THE OYSTEE AI^D THE OYSTER INDUSTRY. 13 



Fertilization of the eggs occurs in the water. The oysters, male 

 and female, lying about over the bottom, at spawning time discharge 

 the reproductive elements into the water where they mingle as chance 

 may bring about. The more numerous the oysters on a particular 

 bed, the greater the chance of the actively moving spermatozoa 

 meeting the eggs. The spermatozoa swarm around the eggs, many 

 about each one, until a spermatozoon penetrates the egg membrane, 

 the head only of the spermatozoon passing on in, the tail dropping 

 off. The material of the head unites with that of the egg, and 

 important changes in the latter are thereby initiated. 



The single cell of the egg begins to divide into many cells and to 

 change its form and in the course of from 5 to 10 hours develops 

 into a small oyster larva, which swims by means of fine hairs or cilia 

 on the outside of its body. A shell then begins to develop and soon 

 covers the entire body, so that the larva resembles a tiny hard clam. 

 A definite organ of locomotion also appears, consisting of a disk, 

 known as the velum, borne on the end of a thick stalk which is pro- 

 truded from between the valves of the shell in front. The disk 

 bears cilia which by their movement enable the larva to swim about 

 rapidly (PI. Ill, fig. 4). When the velum is retracted the larva 

 settles to the bottom. 



The larva is now about two days old and measures about 0.08 mm. 

 in length. As it increases in size certain elevations, the umbones, 

 can be noted on the upper part of the hinge, one on each side. Shortly 

 one valve becomes much deeper than the other, and the umbo on 

 it much more prominent than that on the right side, and by this 

 characteristic the oyster larva may be readily distinguished under 

 the microscope from the larval form of any other bivalve. The 

 deeper valve is the left one and that by which the oyster later becomes 

 attached. Diu-mg the advanced stages of the larval form, the left 

 umbo is very conspicuous, jutting back in almost the form of a 

 hook (PI. Ill, fig. 3). 



The period passed thi-ough by the oyster larva from the develop- 

 ment of the cilia, a few hours "after fertilization, until it "sets" or 

 "strikes" is known as the free-swimming stage. Although the 

 larva swims about freely in the water, being so small, its move- 

 ments and location at any particular time are largely subject to the 

 tides and currents. The free-swimming period lasts from about 14 

 to 18 days in the more northern waters and a somewhat shorter 

 time in the southern. The warmer the water the more rapidly 

 development occurs and consequently the shorter the free-swimming 

 period. 



At the close of the free-swimming period, when the oyster is about 

 one-third of a millimeter (one seventy-fifth of an inch) long, it "sets" 

 if the proper conditions are present. It attaches itself by the left 

 valve to some siu-f ace in the water, a rock, shell, stake, in fact almost 

 any object (PL IV). 



The first essential is that the surface should be clean and that it should remain so 

 a sufficient length of time to enable the young oyster to firmly establish itself. So 

 long as this condition obtains, the nature of the material seems to matter but little. 

 In most bodies of water the spat fixes itself at all levels from the surface to the bottom 

 but in certain parts of the coast its place of attachment is confined to the zone between 

 high and low water, the midtide mark being the place of maximum fixation.^ 



o Moore, H. F. (1897, p. 274.) 



