STUDIES ON REACTIONS TO STIMULI. 229 



4. Anterior one third. This is very rapid in its movements, 

 and circles almost continuously to the right. In the free water 

 it revolves in the usual way. When stimulated, it turns to the 

 right, but docs not jerk back. 



5. Middle one half. - -This was obtained by cutting off the 

 anterior and posterior fourths of the body. Its movements and 

 reactions are essentially similar to those of the uninjured specimen. 



6. Right and left halves.- -These were obtained by splitting 

 Stylonyclda lengthwise. They move and react (after the shock 

 effects have ceased) in nearly the normal manner. Thus, when 

 stimulated mechanically or chemically, they jerk backward, and 

 the right half turns toward its right (uninjured) side, while the left 

 half likewise turns toward its right (injured) side. 



A specimen was cut lengthwise in such a way that the left 

 piece was a strip comprising about one third the body. This 

 moved somewhat irregularly, but reacted in the same manner as 

 the left one half just described --though it evidently bore no 

 part of the peristome. 



7. A small, left posterior corner, comprising less than one 

 fourth the animal, reacted, after the effects of the shock had 

 ceased, in the usual way by backing and turning toward the 

 right. 



Thus, on the whole, pieces of Stylonyclda from any part of 

 the body, if amounting in size to as much as one fourth to one 

 half of the animal, move and react essentially like the entire speci- 

 men. In very small or very irregular pieces the movements be- 

 come irregular, as might be expected. 



Similar results were obtained with OxytricJia falla.v, and with 

 a number of unidentified Hypotricha, though the smaller size 

 makes most other species less favorable for such experimentation 

 than is Stylonyclda. 



II. Stciitor cceruleus.--\\\ Stcntor the oral (or adoral) cilia or 

 membranellae form nearly a circle at the broad anterior end of 

 the body. Stcntor, when free swimming, follows a spiral path, 

 revolving from right over to left. It reacts to chemical and me- 

 chanical stimuli by backing a little, then turning toward the right 

 aboral side. It is evident to observation that the adoral cilia play 

 a large part in the turning ; from their large size and their posi- 



