Nervous System of the Razor-shell Clam 313 



which water is constantly passing, and there is no reason why water 

 should not enter this opening as well as the branchial siphon. 

 When the animal is embedded in the mud however, free admis- 

 sion of w^ater through this opening is not to be expected. The 

 opening is surrounded by well developed tentacles that are similar 

 in appearance to those around the siphons and, like them, very 

 sensitive to tactile stimulations. Stimulation of these tentacles 

 always cause the animal to close its shell and usually, this may be 

 the mechanical effect of suddenly closing the shell, the slight 

 protrusion of the foot. The foot is almost immediately retracted 

 into the shell again and remains retracted unless stimulation is 

 continued. When the stimulation is continued the foot is alter- 

 nately protruded slightly and retracted, and occasionally, when the 

 animal is held anterior end downward, burrowing movements are 

 started. 



On each side of the Hne of fusion of the mantle lobes are very 

 small papillae that are probably also very sensitive to touch. The 

 whole region is very sensitive but whether sensation is more acute 

 on the papillae than on the general surface was not determined. 

 Posteriorly, from the ventral opening to the branchial siphon, the 

 fused mantle margins are very thick and muscular. Anteriorly, 

 to the opening through which the foot is protruded, the margins 

 are loosely attached by their epithehal cells. The extensive fusion 

 of the margins of the mantle keeps mud out of the mantle cham- 

 ber during burrowing, and forms a device for expelling strong 

 jets of water. 



Around the opening through which the foot is protruded the 

 margins of the mantle are much enlarged to form muscular, 

 thin-edged scrapers or valves. Figs, i and 2, c, that keep mud from 

 being drawn or forced into the shell when the foot is withdrawn 

 and the shell is forced down into the mud. It will be convenient 

 to refer to this portion as the collar. The collar is very sensitive 

 to touch and when stimulated is drawn tightly against the sides 

 of the foot. When the foot is withdrawn it turns in over the end 

 and so closes the shell. Strong stimulation of the collar when in 

 this position, causes the margins to be drawn still further in and 

 thus reflected into the shell. 



