Re actions of J so pods 467 



the response was local; in the former the stimulation was general 

 but the response was local. Hence it would seem that inasmuch 

 as the first and most common response to a general stimulation 

 of the animal by mechanical vibration is in movements of the 

 antennae, accompanied by movement of the gnathopod, which 

 response is the same as that produced by local stimulation of the 

 antennae by pure tactile stimulation, that the a:ntennae are par- 

 ticularly sensitive to vibrations, and that we here have a case of 

 specialization in which the antenn.Te, though onlv slightly sensitive 

 to pure tactile stimulation, are decidedly sensitive, and, in fact, 

 the parts most sensitive to mechanical vibrations. 



It was noticed repeatedly that Asellus and Caecidotea were very 

 sensitive to any movement in the water. This sensitiveness was 

 much the more marked in Crecidotea. Even dipping a small 

 camel's hair brush or the end of a pencil into the water would 

 often rouse a Caecidotea to sudden movement, notwithstanding 

 that it might be 40 to 50 cm. away from the point where the dis- 

 turbance started. A few times during experimentation with 

 Caecidotea when I accidentally breathed upon the surface of the 

 water, the animals responded vigorously. This acute sensitiveness 

 to a disturbance in the water was noted, to, in collecting Caeidotea 

 in the caves, where slight and unusual movements in the water at 

 one side of a pool would arouse to active movements every C?eci- 

 cidotea within a radius of 2 or 3 meters, the animals generally 

 leaving the borders of the pools and moving toward the deeper 

 water where they soon became concealed under the edges of stones 

 or in small depressions in the bottom. 



This greater sensitiveness of Caecidotea to mechanical stimula- 

 tion of whatever sort and its relatively slight response to light 

 is a good illustration of the principle of compensation in special 

 senses. 



II. EXPERIMENTS WITH THE ANIMALS IN CURRENTS OF WATER 



A special trough was constructed for experiments on the effects 

 of currents of water on Asellus and Caecidotea. Two thin trips of 

 glass, 46 cm. long by 2 cm. wide, were cemented in vertical par- 



TBE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 8, NO. 4. 



