Hormones and Heredity by J. T. Cunningham
page 33 of 228 (14%)
page 33 of 228 (14%)
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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 |
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