Amphioxi, which are also often referred to as lancelets, are invertebrates that were mistaken for slugs when they were first discovered in 1778 by Piotr S. Pallas. The misidentification is understandable since the marine animals have tapered forms without eyes or distinct heads. The creatures also do not have well developed brains or hearts and are not buoyant. They can swim side to side by contracting the staggered muscle blocks that line the flanks of their bodies, but whenever they cease to do so, they sink.