36 THE AMERICAN NATURALIST [Vol. XLIH 



and with a moderate umbo, while the umbos have a 

 postero-dorsal position, projecting backwards and up- 

 wards and making the shell broader, deeper, squarer be- 

 hind and tapering but rounded in front. The largest 

 measure about .358 by .365 mm. in height and length, but, 

 owing to the different convexities of the valves, the 

 greater breadth above and behind, and the different de- 

 grees to which it may be tilted in this way or that, the 

 same larva may vary much in apparent size and shape 

 according as to how it is presented to the observer. 

 The following are measurements of half a dozen larvae 

 at different ages selected from a large number of records : 

 .131 x .138 mm., .138 x .144 mm., .207 x .241 mm., .241 x 

 .276 mm., .296 x .345 mm., .345 x .372 mm. The larval 

 shell of the young spat (Fig. 5) measures .369 x .384 mm. 

 and may be taken to represent the maximum size. 



When mounted on a slide the larva? are accustomed to 

 remain quiescent, and from their deep coloration are diffi- 

 cult to examine, but sometimes a more transparent one 

 permits certain organs to be traced. When freshly col- 

 lected and examined in a watch-glass of pure, cool water 

 from their native habitat, many of them exhibit the 

 greatest activity, swimming hither and thither or circling 

 round and round by means of the velum (Figs. 9, 10, 11, 

 12), a swimming organ which they protrude between the 

 antero-ventral margins of the shell-valves and expand in 

 a manner resembling the opening of an umbrella. The 

 margin of this is densely covered with large cilia, the 

 violent napping of which propels the animal forwards 

 with the heavy body and shell suspended beneath the 

 velum. Jarring the watch-glass will cause the animal 

 to immediately withdraw its velum (Figs. 7, 8), at the 

 same time snapping the valves of its shell together and 

 dropping towards the bottom. Such observations illus- 

 trate the ordinary mode of locomotion and the response 

 to violent movements in the sea, for during heavy gales 

 a plankton net will take few or no larvae near the surface. 



An organ of immense interest to zoologists and of vast 



