。