EMBRYOLOGY AND EVOLUTION
form have larvae that are strikingly alike. Zoölogists had long classified barnacles as mollusks, until a study of their larvae showed conclusively that they were crustaceans. Many parasitic crustaceans are mere sacs of eggs or spermatozoa in the adult stage, but they have typical crustacean larvae. Ascidians, or sea-squirts, were classified as mollusks until Kowalevsky showed that their larvae are little tadpoles with notochord, dorsal nerve-tube, and gill-slits like any typical vertebrate. These and many other examples show that natural affinities or ancestral relationships are often shown in embryos and larvae long after they have been obscured or lost in adult stages.
All vertebrates, from fishes to mammals, pass through a fish-like stage in their development in which they have (1) gill slits, (2) five or six pairs of aortic arches, (3) a simple tubular heart with one auricle and one ventricle, (4) a notochord (the basis of the backbone in higher vertebrates), (5) a primitive type of kidney (the pronephros), etc. These organs persist throughout life in the lowest fishes, but they undergo many changes in higher forms.
Entirely similar conditions are found in the development of the brain and central nervous system; the eye and ear; the limbs and muscular system; the digestive, respiratory and reproductive systems. In fact, nearly all the important organs and systems of higher vertebrates pass through stages in their development which in lower vertebrates remain permanently.
It is true that some of these embryonic reminiscences of lower forms have been modified and greatly abbreviated, but they are nevertheless all there and are recognizable. Among higher forms there are many adaptations to peculiar conditions of embryonic or larval life which have no counterpart in lower forms ; such are the embryonic membranes of higher
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