BEACTIONS TO LIGHT IN LARVAE OF ASCIDIANS 183 



Garrey ('17, '18) maintains that in insects orientation is due 

 to difference in the tonus of Hke muscles in the legs on opposite 

 sides of the body. He holds, if I understand him correctly, 

 that the degree of tonus bears a specific quantitative relation to 

 the amount of light received by the eyes, such that if one eye re- 

 ceived more light than the other the tonus of the muscles in the 

 legs on one side will be proportionally greater or less than that of 

 like muscles in the legs on the opposite side, that difference in 

 tonus, by inducing difference in flexure, results in difference in the 

 length of the steps on opposite sides, thus causing the body to 

 turn until both eyes are equally illuminated and the animal is 

 oriented. He, consequently, maintains that, in the process of 

 orientation, light acts continuously in proportion to the amount 

 received by the photoreceptors. 



Certain facts recently discovered by Dolley ('20, '21) and 

 others discovered by myself ('20, not yet in press) seem to prove 

 conclusively, that light in the process of orientation in insects 

 (mourning-cloak butterfly, tachina fly, drone fly, robber fly) 

 does not act continuously; and that while the effect of light 

 on the tonus of the muscles may under certain conditions be a 

 factor, it is normally of little or no importance, for certain insects 

 turn toward the light even if the tonus of the muscles is such 

 that its effect tends to make them turn in the opposite direction. 

 This matter will be taken up in detail in a subsequent paper. 

 I should, however, like to add here that it is difficult to see how, in 

 a reversible system like a muscle, there could be a continuous 

 action of the stimulating agent such as to produce a specific 

 proportional relation in magnitude between the energy received 

 and the reaction produced, for in periodic reversible systems Uke 

 muscles, action in one direction cannot continue indefinitely. 

 There must be time for restitution. All parts of a muscle cannot 

 work in one direction all of the time. Either some parts must be 

 at work while others are at rest or there must be periods of rest in 

 the whole muscle, periods during which the stimulus has no 

 effect, refractory periods. And, if this is true, the magnitude 

 of the reaction in relation to the energy received from the stimu- 

 lating agent depends upon the rate at which the energy is re- 



