VIII 



ORGANS OF THE SPAT 



The Shell of the younger spats (Plate II, figs. 1-4) is longer than 

 high, like that of the larva, and this is true not only for each valve but also 

 for the whole shell, even when the far umbo, through tilting of the shell, 

 stands up above the near one. At first the addition of spat shell to the 

 anterior and posterior angles is liable to be greater than that to the 

 rounded part below, but when about 1 mm. in height (Plate II, fig. 5; 

 Plate III, fig. 1) the proportions become reversed, and from this time 

 forwards the shell grows fastest below and at the postero-inferior angle. 

 The growth is not very regular so that specimens of equal length seldom 

 have exactly the same depths. The following are a few measurements: 



55 x 55 80 x 76 



60 x 59 90 x 79 



70 x 67 105 x 95 



The spats caught on glass exhibit the characteristic colour of the 

 pelagic larva, the smallest varying towards pink, the larger towards 

 brown. Jackson described his spat as "yellowish-green". Those taken 

 on opaque objects, on the other hand, present a different appearance; 

 instead of having a pink, reddish, or brown coloration as one would ex- 

 pect from comparison with the larva, or, instead of having a white appear- 

 ance as might be looked for by comparison with the older spat and adult 

 oysters, they preserve a shining, dark, metallic lustre with a few faint 

 radial lines (Plate IV, figs. 4, 5, 6). In the centre of the dorsal region 

 can be distinctly recognized the larval shell (prodissoconch) of the oldest 

 free-swimming stage, presenting a uniformity of appearance in all the 

 specimens, and measuring in the neighborhood of 55 (=.38 mm.) in 

 length and slightly less in height. 



There is another way of verifying the size of the larva at fixation: viz., 

 to measure the prodissoconch of young spats; but it is more liable to give 

 small variations because of the pushing of the left umbo higher upwards 

 along the substratum than the right, the addition of spat-shell overlying 

 the edge of the larval shell, and also because this addition causes the edges 

 of the larval shell to be opened away from each other and the observer has 

 to look at a different angle upon their surfaces in different cases. A few 

 examples of measurements are: 



