When Piotr S. Pallas discovered amphioxi in 1778, he misidentified the marine animals as slugs, an understandable mistake most likely caused by their tapered, sheathed forms without eyes or defined heads. The creatures, which do not have brains or hearts, most often swim at night. To do so, amphioxi contract the staggered muscle blocks that line both sides of their bodies, moving from side to side. Since they are not buoyant, whenever the invertebrates cease to swim, they sink. During the daytime, amphioxi usually remain buried under the bottom of the ocean, but allow the anterior end of their bodies to protrude during feeding so that they can filter particles that pass through their gill slits.