THE STORAGE OF FOOD AND ITS SUBSEQUENT USE

Amoeba and Paramecium

The granules in the endoplasm of these Protozoa include particles of protein reserve and droplets of oil. The proteins no doubt act as food reserves, but it is doubtful whether the oil droplets can do so, for apparently the Protozoa are unable to digest fats and find some difficulty in digesting carbohydrates— proteins forming their chief food substance.

Hydra

No definite food reserve substances are detectable in the ectoderm or endoderm of Hydra. This may be due to the fact that the animal is extremely active in ponds during the summer when its prey are numerous, but passes the winter months in an inactive condition, as an encysted zygote in the bottom of the pond, requiring no food beyond that which has been provided in the egg.

Earthworm

There are no definite food-storage organs or tissues in the earthworm. The animal, provided that its habitat is moist, is normally quite certain to get sufficient food, living as it docs on the organic debris in the soil all the year round.

Insects

In general, the stored food in insects is the mass known as the fat body, a whitish mass occupying most of the space around the gut in the abdomen. The material present consists largely of fat, but contains uric acid which has been deposited there by the Malpighian tubules and which is therefore an excretory product.

Vertebrates

The portal vein present in all vertebrates carries blood from the small intestine to the liver. Here any excess of carbohydrates is stored as glycogen or animal starch, and the remainder passes in the blood stream to the muscles to be utilized as glucose, or is stored in them as glycogen.

The lacteal vessels convey lipides in an emulsion as microscopic droplets of oil. Any excess of this material is stored in the liver and around the muscles under the skin, being in fluid form at the normal temperature of the animal. Other important fat storage ’depots ’are around the kidneys in mammals and the fat bodies in the frog, which latter animal does not appear to store lipides under the skin. There are no specific protein storage organs in vertebrates, for all living cells contain proteins from which their protoplasm is built up.

Thallophyta

In Protococcus and Spirogyra the pyrenoids characteristic of this group of plants are protein bodies around which starch is formed and stored. The starch is quite readily shown around the pyrenoids in the chloroplast of Spirogyra by the iodine test.

In seaweeds the outer layers of cells known as the cortex acts as a storage layer, the materials being chiefly carbohydrate. The fungi do not usually store carbohydrates but oils instead, many of which are highly poisonous. The storage of reserve food by them is not important, since a characteristic feature of the life cycle of fungi is a rapid-growing vegetative stage followed by a dormant resistant dispersal stage as a spore.

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