382 



Comparative Aniynal Physiology 



Diffuse sensitivity to light exists in several phyla of animals. Protozoa, 

 such as Amoeba, Stentor, and Peranema, respond to changing intensity of 

 illumination, without possessing any visible light-sensitive structures. Amoeba 

 proteus responds to intense illumination of any part of its body by a 

 cessation of movement resulting from gelation in the illuminated region 

 (Fig. 100). If the whole body is illuminated, locomotion is greatly retarded 

 or stopped, because of gelation at the tips of the pseudopods (Fig. 100). How- 

 ever, if illumination is continued, locomotion gradually increases and be- 

 comes normal in 6 to 30 minutes, depending on the intensity.^-^ In the 

 absence of light-sensitive organelles and in view of the periodically fluid 

 condition of amoeba protoplasm, there is probably a direct effect of light 

 on sol-gel reversibility (Ch. 17). 



Diffuse sensitivity to light also exists among the echinoderms. In these 

 animals the pigment cells of the integument are believed to be sensitive to 



Fig. 100. Responses of Amoeba to localized illumination (rectangular areas). Arrows 

 show the direction of protoplasmic streaming. Broken lines in B, C, and D indicate 

 position of animal shortly after illumination. F shows the position and direction of 

 streaming assumed by E after illumination, v, Contractile vacuole; n, nucleus. From 



Mast.^"^ 



light. Von UexkiilP^'^ noted the Hght sensitivity of the violet spicules 

 around the anal orifice of the sea urchin, Centroste-phanus longispinus. He 

 extracted the purple pigment with alcohol, and found that it decomposed 

 when exposed to light. The entire surface of Holothuria is photosensitive, 

 and two pigments are present, one of which is a fluorescent greenish yel- 

 iQyy 25, 2« yfig reaction of three species of Holothuria varies with the amount 

 of this pigment present, and young specimens of H. captiva which have little 

 pigment are less reactive than the heavily pigmented adults. Therefore, the 

 greenish yellow fluorescent pigment is believed to be either the photosensitive 

 pigment or a sensitizer. 



Jennings^^^ in his monograph on the behavior of the starfish described 

 reactions to light. In general, if the direction of illumination is such that 



