Lancet for one. An order of primitive chordates.
A lancelet, also known as a cephalochordate, is a small, fish-like marine animal belonging to the subphylum Cephalochordata. Lancelets possess a notochord and a simple body structure, lacking a true backbone, and are characterized by their elongated, lancet-shaped bodies. They are filter feeders, primarily found buried in sandy substrates, and play a vital role in understanding the evolution of vertebrates.
Amphioxus, a type of cephalochordate, is not as adept at swimming as many aquarium fish. While they can move through water using a simple lateral undulation, their swimming capability is limited compared to the more advanced muscle structures and streamlined bodies of fish. Fish possess fins and a more efficient propulsion mechanism, allowing for greater agility and speed in the water. Thus, aquarium fish generally outperform amphioxus in swimming ability.