Definition: a thin membranous, sometimes alveolated sac in the dorsal portion of the abdominal cavity. Contains a varying mixture of gases, not identical to the composition of air.
Definition: Active swimming organisms that live in the water column and are able to move independently of the water mass (adapted from Lincoln et al., 1998).
Definition: 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.
Definition: 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.
