Commentators explain that this verse prohibits three categories of creatures: 1) those that move on their bellies (like snakes, worms, and slugs), 2) four-legged vermin that crawl (like lizards, mice, and weasels), and 3) creatures with many feet (like caterpillars, centipedes, and spiders). This was a comprehensive ban on what were considered low, swarming life-forms.