580 journal of Comparative Neurology and Psychology. 



grouped as primary and secondary. The term secondary is applied 

 to those tropic reactions occurring as a final phase of a complex 

 reaction. 



The Two-phased Character of the Reaction Types. — The weakest 

 reaction type is single-phased unless the checking of movement and the 

 commonly occurring pause after the stimulus is counted as standing 

 for an initial phase. All the others include an initial, shrinking move- 

 ment and a concluding negative turn. Types III and IV include also 

 some intermediate movements. They are nevertheless properly classed 

 as two-phased. The backward locomotion in type III is only an inci- 

 dent during the recovery of extensibility by the anterior end. In type 

 IV the rolling is due to the continuation of the power of extension in 

 the middle during the period in Avhich the more irritable ends are 

 recovering from the effects of the stimulus sufficiently to extend. In 

 type V these intermediate forms of movement are cut out of the reac- 

 tion owing to the fact that the stimulus has made itself more generally 

 felt and the natural outlet for extension movements is the anterior 

 end, when the whole body is involved in the inhibition due to shrink- 

 ing. The possibility of extension of the posterior end and the mid- 

 dle of the worm and the consequent locomotion in types III and IV 

 are then to be regarded as due to an incomplete inclusion of these 

 regions in the inhibition of extension under submaximal stimulation. 

 With a maximal stimulation they may be inhibited from extension 

 more thoroughly than the anterior end, which is simply the result 

 of the fact that the anterior is the best adapted for extension move- 

 ments. 



"Unterschiedsempfiiidlich" respotises. — The distinction between 

 unterschiedsempfindlich and tropic reactions originally made by 

 Loeb in the example of certain tube-dwelling annelids may be quoted 

 here. Serpula uncinata, a tubicolous annelid, bends toward the light 

 and also withdraws suddenly into its tube from the stimulus of a 

 shadow cast upon the oral end. There is a family resemblance 

 between the reactions of this annelid and the earthworm, except for 

 their opposite forms of heliotropism. The earthworm turns away 

 from all but the weakest light (Adams, '03) and also exhibits the 

 familiar reaction of withdrawing into its burrow on illumination of 



