272 



MOTOR RESPONSES 



Responses to Light 



A. RHIZOPODS 



Many of the rhizopods respond to light and some orient if they are 

 exposed in a beam of light. Some of the responses are correlated with 

 the rate of change in intensity, others are not. They are fundamentally 

 the same in all the species which have been investigated, but they have 

 been more thoroughly studied in Amoeba proteus than in any of the 

 other species. The following considerations therefore refer largely to 

 this species. 



A. proteus consists of a thin elastic outer membrane, the plasmalemma, 

 a central relatively fluid granular mass, the plasmasol, surrounded by a 



G 



- 0" ' w 



Figure 93. Camera sketch of horizontal optical section of Amoeba proteus. Ps, plas- 

 masol ; Pg, plasmagel ; PI, plasmalemma; HC, hyaline cap; Pgs, plasmagel sheet; L, 

 Liquid layer; S, region of solation; G, region of gelation. (After Mast, 1926.) 



relatively solid granular layer, the plasmagel, and a thin fluid hyaline 

 layer between the plasmagel and the plasmalemma (Fig. 93). During 

 locomotion, the plasmalemma is attached to the substratum and to the 

 adjoining plasmagel; the plasmagel at the posterior end is transformed 

 into plasmasol, which flows forward to the anterior end and is there 

 transformed into plasmagel. The forward flow of the plasmasol is due 

 to contraction of the plasmagel at the posterior end and expansion at the 

 anterior end, owing to difference in its elastic strength in these two re- 

 gions. In some species surface tension is probably also involved in locomo- 



