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. The argument's conclusion follows logically if which one of the following is assumed? ['Some primitive animals other than lancelets have what is widely held to be a heart.', 'A vessel whose structure and actions closely resemble those of other animal hearts is a heart.', 'No animal that has a heart lacks an artery.', 'Only animals that have contracting vessels have hearts.']