REACTIONS OF BIVALVE MOLLUSKS 317 



and decrease in light intensity (e.g., Mya) ; (b) those sensitive 

 to decrease only (e.g., Pecten) ; and (c) those sensitive neither 

 to decreases nor to increases (e.g., Cumingia). 



2. A perfect correlation was found for the animals studied, 

 between sensitivity and the presence of pigment in the epithelium 

 of the sensitive areas. 



3. In the case of Mya arenaria, and some others, reaction to 

 increases resulted in a withdrawal of the siphon tubes, while 

 reaction to decreases resulted in a closure of siphonal openings, 

 often unaccompanied by any withdrawal of the tube. * 



4. Experiments on Pecten, which is sensitive only to decreases 

 (with exceptions noted in the text), demonstrated that: (a) 

 vigorous reactions by closing the valves may be obtained when 

 a white card is moved over a black background, even when 

 the movement involves an increase in the intensity of the light 

 falling on the animals. This is believed to show that the Pecten 

 eye may form an image, (b) The smallest white card the move- 

 ment of which produced a response at a distance of 35 centi- 

 meters was 15 millimeters square, (c) When bright illumina- 

 tion was employed reactions occurred on decrease in intensity 

 even if only two eyes were affected. Response in this case 

 was local. 



V. LIST OF PAPERS CITED 



Bauer, V. Zur Kenntnis der Lebensweise von Pecten jacobaens L. in beson- 



1912. deren iiber die Funktion der Augen. Zool. Jahrb., Abt. f. allg. Zool., <, 



Bd. 33, pp. 127-149. 



Beer, T. Ueber primitive Sehorgane. Wiener klin. Wochenschr., Jahrg. 14, 

 1901. pp. 255-261, 285-293, 314-324, 30 Textfig. 



Dakin, W. J. Pecten. Liverpool Marine Biol. Com., Mem., XVII, pp. viii + 136, 



1909. 9 pi., 4 text fig. 



1910a. The Visceral Ganglion of Pecten, with some Notes on the Physiology 

 of the Nervous Svstem. etc. Milteil. Zool. Sta. Neapel, Bd. 20, 

 pp. 1-40, pi. 1-2, 1 text fig. 

 1910fc. The Eye of Pecten. Quart. Jour. Mia. Sci., Vol. 55, pp. 49-112, pi. 

 6-7, 2 text fig. 

 Drew, G. A. The Habits, Anatomy and Embryology of the Giant Scallop, Pecten 

 1906. toiuicostatiis Mighels. Univ. of Maine Studies, No. 6, pp. 1-71, 

 pi. 1-17. 

 Garner, R. On the Nervous Svstem of Molluscous Animals. Trans. Linn. 



1837. Soc, Vol. 17, pp. 485-501, tab. 24-27. 

 Grube, A. E. Ueber Augen bei Muscheln. Arch. f. Anal., Physiol., und wiss. 



1840. Med., pp. 24-35. 

 Hensen, V. Ueber das Auge einiger Cephalopoden. Zeilschr. f. wiss. Zool., Bd. 



1865. 15, pp. 155-242, Taf. 12-21. 

 Hess. C. Neue Untersuchungen uber den Lichtsinn bei wirbellosen Tieren. Arch. 



1910. ges. Physiol., Bd. 136, pp. 282-367, 6 Taf. 



