46 COMMISSION OF CONSERVATION 



settle down at an early age to a fixed mode of existence like its parents. 

 The fact that both of these conceptions have been proved wrong by my 

 study of plankton stages justifies the correctness of the view with which 

 I started out, viz., that plankton stages had been neglected, embryo- 

 logists having jumped from early veliger to young spat periods. 



On the other hand there appears to have been a predisposition to ex- 

 pect some vestige of this organ which will account, perhaps, for a few 

 references to it in the literature, although the early stages to which these 

 references are applied possess no organ that can justly be regarded as such. 



The first appearance of the foot in development is difhcult of recog- 

 nition on account of close union with the abdomen, of which it is a ventral 

 outgrowth. It is further obscured by the overlying shell, mantle, gills, 

 and by the lack of colour and of movement at this period. It is not until 

 the larva passes into the umbo stages that the foot becomes recognizable 

 from its movements. By the time the shell measures 35 and over, the foot 

 is becoming a well developed, active, and most capable organ. It is 

 formed as a muscular creeping surface along the ventral edge of the 

 abdomen, behind the velum and mouth, and grows forwards and down- 

 wards until its distal end acquires an amount of freedom sufficient to per- 

 form feeling movements over all parts of the body of the larva, both in- 

 side and outside of the shell. (Plate I, figs. 19, 20; Plate V, figs. 30-32; 

 Plate VI, figs. 2, 4). These movements are quick, nervous and wormlike, 

 always ready to instantly jerk the organ back to its position of rest within 

 the shell, where it lies closely tucked away behind the velum and between the 

 gills. When being protruded, it is at first short and tongue-shaped, but 

 as it becomes further extended it assumes a narrow ribbon-like form as 

 long as the length of the shell. In creeping locomotion, the foot is stretched 

 forwards and flattened against some object to which it clings, and then, 

 by contraction, the body and shell are dragged ahead and the process 

 repeated. To facilitate adhesion the under surface is flattened and some- 

 times appears grooved even to such an extent as to permit the two halves 

 to fold against each other longitudinally. It is possessed of relatively 

 great muscular strength, as evinced by the way in which it jerks the shell 

 about and flops it from side to side. The surface is uniformly covered with 

 fine cilia in active motion. Near the base of attachment there is a 

 posterior heel-like projection w^hich, when the foot is fully extended, is 

 likewise carried beyond the limits of the shell. The heel is the papilla 

 upon which opens the duct of the byssus gland, situated along the axis 

 of the proximal part of the foot (Plate VI, fig. 5). In each side of the 

 base of the foot is an otocyst in apposition to the surface, and between 

 these a pair of pedal ganglia. 



When the animal is at rest or swimming, the foot is shortened, with- 

 drawn and compactly folded away so close against the abdomen as 

 hardly to be observable. It is rarely to be seen protruded at the same 



