126 Causes and Course of Organic Evolution 



summed up in Jenning's recent statement (p. 242): "In many 

 free STvdmming Rotifera the cliief methods of movement and 

 reaction are similar even in details to those of the free-swim- 

 ming Infusoria. Like the Infusoria, these rotifers swim by 

 means of cilia, revolve on the long axis, and swerve toward 

 one side (usually dorsal) as they progress." 



But as indicating an increased tendency to localization of 

 rotatory sensation at the anterior end of the organism, as 

 compared T\dth the diffuse sensation in infusors, he adds: "It 

 is interesting to observe that in the Rotifera, o^dng to the 

 concentration of the cilia at one end of the animal, there is 

 no such incoherence and lack of coordination, in the reaction 

 to the constant electric current, as is found in Infusoria. The 

 rotifer {Anurcea cochlearis) becomes oriented with anterior end 

 to the cathode by the same method as in reactions to light and 

 other agents." 



The researches of Pearl on the flatworm Planaria, also of 

 several authors on the earthworm and leech, suggest that, 

 with increasing complexity and correlation of the nervous 

 system, the circumnutating or rotatory movements tend to 

 become somewhat restricted to an ellipse or even side to side 

 movement. But these differ little from zigzag ellipses or circles 

 that our students often record on glass for the groTN-ing apices 

 of plant stems and roots. The movements of the head in 

 caterpillars, of the antennae and eyes in lobsters, as well as 

 other related actions, seem all to form a consecutive whole, 

 that is explained on the basis of the existence, from lower 

 nucleate plants to highest animals, of a rotatory sense. This 

 in vertebrates becomes evidently so poised or equilibrated to 

 average environments that it is only when the equilibrating 

 nerves are severed that want of correlated movement in a 

 straight line becomes evident. But it suggests also that such 

 movement in a straight line has resulted from evolving con- 

 densation of the i)riniitively spiral rotatory course. 



But, as discussions on circumnutation in plants and as the 

 constant intimate connection between the geotactic and gyro- 

 tactic sense-areas in animals demonstrate, there seems to be a 



