[15] THE OYSTER AND OYSTER-CULTURE. 697 
contains about 3 per cent. of salt; and since not only over our oyster- 
beds, but over our entire sea-flats, the water possesses this degree of salt- 
ness, neither a lack nor an excess of salt can hinder the extension of 
the beds over the whole area. Even less can the temperature of the 
water hinder their extension, for the variation is the same over the 
oyster beds as at other points, and it fluctuates, during the course of the 
year, from 20° C. above zero to 2° C. below. Norcan alack of motion of 
the water or of nutriment be the cause why the oyster-beds have not 
during the past hundreds of years extended themselves beyond certain 
definite limits, for floating everywhere, in the ebbing and flooding 
water, are microscopic plants and animals, and much dead organic mat- 
ter, which would nourish large numbers of oysters, just as they do mul- 
titudes of soft clams (J/ya arenaria), edible mussels (Mytilus edulis), and 
cockles (Cardium edule). There remains, then, as the single natural hin- 
derance to a further extension of the oyster-beds, the unfavorable condi- 
tion of the ground over the greater portion of the sea-flats. Oysters 
cannot thrive where the ground is composed of moving sand, or where 
mud is being deposited, and one of these conditions or the other is found 
over the greater part of the sea-flats. The number and size of those 
places where, notwithstanding the daily ebb and flood currents, the 
ground remains unchanged and free from mud are very limited. Only 
along the slopes of certain channels to the north of the mouth of the 
Hider do we find united all the conditions favorable for such places, and 
only within these limited districts can young oysters grow to complete 
maturity. 
When the young oysters attached to the beard of the mother have 
reached a diameter of 0.15 to 0.18 of a millimeter, when their digestive 
organs have reached such a stage that the young animal can receive 
nourishment through them, and when the velum, by means of its cilia, 
is in a condition to enable them to move about, they leave the brood- 
cavity, swarm at the surface, and after swimming about for a short time 
finally sink once more to the bottom. If the swarm of young oysters 
settles upon a spot covered with clean stones or mussel shells to which 
they can become attached, they have a prospect of growing to maturity ; 
but if, on the contrary, they settle upon a changing sand-bank or upon 
a muddy bottom, they will surely be lost; for at the close of their swarm- 
ing period their velum, which is their swimming organ, is absorbed, and 
September, and October, and often catch, at a single drag of the dredge, as many as 
1,000 oysters. Sometimes great bunches of oysters growing attached to one another 
are gathered into the net. 
The deep-sea oysters grow much larger than those found along the coasts. Speci- 
mens are taken with shells 13 centimeters broad. Their flesh is tough, yet large 
numbers are consumed in England, France, and Germany; in England and Trance 
chiefly in pastries and sauces, but in Germany many are eaten fresh, especially in 
Hanover and Bremen. For general winter use they are kept under water in certain 
places adapted to them, especially near the island of Wangeroog. (S. Metzger’s Bei 
triige zu dem Jahresbericht d. Commiss. zur Unt. d. deutschen Meere, 1873, page 171, 
u. 1875, page 252.) 
