§4] PHOTOTAXIS AND PHOTOPATHY 207 



d. The Mechanics of Response to Light. — Under this head 

 I shall speak of the part of the protoplasmic body most sensitive 

 to light, of the immediate effect of the light, and of the cause 

 of this immediate effect. 



There can be no question that radiant energy with rapidly 

 vibrating waves produces upon all protoplasm a profound effect. 

 The question arises, however, to what extent in organisms a 

 special kind of protoplasm is differentiated for the reception of 

 rays which result in a discharge of the locomotor response. 

 Certainly such a differentiated protoplasm can hardly be con- 

 sidered necessary to the discharge of such a response, since 

 there is no morphological evidence of its existence in the 

 responsive amoeba. However, even in the swarm-spores and 

 Flagellata such a specialized protoplasm is clearly indicated. 

 Thus Engelmann ('82% p. 396) found that when a dark band 

 fell across the body of a swimming Euglena, no reaction occurred 

 so long as the hinder chlorophyllaceous part alone was shaded. 

 When, however, the clear area at the base of the flagellum was 

 shaded, a marked reaction occurred. Here, near the pigment 

 spot, if not at it, is the specialized light-perceiving protoplasm. 



Similarly specialized protoplasm occurs extensively in the 

 higher groups in the form of retinas ; but there is much evi- 

 dence that in many eyeless Metazoa the whole surface contains 

 such light-perceiving substances. This is well known to be the 

 case in the earthworm (cf. Hesse, '96). According to Dubois, 

 ('89, p. 233), the siphon of the boring mussel Pholas dactylus 

 contracts at the least -variation of light intensity upon the skin. 

 Similarly, other Lamellibranchia (Ostea, Unio, Venus) close 

 their valves (Nagel, '96, p. 58). Blinded Helix are said by 

 WiLLEM ('91, p. 248) and Nagel ('96, p. 19) to be similarly 

 sensitive. The lamellibranch Psammobia, the blind Proteus 

 anguiiius, and the blinded Triton cristatus are irritated by rays 

 of light, especially the blue rays, falling upon the skin (Nagel, 

 '96, p. 22 ; Grabee, '88, p. 238; Dubois, '90, p. 358). Thus, 

 in many Metazoa, protoplasm sensitive to light is of widespread 

 occurrence, outside of the retina.* 



The immediate visible effect of light upon the organism differs 



* For an extended list of cases of such dermatoptio reaction, see Nagel, '96. 



