Yerkes, Aiiunal Behavior. 237 



lowest stimulation (negative phototaxis, phototaxie negatif). When theHedista 

 have undergone hydration, light inhibits muscular movements and the reactions 

 are of the opposite sign (No. 28, p. 63). 



In Hedista "several systems of muscles may enter into an action and determine 

 sinuous undulatory movements of different kinds. Sometimes lateral undulations of 

 slight length and amplitude determine in the anterior region ot the body, the 

 balancing of the parapodia and consequently progress; sometimes other lateral 

 undulations, in number from eight to two and ot greater and greater amplitude, 

 run from behind forward over the length of the body, and determine the swimming 

 movement, if the amplitude be sufficiently great. Sometimes sagittal waves of 

 medium length and amplitude, determine, if the annelid is in the sand, the circu- 

 lation of the water. 



"The light received by the eyes has a manifest influence upon these diverse 

 undulatory movements. The suppression of the illumination of the whole of the 

 eyes (obscurity, section of the head) brings about the supression of the lateral waves 

 and the exaggeration of the sagittal ones, etc." (No. 25, p. 173). 



Paper No. 31 is the author's second memoir relative to "phototactisme"' and 

 "phototropisme." It describes the results of experiments with Crustacea. The 

 trend of results may be gathered from the following summary statements. The 

 phenomena of the tropisms are considerably more complex in the Crustacea than 

 in the worms and molluscs. The tropic (troptques) actions of light are the results 

 of a series of rotatory movements of several diff^erent kinds. All of these rotations 

 ire due to the influence of illuminated surfaces, and they can be easily determined 

 by the use of black and white screens. The phenomena observed, in reality, 

 depend upon the illumination of the surfaces of the eyes. 



I wish now to call attention to Bohn 's discussion of tropism theories and the 

 bearing of his results on the same. After pointing out several inconsistencies in 

 the orthodox theory, he gives a critique of Jennings' recent theory, mentions the 

 sad confusion of terms and finally concludes that it is better worth our while to 

 study the form and mechanism of reactions, than to discuss a terminology for 

 what is very superficially known (No. 28, p. 79, et seq.). 



In their relation to our conception of the nature of phototropism, the following 

 facts discovered by Bohn are of unquestionable importance: (l) In certain 

 annelids and molluscs the light received by the eye exercises an excitory or 

 inhibitory influence, according to the state of hydration of the animal, on the 

 muscles of the same side of the body. (2) When an animal is subjected for a 

 sufficiently long time to the influence of light, that influence gradually changes 

 from excitation to inhibition; fatigue for light manifests itself. (3) While the 

 light is still an excitant, a slight diminution in its intensity produces an arrest 

 of locomotor activity. (4) Similarly, while the light is an excitant, a slight de- 

 crease in the illumination of one eye in relation to the other induces asymmetrical 

 action of the muscles of the two sides of the body, and therefore a turning move- 

 ment which results in bringing the eyes into regions of equal illumination (No. 28, 



These and similar facts indicate that Jennings' theory does not cover all 

 phototropic reactions. Jennings argues, from the results of his studies of uni- 

 cellular organisms, that in the tropisms, orientation is a secondary phenomenon, 



