44 COMPARATIVE PHYSIOLOGY 



cloth and two of porcelain. For one of the former and two of 

 the latter we provide covers of white or black paper pierced 

 with several pin-holes. As the animals are caught they are 

 distributed between the four jars. When brought into the 

 laboratory and examined, the animals in " open " porcelain 

 and in " pin-hole " porcelain are found to be in the fully 

 contracted phase. Now the light intensity in the open jars 

 is far higher than that to which the animals are subjected before 

 capture, and the intensity in the pin-hole jars is probably far 

 lower. Any effect which a raised or lowered light intensity 

 might produce is swamped by the background effect. The 

 latter is of such a nature that an absorbing (black) background 

 induces expansion, and a scattering (white) background produces 

 complete contraction, and these effects are produced in the 

 faintest (pin-hole) Hght." 



Apart from the intensity of illumination, an essential 

 difference between the condition of a shrimp exposed to black 

 (absorbing) or white (scattering) background is to be sought in 

 the direction of the incident rays impinging upon the eyes, which 

 as we have seen are the receptor organs involved in the back- 

 ground response. When placed against a mirror background 

 which reflects incident light mainly in one direction, the animals 

 display a state of partial expansion and not the complete 

 contraction characteristic of exposure to a white surface which 

 scatters light rays in all directions. It would appear, then, to 

 quote the same authors once again, that *' it is, in some way or 

 other, the ratio direct/scattered light which determines " the 

 background reaction. This interpretation impHes a dorsi- 

 ventral differentiation of the photoreceptive elements of the 

 eye. But whatever be the nature of this dorsiventrality, it 

 does not reside in any permanent structural arrangement of 

 the retinal elements, for illumination of the animal from below 

 against a white surface calls forth the characteristic transparency 

 of the contracted phase. 



Keeble and Gamble have, in addition to the foregoing 

 analysis of the more rapid responses to light, darkness, and 

 light or dull backgrounds, attempted to analyse the factors 

 involved in the production of the different colour forms of 



