10. 



but little doubt that the siphonal tentacle is a greatly enlarged 

 atid specialized marginal tentacle. 



Both stnacture and experiment agree in assigning to 

 these tentacles a tactile fimction. The siphonal tentacle is 

 not, as might be supposed, the most sensitive to toucli of any of 

 the organs. It seeins to be rather more sensitive than the mar- 

 ginal tentacles, about as sensitive as the expansions of the man- 

 tle opposite the extremities of the posterior shell stripes, and 

 rather less sensitive than the projections opposite the extremi- 

 ties of the anterior shell stripes, the foot, and the siphons. It 

 may be moved about with a pencil-point without causing much dis- 

 turbance, but if it be pressed slightly, moved qxxickly, or Jarred, 

 it is quickly withdraTOi, and the withdrawal may be accompanied by 

 the partial or complete retraction of the siphons, the closing 

 of the shell, and, in some cases, by the activity of tlie foot. It 

 seems especially sensitive to sudden movements or Jars, and its 

 special function may lie in this direction, in enabling the 

 creature to detect the approacJi of enemies, of -^^lich flounders 

 seem to be a^.ong the most dangerous. 



If a vessel, containing several specimens partially 

 buried in the mud, is not disturbed for some hours and^^then Jar- 

 red, all will generally disappear with astonishing rapidity. 



