MOVEMENTS, ET(!., OF FIlESH-WATEIi PLANAKIANS. 617 



worm only ufier a uuw half lias been regenerated along its 

 cut edge. Tlie various stages in the change from the 

 reactions as a half-worm to those as a whole worm can be 

 followed step by step as regeneration proceeds. The new tissue 

 formed along the cut edge very quickly takes on some of the 

 functions of a side. Wiien only a narrow strip has been 

 formed it serves for the reception of the stimulus, and hence 

 stops the reaction of the opposite side, as in the experiment 

 last discussed. To make the meaning more clear, reference 

 may be made to diagrams h and c of Fig. 2o. In h is repre- 

 sented, in a straightened position, the half B of a normal worm 



B 



A c 



Fig. 23. — Diagrams to show the relations of the halves of the body 

 of Planaria to the reception of st.imuli, ant] the reactions 

 tliereto. See account in text. (Tlie pharynx is omitted for 

 the sake of clearness.) 



immediately after being separated from the other half, while 

 c represents the same half after regeneration has begun and 

 a strip of new tissue has been formed down the cut edge. 

 Now stimulation of the cut edge of h causes the anterior end 

 of the piece to turn towards the stimulus, i. c. to give its 

 own proper negative reaction (cf. experiment given above on 

 slitting anterior end). This is because in this case it is 

 side B that is stimulated, although along its inner edge. 

 Stimulation along the right-liand edge of c does not cause 

 the turning towards the stimulus, because in order that this 



