Passage Summary: Even though most people think lancelets don't have hearts, the author argues they do. The author's proof is that the vessel in question looks and acts exactly like the hearts found in other animals.

Conclusion: The contracting vessel found in lancelets is actually a heart.

Reasoning: The vessel's physical structure is very similar to the hearts of other sea creatures, and its muscular contractions also mirror those of other animals' hearts.

Analysis: The author is making a leap from 'it looks and acts like a heart' to 'it IS a heart.' To make this argument logically airtight, we need a rule that guarantees this classification. A sufficient assumption would be something like: 'If a vessel resembles the structure and contraction patterns of known hearts, then it must be classified as a heart.' We are looking for a bridge that turns those physical similarities into a definitive identification.