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.

During early youth, however, they stand opposite to each other, and the whole body is then symmetrical, with both sides equally coloured.

Soon the eye proper to the lower side begins to glide slowly round the head to the upper side; but does not pass right through the skull, as was formerly thought to be the case.