PHYSIOLOGY OF SYNAPTULA HYDRIFORMIS 357 



The 'heads' in every case went straight up the (inside) wall of 

 the bowl directly away from the supply of oxygen. 



In contrast to the sand-inhabiting synaptids, like Synapta 

 inhaerens (Semon, '87), but in agreement with others, like 

 Leptosynapta bergensis (Becher, '07, '09), Synaptula hydri- 

 formis appears not to respond to vibrations. In individuals 

 placed in a dish resting on an arm of a tuning-fork (run by 

 electricity) whose vibration rate was 256 vibrations per second, 

 no response was observable either when the fork was suddenly 

 started or stopped, or during uninterrupted vibration. Tap- 

 ping lightly with a metal rod on a dish containing synaptulas 

 failed to call forth a response. Not even when the blow was 

 so severe that there was danger of breaking the dish could 

 any response be detected. Nevertheless a drop of water 

 falling through a distance of 5 cm. on to the surface of water 

 in a finger-bowl containing synaptulas was sufficient, in most 

 cases, to cause partial closing of the tentacles and a dropping 

 of the specimens to the bottom of the dish. A fall of a single 

 drop of water through less than 5 cm. produced, in general, no 

 effect. One individual did not respond until the drop had 

 fallen through 10 cm. 



These experiments show (1) that Synaptula hydriformis 

 does not respond to vibrations of relatively high frequency, (2) 

 that it is distinctly negatively geotropic, and (3) that the or- 

 gans which respond to the force of gravity are located at the 

 anterior end of the animal very near the calcareous ring. Now, 

 this is exactly the position where the statocysts are to be found. 

 These organs are so small and in such a location that the dif- 

 ficulties of removing them for experimental purposes are great. 

 Indirect evidence of the kind brought out in the above experi- 

 ments must therefore decide the question as to whether the 

 statocysts are truly organs of orientation. 



Since Clark ('98) has shown that the structure of these stato- 

 cysts meets the requirements for 'positional organs,' and since 

 my experiments show that the animal does orient to gravity, 

 and that the organs concerned with this response can lie only in 

 the region where statocysts are to be found, I think it safe to 



