MOTOR RESPONSES IN INVERTEBRATES 593 



ing the minimum latent period, is inversely proportional to the intensity 

 i, that is, ti is constant and the response is therefore in accord with the 

 Bunsen-Roscoe law. 



With a given intensity, the exposure period is practically independent 

 of temperature, but the latent period varies inversely with the tempera- 

 ture. The value of m in the Arrhenius equation is 19,700 for Mya, 

 18,300 for Pholas, and 16,200 for Ciona. 



During dark-adaptation the reaction time for constant intensity 

 increases rapidly at first, and then more slowly until it reaches a minimum, 

 after which it is constant. The rate of dark-adaptation varies directly 

 with temperature. It is a slow process, requiring half an hour or more. 



The maximum stimulating efficiency in the visible spectrum is at 

 5000 A for Mya and at 5500 A for Pholas. From these points it decreases 

 rapidly in either direction. 



During exposure to light, a substance S in the photoreceptors, is 

 broken down into two or more substances (P, A,C). One of these, C, is a 

 catalyst, which during the latent period acts on another substance L and 

 produces still another substance T. When this substance becomes 

 sufficiently concentrated it stimulates the nerves and causes the siphon 

 to contract. In darkness, P and A unite to form S. 



The first reaction is a reversible photochemical reaction independent 

 of temperature. The second is dependent upon temperature. It is not 

 a photochemical reaction. It is probably an oxidation reaction catalyzed 

 by iron. 



The processes involved are identical in the three forms investigated, 

 but the substances involved differ. 



Pieron (185) and Folger (66) repeated a portion of Hecht's observa- 

 tions on Mya. Both maintain that the results obtained do not support 

 Hecht's contention that the energy ti, necessary to induce retraction of 

 the siphon, is constant for different intensities. In view of these con- 

 tradictory contentions it is highly desirable to have the observations on 

 which they are based, repeated and extended. 



Hecht and Wolf (95) compared the stimulating efficiency for Mya, of 

 intermittent light with a flash frequency of 130 per sec, with that of 

 continuous light and found it to be the same. They conclude (page 388), 

 "the results demonstrate the validity of Talbot's law for Mya." 



PECTEN 



Pecten is remarkable for its numerous well developed eyes on the edge 

 of the mantle. It responds vigorously to rapid reduction in light by 

 closing its valves but it does not respond to increase in light (Wenrich, 

 219). There has been much discussion but very little experimental 

 work concerning the function of the eyes. Wenrich maintains that if a 

 highly illuminated small white cardboard is moved over a large vertical 



