MOVEMENTS, ETC., OF FRESH-WATER FLANARIANS. 567 



very active pass on tlii-ougli the region, but necessarily iu 

 course of some time several individuals will have stopped, 

 and a group will have been begun. When once started 

 another reaction apparently enters to assist in enlarging it. 

 This reaction appears to be due to some chemical substance, 

 and belono-s to the class o£ reactions which Ene'elmann has 

 suggested should be called "kinetic," in this case chemo- 

 kinesis. It would appear that planarians excrete or secrete 

 some chemical substance towards which they are themselves 

 positively chemotactic, and which also causes them to come 

 to rest. When several individuals remain quiet in a small 

 area this substance, of course, accumulates aud affects other 

 individuals passing. That some such a substance is sepa- 

 rated from the bodies of the animals is evidenced by two 

 phenomena. First, in the case of the food reaction, which 

 will be taken up in detail later, it is found that after one or 

 two individuals have attached themselves to a piece of food 

 material and begun feeding the mass of food and planarians 

 is a much more effective stimulus to positive chemotaxis than 

 is the same food substance alone, even though it may have 

 remained in the water a greater length of time. The "zone 

 of influence " (vide infra, p. 626) of the food and feeding- 

 individuals together is much wider than that of the food 

 alone. Specimens are affected at a greater distance from 

 the food and react more sharply. As a result of this, dense 

 aggregations of planarians will be foi-med in a comparatively 

 short time after the first two or three individuals have found 

 a bit of food. As there is no reason to suppose that the 

 action of the food itself is different in the two cases, we 

 must conclude that the greater effectiveness of the food and 

 feeding individuals is duo to some chemical substance coming 

 fi-om the organisms themselves. 



The second line of evidence for the existence of a reaction 

 to a chemical in the formation of collections is found in the 

 behaviour of specimens coming near a group of individuals 

 resting on the bottom of a dish. When some distance away 

 from the outer boundary of such a group a gliding animal 



