We should bear in mind that each species of duck is subjected to a severe struggle for existence, and that the structure of every part of its frame must be well adapted to its conditions of life.

The Pleuronectidae, or Flat-fish, are remarkable for their asymmetrical bodies.

They rest on one side--in the greater number of species on the left, but in some on the right side; and occasionally reversed adult specimens occur.

The lower, or resting-surface, resembles at first sight the ventral surface of an ordinary fish; it is of a white colour, less developed in many ways than the upper side, with the lateral fins often of smaller size.

But the eyes offer the most remarkable peculiarity; for they are both placed on the upper side of the head.