PSYCHOLOGY, COMPARATIVE ; ANIMAL BEHAVIOUR).
Examples of chemical co-ordination in animal responses are set forth in the article ENDOCRINOLOGY, where the role of internal secretion in the developmental processes is also indicated. It can be said that everything of any general significance that is known about the physiology of development has been based on the study of lower forms of life, more especially those which display phases of abrupt development (metamorphoses, q.v.), as do sea-urchins, salamanders, etc. (see EXPERIMENTAL EMBRYOLOGY). Of recent developments in comparative physiology, one field in which the comparative method has proved invaluable is the physiology of the cell or microscopic units of the animal body (see CYTOLOGY : ExPerimental Cytology and Cell Inclusions). The germ cells of marine organisms, worms, starfishes, and the like have provided much of the most favourable material for manipulation in experi mentation on isolated cells. The researches of Pantin, Chambers and others on the movements of the Amoeba are worthy of men tion in this connection. In the study of nervous co-ordination and muscular activity the most favourable materials have hitherto been found among animals dealt with by the medical physiologist. But considerable attention has been paid of recent years to the mechanism of ciliary activity, bioluminescence, colour change and other forms of response which are specially illustrated by the lower organisms. One field in which significant advance has been made through the researches of Krogh, Barcroft and their co workers, is in the study of the variety of respiratory pigments which in different groups of the animal kingdom serve, like the haemoglobin or red colouring matter of human blood, as a means of taking up oxygen and transporting it to the tissues.
Though the influence of comparative physiology has largely been manifest in relation to human physiology, it is probable that it has unrealized possibilities of practical application in relation to agriculture, fisheries and other applied sciences. At present the experimental study of breeding is the only branch of com parative physiology which has notably contributed to the solu tion of commercial problems. Comparative physiology has still to shoulder the task of interpreting the mechanism of animal dis tribution in terms very different from the teleological assumptions of the evolutionists of the i9th century. (See ECOLOGY, ANIMAL ;