ORGANS OF THE LARVA 



43 



8) 



It will be seen that for the same length there are small variations in 

 depth and in the length of the hinge-line. Most of these are not true vari- 

 ations, but due to slight differences in position, which are more common in 

 proportion to the growth of the umbos. I have not given measurements 

 for all of the even numbers above 25, because from this size up there is 

 no excuse for mistaking the oyster larva for that of any other bivalve. 



The colour of the straight-hinge shells is light gray, but the older 

 umbo-stages become darker horn-coloured. A few irregular teeth may 

 be seen along the hinge-line, which is as a rule rather straight, but may be 

 slightly hollowed, or have anterior and posterior hollows, being higher in 

 the middle and at each end. With the growth of the umbos the charac- 

 ters of the hinge become obscured, from the fact of its having to be seen 

 through one of the valves, or else from above, which changes the direc- 

 tion of observation. 



The colour of the living larvse changes in passing from the younger 

 to the older stages. The youngest are uniformly grayish, but even at a 

 length of twelve there may be a faint tinge of pink, and at 14 a shade 

 of yellow in the position of the liver, which at 17 has deepened to brownish. 



* Explanation: 10 x 8 

 micrometer units). 



7, i. e. length 10, depth 8, length of hinge 7 (measured in 



