With the assistance of so-called accessory 

 hearts the blood from the kidneys is pumped 

 into the vessels of the mantle, and eventually 

 this blood, together with blood {rora the gills, 

 is returned to the heart by way of the auricles. 

 The blood cells are colorless and contain 

 neither the red hemoglobin that is found in 

 blood cells of higher animals, nor hemocyanin, 

 a blue-colored substance found in other mol- 

 lusks, such as some snails, squids, and octo- 

 puses. 



The nervous system of the oyster is com- 

 paratively simple. It consists of two knots, 

 or ganglia, of nervous tissue situated near the 

 mouth, from which two nerves pass to another 

 pair of ganglia lying under the adductor muscle. 

 From these two pairs of ganglia small nerves 

 extend to various parts and organs of the body. 

 Oysters, like other mollusks, do not possess 

 the type of centralized nervous system which 

 is characteristic of vertebrates. 



GROWTH 



Oyster growth varies considerably with 

 size, temperature, quantity and quality of 

 food, and seasons of the year. As a rule, 

 growth is more rapid in warm waters, such 

 as the Gulf of Mexico, where a marketable 

 size of 3- 1/2 inches may be reached on some 

 beds in 2 years. In northern waters, such as 

 Long Island Sound, characterized by shorter 

 summers and generally lower water tempera- 

 tures, oysters reach this size in 4 or 5 years. 

 The average size of a Long Island Sound 

 oyster at the end of the first, second, third, 

 fourth, and fifth growing periods is 3/4, 2-1/4, 

 3, 3-1/2, and 4 inches, respectively. Speci- 

 mens measuring more than 14 inches long 

 have been found on old natural beds. 



The exact maximum age attained by oysters 

 cannot be stated definitely, but the number of 

 layers composing the shells of some unusually 

 large individuals indicates that they may reach 

 an age of 40 years. 



REPRODUCTION 



Gonad Derelopment and Spawning 



Oyster reproductive organs, or gonads, 

 consist of a mass of tissue made of nriicro- 

 scopic tubules, sex cells, and connective tissue 

 which envelops the stomach, digestive diver- 

 ticula, and the fold of the intestine, and, during 

 the period of ripeness, constitutes a significant 

 portion of the entire oyster body (fig. 2). The 

 gonads become larger and thicker as their 

 eggs and sperm mature. When ripe, the gonadal 

 layer in the region of the stomach of an oyster 

 about 4-1/2 inches long may be as thick as a 

 quarter of an inch. 



The Eastern oyster exhibits alternative 

 sexuality; i.e., adults function seasonally as 



separate sexes. In this respect, it is like the 

 Japanese oyster, the Australian oyster, 

 Crassostrea commercialis . and the Portuguese 

 oyster, C. angulata . It differs, however, from 

 another group typified by the European oyster, 

 which has a series of alternating male and 

 female phases throughout its life. 



During the first spawning season of the 

 Eastern oyster, most young individuals function 

 as males. This condition is known as protandry. 

 Nevertheless, the gonads of young oysters may 

 show all gradations from true males, in which 

 no female cells can be found, to those that 

 develop directly into ovaries containing grow- 

 ing eggs. After the second spawning season 

 the numbers of individuals of each sex are 

 almost equal. 



The adults function seasonally as separate 

 sexes, but, nevertheless, the sex of an oyster 

 is generally unstable and, therefore, a change 

 of sex from male to female and vice versa 

 sometimes occurs. This change usually takes 

 place in the intervals between the two spawn- 

 ing seasons. In experiments with Long Island 

 Sound oysters, about 9.7 percent reversed 

 their sex. The percentage of reversals was 

 considerably higher among the females (13 

 percent) than among the males (8 percent). 

 Functional hermaphrodites are found among 

 adult oysters, but, as a rule, they are un- 

 common, constituting less than 1 percent of 

 the adult population. 



Gonads of Eastern oysters change markedly 

 during the year. Such changes may vary con- 

 siderably from one geographical area of the 

 Atlantic or Gulf coast to another, but, never- 

 theless, they correspond to definite seasons 

 of the year and, in general, nnarkedly affect 

 the physiological behavior of the oysters and 

 the chemical composition of their meats. 



During the cold season, the gonad follicles 

 of oysters of northern waters, such as those 

 of Long Island Sound, are small and contain 

 sex cells only in early stages of development. 

 With the end of hibernation, however, as soon 

 as the temperature of the surrounding water 

 increases, the gonads develop at first slowly 

 and later rapidly, and by the end of June many 

 oysters are already ripe (fig. 3). 



Oysters begin to spawn as soon as the water 

 temperature is sufficiently high. Gonads of 

 partially spawned oysters are characterized 

 by contraction of the follicles, invasion by 

 phagocytes, and rapid increase of connective 

 tissue cells that contain much glycogen. Re- 

 sorption of the gonads is completed inOctober. 

 After October they enter a brief indifferent 

 stage during which the sexes are almost 

 indistinguishable. It is assumed that sex re- 

 versal, if it occurs, takes place during this 

 period when the sex is least defined. At the 

 end of the indifferent phase, gonad develop- 

 ment begins again but it is soon stopped by 

 the low temperatures of approaching winter. 

 In the spring, gonad development is resumed. 



