THYROIDAL FUNCTION IN THE FISHES FUNDULUS HETEROCLITUS F. MAJALIS, AND F. DIAPHANUS¹

Abstract

IN THE evolution of organs or systems it is generally believed that successive changes in them persist principally because they endow the organism with new properties important in the adaptability and continuance of the organism and species. In this sense the lack of what would appear to be an important function for the thyroid gland in fish presents a real challenge to the comparative endocrinologist. There is no clear indication of the nature of the value of the thyroid gland to the most primitive of living vertebrate groups that would have led to the evolution of this fairly complex gland as well as its system of functional control by the pituitary. It has been shown that the thyroid thickens the epidermis (Harms, 1929; LaRoche, 1952), may affect the pigmentary pattern (La- Roche, 1952; Robertson, 1948) and increase the retrobulbar connective tissue in various species of fish (LaRoche, 1952).