DIURNAL MIGRATION OF PLANKTON 



23 



the light, then clearly the geometry of the compound eye and the plane of 

 vibration of the light control which lenses shall have light penetrating to 



Fig. 1. A sixty-degree projection of only three cone lenses of 

 a daphnid compound eye showing various light paths. At A light 

 is nonpolarized. At B the light vibrates in the transverse plane 

 of the daphnid. At C the light vibrates in the dorsal-ventral plane 

 of the daphnid. At D a surface parallel to the light vibrating in 

 the transverse plane of the animal permits reflection of this light 

 to photoreceptors at the tip of the cone. At E the surface is vir- 

 tually perpendicular to the plane of vibration of the light and 

 hence the light is refracted through the lens to the outside again 

 without reflection to the inner tip of the cone. 



the tip of the cone. The important exceptions to this statement are those 

 lenses directly facing the light source. 



For a horizontal beam of polarized light vibrating in the transverse 

 plane of the eye, the lenses permitting penetration of light to the tip are 

 those directed anteriorly, dorsally, and ventrally. The cone lenses directed 

 laterally do not present parallel surfaces for internal reflection and there- 

 fore are not illuminated by a horizontal beam of polarized light vibrating 

 in the transverse plane of the animal. Operationally the behavior pattern 

 in response to polarized light would appear to orient the animal so that 

 the light can enter the ventral, anterior, and dorsal lenses only and would 

 overtly exclude light from the lateral lenses. 



