384 BULLETIN OF THE UNITED STATES FISH COMMISSION. 



continuous current and in a closed or interrupted circulation of water. 

 Putrescence or the development of deleterious organisms did not, I 

 believe, interfere with our experiments. At all events, they remained 

 of about the same size as the eggs with which we started, although 

 food was already perceptible in the stomach rotating under the impulse 

 of the cilia which lined the gastric cavity. They had the power to 

 retract the velum and mantle, but not wholly. The pallial and velar 

 muscles were therefore developed as well as the adductor, which could 

 be seen to actuate the valves. That these embryos were developed 

 from the eggs put into our apparatus there could not be the slightest 

 doubt, since the sea-water used was first carefully filtered through a 

 large dense mass of cotton wool to remove impurities and small, hurt- 

 ful organisms, and no additional water from the open bay was afterwards 

 introduced. 



The deflected border of the mantle in this fry seems to me an im- 

 portant fact in its bearing upon the manner in which the fixation 

 of the young animal is accomplished. Though it is true there was 

 as yet no umbo developed upon the shells of these larvae, such as may 

 be seen when the larval shell measures from an eightieth to a nine- 

 tieth of an inch in diameter, yet the hinge-line was straight as if the 

 shell at this point was truncated. In the last stage of development of 

 the larval shell, which I shall call the umbo stage, this apparent trun- 

 cation disappears, the umbos projecting somewhat past the level of the 

 hinge, which is still approximately straight and without teeth, contrary 

 to the statement of Lacaze-Duthiers. Immediately following the umbo 

 stage, the larval shell is converted into the spat, the valves of the latter 

 growing outwards from the borders of the valves of the fry shell, or, 

 rather, speaking more correctly, the calcareous deposits which are laid 

 down by the young developing bivalve project past the free edges of the 

 valves more and more, and immediately thereupon exhibit a prismatic 

 arrangement of the shelly substance wholly different from that seen in 

 the fry, which is laminar, homogeneous, and not prismatic. The fry 

 shell is perfectly symmetrical and very convex up to the time when it 

 is converted into that of the spat, which is at once developed in an 

 unsyinmetrical manner, but at first tends to simulate the rounded out- 

 line of the fry, except at the hinge, where no lateral growth of shelly 

 matter takes place. As growth of the spat goes on, and in fact from 

 the very 'first, the hinge of the fry shell is inclined slightly upward, the 

 fixation evidently having taken place at its edge. 



This I regard as the most important step which I have made in de- 

 veloping the history of the shell, and it is probable that in it we have 

 a clew to the manner in which the young oyster becomes fixed to station- 

 ary objects. It is important to note in this connection that the whole 

 of the lower surface of the under valve of the young oyster is at first 

 firmly attached by an organic cement. So firm is this substance that 

 the young shell can rarely be removed from its attachment without 



