ORGANS OF THE LARVA 45 



corresponding with the slit between the valves, its two halves turning 

 forwards and then crumpling into smaller, secondary folds. The surface 

 of the velum is ciliated, its front margin carrying rows of especially large, 

 powerful cilia, which are the chief propelling agents. Their movement 

 can not ordinarily be observed in active life, exhibiting only a shimmering 

 of the surface, but sometimes in enfeebled, injured, or dying individuals 

 the strokes of the cilia are so slow that it is an easy matter to see how the 

 locomotory effect must depend upon a simultaneous, vigorous, bending 

 stroke in one direction, followed by a slow return to the original position. 

 The velum precedes in locomotion, dragging the heavy shell and con- 

 tained body suspended beneath and behind it. In viewing the move- 

 ment under a low power, it is very difficult to follow the movements of 

 a swimming larva. It generally circles round and round or in a spiral, 

 but sometimes goes straight ahead until it bumps into something resis- 

 tant, when it jerks in its velum and snaps its shell-valves together, drop- 

 ping to the bottom. In a watch-glass one can sometimes find a larva 

 floating, with its velum expanded on the surface of the water and shell 

 suspended below (Plate V, fig. 29) , or creeping, with its velum pressed 

 flat against the glass and shell carried above (Plate V, fig. 28). In such 

 positions the shape of the velum is elliptical or oblong-elliptical, thin at 

 the margins, but thick towards the centre, where it is attached by a broad 

 stalk to the body. 



The velum has been known since Leeuwenhoek (1695). 



Foot. — A second organ of locomotion, capable of protrusion from 

 the open shell, is the foot — an organ unknown for the oyster until 1904, 

 when it was discovered by myself. In adult bivalves generally the foot 

 is such a characteristic organ as to have suggested one name (Pelecypoda) 

 for the class, notwithstanding the fact that there are a few genera in which 

 it is absent or greatly reduced. Of these the oyster and the silver-shell 

 are the commonest. Spat and adult oysters are normally fixed to rocks 

 or shells, to which their left valves are solidly cemented. Under these 

 circumstances a creeping foot, such as is possessed by a clam, a quahaug, 

 or a mussel, would be of little or no service to an adult oyster, which as a 

 consequence has failed to retain it. Its absence from all sizes of oysters 

 down to microscopic spat stages is doubtless chiefly responsible for the 

 hitherto universal belief that the oyster does not possess a foot at any 

 stage of its life. If the grown stages differ so widely in habit and struc- 

 ture from the polecypod type, why should not the larva also? The young 

 stages of the larva known to and figured by various investigators do not 

 possess a foot. 



Influenced by such facts of morphology, physiology, and embryology, 

 zoologists could not resist the conclusion that the oyster larva is very 

 different from other common bivalve larvae, that were known to possess 

 a foot, in that this organ is absent in the oyster larva, which must promptly 



