PHYSIOLOGY OF SYNAPTULA HYDRIFORMIS 353 



current on the tentacles caused shortening of the body only, 

 while a still stronger current caused, in addition to this reaction, 

 a partial withdrawal of the tentacles and consequent dropping 

 to the bottom of the dish. Stronger currents on the mid-body 

 or the posterior end called forth the same reactions as the weaker 

 currents, though more pronounced. When an individual was 

 crawling along the bottom of the dish, no response to any ordi- 

 nary current was shown, no matter what portion of the body 

 received the stream. 



In these experiments with water currents, the tentacles seem 

 to behave contrary to the general rule, since they appear to be 

 less sensitive than the other parts of the body. But this becomes 

 intelligible when one considers that the constant motion of the 

 tentacles must have the same effect as gentle currents impinging 

 on them, and must also cause small currents to continually move 

 over the anterior end of the animal, while the other parts of the 

 body are not subjected to these currents. Once the threshold of 

 stimulation is reached, there is a much more vigorous and com- 

 plete reaction to a strong current on the tentacles than on other 

 parts, a fact which brings these results into line with the effects of 

 other mechanical agents. 



2. The effect of gravity and of vibrations 



a. Historical. Experiments like those of Kreidl ('93) on Palae- 

 mon have shown that those structures which earlier investigators 

 called 'otocysts/ to which we now apply the term 'statocyst,' are, 

 in many of the invertebrates, organs of orientation, since they 

 react to the force of gravity and serve to keep the animal in 

 equilibrium. Reasoning from the well established cases, it is 

 only natural to infer that similar structures, when found in 

 other animals, must serve the same purpose. Such structures 

 have been described in synaptids by Johannes Miiller ('50b, p. 

 226), Baur ('64, p. 46), and others. Hamann ('84, p. 25) con- 

 sidered them larval structures only, and therefore functionless in 

 the adult. They were, however, given the name 'Gehorblaschen, ' 

 vv'hich implies their relation to the vertebrate organ of hearing. 



