light, absence of colour to absence of light. Position of eyes is also
related to light; they are situated where they can see, absent from the
side which is shut off from light. The marginal fins are extended where
their movements best support and move the body.

It is to be noted also that these adaptations of different organs of the
body, eyes, fins, colour, are entirely independent of each other
physiologically. It may appear on first consideration that eyes and
colour, being both on the upper side, may have been somehow connected in
the constitution of the body, whereas the only connexion is external in
their common relation to light. This independence is well shown in the
modification of the dorsal fin: if this were physiologically affected by
the change in the eyes, which is brought about by the twisting of the
interorbital region of the skull, the anterior end of the fin would be
between the two eyes, since the morphological median line of the body is
in that position. In fact, on the contrary, the attachment of the dorsal
fin is continued forward where it is required for its mechanical function,
regardless entirely of the morphology of the head.

This is even more clearly evident in the structure of the jaws and teeth.
These are entirely unaffected by the torsion of the interorbital part of
the skull. In cases where the mouth is large and teeth are required on
both sides, the prey being active fish of other species, as in Turbot,
Brill, and Halibut, the jaws and teeth are equally developed on the upper
and lower sides, and there is almost complete symmetry in these parts of
the skull. In Soles and Plaice, on the other hand, whose food consists of
worms, molluscs, etc., living on or in the ground, the jaws of the lower
side are well developed and strong, those of the upper side diminished,
and teeth are confined to the lower side. Here it is not a question of the
jaws twisted, but simply unequally developed. There is no general and