定义：motion characterized by wave-like movement patterns that act to propel an animal forward. eg: crawling in snakes, or swimming in the lamprey. Typically utilized by limbless animals
定义：Capable of creating a new organism by combining the genetic material of two gametes, which may come from two parent organisms or from a single organism, in the case of self-fertilizing hermaphrodites.
定义：length from the tip of the snout to the tip of the longer lobe of the caudal fin, usually measured with the lobes compressed along the midline. It is a straight-line measure, not measured over the curve of the body
定义：Relative to pinhole eyes, lens eyes have greatly improved resolution and image brightness. The lens converges incoming rays of light, thereby reducing the angle over which each photoreceptor receives light, and forming an image focused on the retina. Most lens eyes have "accommodation"; they can focus an image either by physically moving the lens toward or away from the retina or by using eye muscles to adjust the shape of the lens.
