that convert starches and some celluloses 

 into the sinnple sugar called glucose and also 

 probably break down fats. These enzymes 

 are needed because many plant cells, which 

 comprise the major items of oyster food, 

 contain large quantities of starch. The nnate- 

 rial of the dissolved style also helps to lower 

 stomach acidity to provide optimal conditions 

 for digestion. Thus, the style helps mix the 

 food particles in the stomach and also pro- 

 vides a continuous supply of digestive en- 

 zymes. 



Since the particles which enter the tubes 

 of the digestive diverticula from the stomach 

 are small, they can be ingested into the cells 

 and go through the so-called intracellular 

 process of digestion, in contrast to the extra- 

 cellular type of digestion taking place in the 

 stomach proper. Both proteins and fats can 

 be digested in this manner. Undigested por- 

 tions of the particles are discharged from the 

 cells. They are finally forced to enter the 

 intestinal groove and are eliminated in the 

 normal way. 



Blood cells of the oysters also participate 

 in digestion. These phagocytic cells pass 

 through the stomach walls into the stomach, 

 where they engulf small particles such as 

 diatoms. Digestion involves the gradualbreak- 

 down of ingested particles, after which the 

 blood cells move back through the stomach 

 lining into the blood stream. 



Part of the absorbed food is stored in the 

 oyster body. Oysters are considered "fat" 

 when their meats are large and plump. This 

 condition, however, may be caused either by 

 development of spawn or by accumulation of 

 reserve food material. A plump condition 

 during breeding time results from the in- 

 creased gonads; the fatness of the oyster 

 after the breeding season, when the oyster 

 feeds actively, to the reserve food. The stored 

 material contains many substances, but it 

 consists mainly of animal starch (glycogen), 

 which is found in the large cells of connective 

 tissue in most parts of the body. 



In oysters of Long Island Sound, accumula- 

 tion of glycogen commences sometimes as 

 early as August and continues until the start 

 of winter hibernation, which is usually early 

 in December when the water temperature 

 decreases to about 41° F. In the warmer 

 waters of Florida, Louisiana, and Texas, 

 fattening may not begin until December. In 

 all cases, the stored food reserves are nor- 

 mally used during the following spring and 

 summer when the oyster grows and forms its 

 reproductive cells. 



The quality of oyster nneats depends prin- 

 cipally upon its solids and amount of stored 

 glycogen. Good quality oysters usually contain 

 between 18 and 20 percent solids; poor ones 

 may contain less than 10 percent. Normally, 

 a high solids content is accompanied by cor- 

 respondingly high glycogen storage. "Fat" 



oysters are normally white, and their meats 

 fill the shell cavity. 



Oyster meat contains large quantities of 

 nutritive substances that people need for a 

 balanced diet. It is high in copper and iron, 

 which are needed for proper composition of 

 human blood, and because of this, oysters are 

 often prescribed for patients with anemia. 

 Oyster meat also contains iodine necessary 

 for normal activities of the thyroid gland. 

 Proteins in oyster meat are especially high 

 in nutritive value, and the carbohydrates, in 

 the form of glycogen, are readily digested 

 and assimilated. Phosphorus and calcium 

 are present in relatively high quantities. 

 Oyster meat also contains most essential 

 vitamins. 



The condition of oyster meats depends on 

 several factors, such as location of oyster 

 beds, quantity and quality of food present in 

 surrounding waters, salinity of the water, and 

 time of year. 



The provision that oysters should not be 

 eaten in months that do not contain the letter 

 "r", while valid for the flat European oyster 

 which incubates its young during summer, 

 does not apply to American oysters, which 

 can be and are eaten at any time of the year. 

 Because they are highly perishable, however, 

 their transportation and storage before re- 

 frigeration was widespread created consid- 

 erable difficulties during warm weather. Fur- 

 thermore, oysters that spawn late in summer 

 become watery and unattractive in appearance 

 and are not as acceptable as during the cold 

 season, when the quantity of stored materials 

 is highest. Nevertheless, there are localities 

 in several States where oysters of fairly good 

 quality may be available during the entire 

 year. 



The circulatory system of the oyster car- 

 ries body fluids from one part of the animal 

 to another. The heart is located above the 

 adductor muscle in the pericardial chamber. 

 It consists of a single ventricle and a pair 

 of contractile auricles, one on each side. The 

 auricles collect blood largely from the gills 

 and push it to the ventricle, which drives it 

 by rhythmical contractions into the anterior 

 and posterior aortas. The posterior aorta is 

 short and supplies only the adductor muscle 

 and the rectal region. The rest of the body is 

 supplied by the anterior aorta, which divides 

 into a series of smaller blood vessels. These 

 vessels open into the so-called blood sinuses, 

 where the blood flows and washes the different 

 organs coming in contact with it. The used 

 blood, now low in oxygen, collects in the veins 

 and is carried into the gills or the organs of 

 excretion, the kidneys, sometimes called the 

 organs of Bojanus. The kidneys, which purify 

 the blood, are located by the adductor muscle 

 and consist of two convoluted tubes connected 

 internally with the pericardium and externally 

 with the exhalant channber. 



