546 William Patten 



mens of Arca Noae closed their Shells wheuever a slight shadow was 

 cast upon them. If, for instauce, an aquarium containing* one of these 

 animals is placed in a room well lighted from both sides, and a band 

 put in front of the g'lass (care being taken to avoid any jar or shock to 

 the water) , a faint shadow will be cast on the animai, sufficient, in spite 

 of its indistinctness, to cause it to dose its shell with remarkable quick- 

 ness, but always one or two seconds afterward, the promptitude depend- 

 ing upon the depth of the shadow. The sensitiveness and rapidity of 

 reaction depend also upon the vitality of the animals, always being- less 

 in those whicb bave been for some time in confinement, yet stili so great, 

 that those kept two or three months in small aquaria, with no special 

 pro vision for food, ne ver failed to dose the shell when a shadow was 

 thrown upon them. Stili other simple experiments show that it is not 

 necessary to cast a shadow upon them, in order to prove that they bave 

 organs specially sensitive to light. If, for instance, an ordinary, black 

 lead pencil, or any other equally small object, is approached with ex- 

 treme caution within two and a half or three inches of the anterior end 

 of the open shell, and in such a position that no perceptible shadow 

 falls upon the animai, it at once closes its shell, and with the same ener- 

 gy as when a deep shadow is cast upon it. If, on the other band, a glass 

 rod is brougbt within the same distance of, or even much nearer to, the 

 anterior end of the open shell, and moved quite rapidly to and fro, no 

 effect is produced upon the mussel. The last experiment shows 

 that, in the trial with the pencil, the closing of the shell was not pro- 

 duced by the disturbance of the water, since in the former experiment 

 care was used to avoid that effect, while in the latter, even a decided 

 agitation produced no result. Ali three experiments — with the shadow, 

 a small dark object, and a small transparent one, — were repeated at 

 least twenty-five or thirty times with dififerent animals, and under dif- 

 ferent circumstances, and invariably gave the results related above. 

 Moreover, I bave watched the animals many times for extended periods, 

 but bave never seen them dose the shell, unless startled by some sudden 

 change in the intensity of light, or by shocks or disturbance of the water 

 in which they were. 



I bave not made any experiments upon Arca barbata or A. tetra- 

 gona. 



It is only reasonable to suppose that the organs of vision, which 

 by the foregoing experiments we bave proved to exist in Arca, must be 

 situated upon that portion most exposed to the light; and, iudeed, such 

 is really the case, for on examiuing- the exposed part of the mautle edge, 



