VI 



ORGANS OF THE LARVA 



Shell. — The shell in growing (Plate I, figs. 10-21) scarcely increases 

 the length of the hinge-line, but it soon shows a tendency to become 

 slightly pointed in front, and to exhibit delicate, concentric, semi-circular 

 lines of growth, apparently under the centre of the hinge, but in reality 

 somewhat laterally from it. With further growth this part of each valve 

 becomes raised into a prominence, the beak or umbo (Plate V, fig. 31, um), 

 which projects upwards, backwards and outwards, and gives to the later 

 stages of the larval shell a different appearance from the earlier ones. 

 The difference becomes distinct at about 25 units length (Plate I, fig. 15), 

 so that this size may be regarded as dividing younger straight-hinge 

 larvse from older umbo-stages. The shell of the largest free-swimming 

 larva I have a record of, measured 56x52 (56 x 6.9 //= .386 mm.). 

 Whole numbers of small units are more easily grasped than fractional 

 parts of large units; so I have used measurements of the shell from 10 to 

 56 (micrometer units), rather than from .069 to .386 mm. From 69 

 to 386 fj. (micro-millimetres) could also be used. 



The straight-hinge stages are thin from side to side and the valves are 

 symmetrical or nearly so; but the umbo-stages become thick from umbo 

 to umbo, although thin anteriorly and below, and the left valve is larger 

 and more convex than the right. On this account specimens mounted on 

 a slide appear of different shape, depending on the size and on the side upon 

 which they are lying, for when one valve is pressed flat against the slide 

 the hinge-line will be tipped towards the observer so much, it may be, as 

 to expose the dorsal portion of the under valve. On account of the greater 

 size and convexity of the left valve, and especially of its large umbo, the 

 tipping is greater when this valve is below and both umbos (umbones) are 

 largely exposed. When, however, the larva is lying on its right side the 

 small right valve is more completely covered by the larger left. Because 

 of the different positions possible from various degrees of tipping or rolling, 

 the measurement of length is safer for comparison than that of depth 

 (height). The following are some measurements giving length, depth 

 and length of hinge-line: — 



