Reactions of I so pods 453 



with the weakest than with the strongest of the four currents used, 

 while the sensitiveness of Asellus almost disappears within the 

 same range of stimulation. 



3. The threshold of stimulation for Caecidotea is considerably 

 below that for Asellus. 



4. The flagella of the antennae m both species are only slightly 

 sensitive; contrary to what is usually true, to water c rrents this 

 organ is more sensitive in Asellus than in Caecidotea. 



5. The most sensitive parts of the body in each species are 

 the head and bases of the antenna, but the uropods and legs of 

 Caecidotea are only slightly less sensitive than such other parts. 



J. With the Concussion Produced by a Falling Solid Body 

 Striking upon a Surface of Wood 



A third method of testing the relative sensitiveness of Asellus 

 and Caecidotea to mechanical stimulation was by trying the effect 

 of a mechanical vibration produced by the concussion of a falling 

 solid body upon a surface of wood. A steel ball and a lead shot 

 were used. The steel ball weighed 3.505 grams; the shot 0.542 

 grams. At first only the steel ball was used and it was dropped 

 through a distance of from 50 cm. to 2 cm. Since the threshold of 

 stimulation was not reached for either species by the concussion 

 produced by the steel ball, the lead shot was used in an endeavor 

 to get a weaker stimulus. The momenta of the two falling 

 bodies were calculated,^ and the falling distances chosen so that 

 the concussions produced by the two bodies would form a graded 

 series. 



The metal balls were dropped upon the flat, smooth surface 

 of a pine board 30 cm. long, 19 cm. broad and 1.2 cm. thick. The 

 balls were dropped from between the thumb and forefinger at 

 various levels, as determined by a meter stick, and made to fall 

 at a particular place on the board. The isopod to be experimented 

 upon was put into a waxed-bottomed stender dish 6 cm. in diame- 



^The effect of the friction of the air and the difference in the hardness and density of the two bodies 

 were not taken into account. 



