146 THE POPULAR SCIENCE MONTHLY. 



that of the sea, but the density is not important, except within wide 

 limits, though the specific gravity should not fall below 1-01 at 

 60 Fahr., distilled water being represented by 100. The main neces- 

 sity is that the water should contain a sufficient amount of lime to 

 furnish the animal with the principal constituent of its shell. 



The mature marketable oyster is so well known that it is unneces- 

 sary to describe it ; but the following will assist the reader, who is not 

 acquainted with the anatomy of the animal, to a correct appreciation 

 of the facts stated and terms used in subsequent pages : 



That part of the oyster usually known as the heart is a muscle, 

 called the abductor muscle ; its office is to keep the valves (or shell) 

 closed, and prevent the ingress of deleterious matter. 



The two valves are hinged at the round, blunt end of the shell, 

 and between this hinge and the abductor muscle lies the body of the 

 oyster or visceral mass, which is made tip of the light-colored repro- 

 ductive organs and the dark-colored digestive ones, packed together in 

 one continuous mass. The mouth of the oyster is that part nearest the 

 hinge, and what is usually called the " beard " of the animal is known 

 as the " gills." 



The oyster lies on its side in the shell, and the minute animal and 

 vegetable organisms contained in the water, and which form the food 

 of the animal, are passed along between the gills to the mouth by the 

 action of myriads of small vibrating hairs, called cilia. These cilia 

 cover the surface of the gills, and, by a rapid and simultaneous motion 

 in one direction and a slow one in the opposite, cause a strong current 

 to set into the lips of the valves, bringing in not only what is suitable 

 for food, but many other minute organisms and particles which thus 

 come in contact with the gills and what they may hold. 



The European oyster ( Ostrea eclulis) and the American oyster ( Os- 

 trea Virginiana) are varieties of the same family, and, though differ- 

 ing in several particulars, are not so dissimilar but that the conditions 

 favorable to the growth and life of one may be considered as equally 

 so for the other. With each variety the formation of the generative 

 matter is gradual, and the spawning-season of both is during the early 

 summer months, its advent depending probably upon the tempera- 

 ture, the higher temperature hastening and the lower retarding that 

 event. No particular temperature can be assigned, as it depends 

 greatly upon the locality, but departures from the normal temperature 

 of the spring and summer months will have the effect described. In 

 the same way an increased or diminished density of the water, whether 

 due to change of temperature or to the addition of water of greater or 

 less specific gravity than that usually surrounding the animals, has 

 probably a similar accelerating or reverse effect upon the spawning, 

 though as yet we can not speak with certainty upon this point. 



Generally, both in Europe and America, the spawning-season may 

 be said to be from June 1st until August 15th, though variations of 



