274 REPORT OF COMMISSIONER OF FISH AND FISHERIES 



attached, beiug usually flat, but afterwards, as a rule, beconiiug deep 

 and strongly concave, through an upgrowing along the edges. 



FIXATION, SET, OR SPATTING. 



At the time of fixation the fry will, under proper conditions, attach 

 itself by its left valve to any hard or firm body with which it may come 

 in contact. 



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 obtaius, 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 cer- 

 tain parts of the coast its place of attachment is confined to the zone 

 between high and low water, the mid-tide mark being the place of max- 

 imum fixation. It has been suggested that this was due to the density 

 of the water preventing the sinking of the fry. There are a number 

 of objections to this theory, but no better one has been offered, and it 

 may receive provisional acceptance. 



GROWTH. 



At the time of its attachment the oyster fry measures about one- 

 eightieth or one-ninetieth of an inch in diameter. The valves of the 

 shell are strongly convex and symmetrical, and are composed of a 

 horny material quite different from the finished shell of the adult. 



The mantle, a thin flap of tissue which envelops the body of the 

 oyster on each side, projects freely from between the lips of the valves 

 and is the organ which secretes the shell. U])on its outer surface suc- 

 cessive layers of horny material are laid down, these becoming impreg- 

 nated with calcareous matter arranged in a prismatic manner, and thus 

 forming the stony shell which characterizes the adult. 



The mantle increases 2itt^'i passu with tlie growth of the soft parts in 

 general, and as it is always caj^able of protrusion a little beyond the lips 

 of the valves, it follows that each successive layer of shell is slightly 

 larger than that which preceded it, and the shell increases in length 

 and breadth as well as in thickness. From the nature of its growth, 

 therefore, the youngest or newest part of the shell is on the inner face 

 and at the edges, the latter always being sharp and thin in a growing 

 oyster. The shell of the young oyster is always thin and delicate, and 

 is generally more rounded than in the adult. The lower valve at first 

 adheres closely to the body to which it is attached, but later its edge 

 grows free and the valve, as a whole, becomes deeper and more capa- 

 cious than its fellow. The small larval or fry shell remains visible at 

 the beak of the spat shell for a considerable time, but becomes eroded 

 away before the oyster reaches the adult condition. 



The soft parts of the oyster assume their adult form in general soon 

 after attachment, although the genital glands do not become functional 

 until a much later period. 



