XVII 



MOTOR RESPONSES TO LIGHT IN THE INVERTEBRATE 



ANIMALS 



S. 0. Mast 



The Zoological Laboratory of the Johns Hopkins University, Baltimore 



Rhizopods. Flagellates. Ciliates. Colonial forms. Coelenterates. Worms: Tuhic- 

 olous annelids — Arenicola larvae — Earthworms — Turbellaria. Molluscs: Mya — Pecten 

 — Helix — Agriolimax. Arthropods: Daphnia. Insects. References. 



A motor response is often defined as a change in rate or in direction of 

 movement of an organism or a portion of it. This definition is accepted 

 in the following review. The space allotted is, however, so limited that 

 it is not expedient to attempt to consider even superficially these responses 

 in all the species of invertebrates. I shall therefore omit several groups 

 altogether and select for discussion only a few" species in the remaining 

 groups, i.e., those which have been most thoroughly studied. 



RHIZOPODS 



Response to light is fairly common among the rhizopods and the 

 results in hand indicate that it is essentially the same in all. It has, 

 however, been thoroughly investigated in only one species. Amoeba 

 proteus. 



Amoeba proteus consists of a thin elastic outer membrane, the plas- 

 malemma, a central relatively fluid granular mass, the plasmasol, sur- 

 rounded by a relatively solid granular layer, the plasmagel, and a thin 

 fluid hyaline layer between the plasmagel and the plasmalemma. 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 trans- 

 formed 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 

 regions. Response in this form is due to changes in the elastic strength 

 of localized regions in the plasmagel, or in the rate of transformation of 

 plasmasol into plasmagel and vice versa, or in the attachment to the 

 substratum (Mast, 148a, 151a, 153). 



There are two types of responses to light in Amoeba, one of which is 

 primarily correlated with the magnitude of change in intensity, and the 

 other with the rate of change in intensity. The latter is closely correlated 

 with adaptation; the former is not. 



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