It is widely believed that lancelets -- small, primitive sea animals -- do not have hearts  Each lancelet has a contracting vessel, but this vessel is considered an artery rather than a heart  However, this vessel is indeed a heart  After all, it strongly resembles the structure of the heart of certain other sea animals
Moreover, the muscular contractions in the lancelet' s vessel closely resemble the muscular contractions of other animals' hearts
A vessel whose structure and actions closely resemble those of other animal hearts is a heart.