In the eye, the pupil is the opening in the middle of the iris.
It appears black because most of the light entering it is absorbed by the tissues inside the eye.
In humans and many animals (but few fish), the size of the pupil is controlled by involuntary contraction and dilation of the iris, in order to regulate the intensity of light entering the eye.
This is known as the pupillary reflex.
In bright light, the human pupil has a diameter of about 1.5 millimeters, in dim light the diameter is enlarged to about 8 millimeters.
The shape of the pupil varies between species.
Common shapes are circular or slit-shaped, although more convoluted shapes can be found in aquatic species.
The reasons for the variation in shapes are complex; the shape is closely related to the optical characteristics of the lens, the shape and sensitivity of the retina, and the visual requirements of the species.