Foods of the class to which oysters, crabs and clams belong, assume a new importance in the light of the nutrition studies which have so clearly shown the inferiority of muscle meats as compared with the secretory organs and tissues which are concerned with the transformation of foods to end-products of metabolism, i. e., the glandular organs (52). In clams, oysters and soft crabs, we eat the entire animal except its exo-skeleton, the shell. This insures the consumption of tissues containing all the uncharacterized food substances, and makes these articles of diet, supplementary foods in the sense that they are likely to add to the diet some principles which it in many instances contains in amounts below the optimum.

While such articles are not necessary in a diet which contains the requisite amount of milk and the leafy vegetables, and must be regarded as somewhat expensive foods to be eaten as an indulgence of the appetite, the shellfish doubtless played a most important role in the nutrition of early man. Numerous shell-heaps or "kitchen middens" of enormous proportions occur on the Atlantic coast of America and Europe. These were the sites of human habitations over long periods and indicate that these shore dwellers depended in great measure on shellfish for their sustenance. Their type of diet represented a peculiar kind of carnivorous feeding which was very satisfactory.