ually destroyed. These observations upon seasonal variations were made with oysters taken from Narragansett Bay; whether the same holds true in the warmer waters farther south does not appear to have been determined. It would seem only a reasonable precaution not to eat shellfish taken from contaminated waters at any season of the year; at least not until after such thorough cooking as to insure the death of any bacteria present.

Oysters are often kept for a time after gathering, on rafts constructed with false bottoms where they remain immersed in the water. This is called "floating." Except where the practice is forbidden by law, it is common for the dealers to "float" oysters in waters of a less salt content than that in which they were grown, with the result that the fresher water enters the oyster, increasing its plumpness and weight and giving it a whiter appearance. If the water in which the oysters are floated is less pure than that in which they were grown, the danger of disease bacteria in the oyster is of course increased, and vice versa. How long it would be necessary to float polluted oysters in pure water in order to make them safe cannot be stated with any degree of certainty at the present time.

The composition of the principal shellfish used for food is shown in Table 31, in which the percentages of nutrients are those given by Atwater and Bryant and the fuel values are recalculated as explained in earlier chapters.

Another shellfish used somewhat for food, especially upon the Pacific coast, is the abalone, the flesh of which is marketed fresh, canned, and dried. Fresh or canned flesh of the abalone contains about 22 per cent of protein; dried abalone meat, about 36 per cent. According to data quoted by Langworthy the fat content is negligible, but the fresh flesh of the abalone contains, like that of clams and oysters, about three per cent of carbohydrate, doubtless chiefly glycogen.

Ash constituents of oysters. The fact that the oyster secretes such a large amount of calcium in its shell suggests that the edible portion may be relatively rich in calcium as compared with other flesh foods, which, as we have seen, are strikingly poor in this element.

According to Albu and Neuberg the edible portion of the oyster is strikingly rich in calcium, but an analysis in the writer's laboratory yielded:

This analysis shows a calcium content somewhat above that of meat but much below that of milk, and a preponderance of acid-forming elements as great as that found in lean meats.

Comparison of Poultry, Fish, and Shellfish with Other Flesh Foods

Attention has been called to the similarity of all these flesh foods and to the fact that the differences in general composition are chiefly attributable to varying fat content.

That there is also a general similarity in the structure of the proteins of shellfish, fish, and fowl and of ordinary meat protein such as beef is shown by the following table based on the work of Osborne:

TABLE 32. PERCENTAGES OF AMINO ACIDS FROM THE FLESH OF WIDELY DIFFERENT SPECIES

The digestibility of poultry and fish has been studied quantitatively by Milner and by Holmes in experiments in which the total percentages of net absorption from the digestive tract (coefficients of digestibility) were determined for protein and fat with the following results:

TABLE 33. DIGESTIBILITY OF POULTRY AND FISH.
(MILNER AND HOLMES)

The digestibility as thus determined is seen to be approximately equal to that of meats and appreciably higher than that of average mixed diet.

While these coefficients represent digestibility in the only sense in which it can be measured quantitatively, it is well known that the term digestibility is also used to indicate the relative ease and comfort with which foods are digested and the readiness with which gastric digestion is completed as evidenced by the time elapsing between the eating of the food and its entire passage from the stomach into the intestine. In these respects oysters, lean fish, and chicken are held to be even more digestible than lean beef, while fat fish, duck, goose, lobsters, and crabs are held to be of about the same order of digestibility with ham and pork. (See, for instance, the Table of Comparative Digestibility in Gilman Thompson's Practical Dietetics.)

Place in the diet. From most standpoints poultry, fish, and shellfish may be regarded as interchangeable with the ordinary meats. As in the case of meats, their chief nutritional significance is as sources of protein. Drummond has found fish proteins to resemble meat proteins in efficiency for the support of