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 statement 'Only animals that have contracting vessels have hearts' is an assumption required by the argument.