22 INVERTEBRATE PHYSIOLOGY 



are correct, but we feel that they are the simplest we can make so far. 

 Actually, the behavior patterns may turn out to be even more complex. 



In general we have concerned ourselves with two types of behavior — 

 orientation to food and diurnal vertical migration. First, we shall take up 

 orientation to food, a series of behavior patterns mediated by the com- 

 pound eye in Daphnia magna, the large water flea, and in Eubranchipus, 

 the fairy shrimp. We have particularly concerned ourselves with those 

 zooplankters which filter suspended phytoplankton. I shall digress mo- 

 mentarily to point out how suspended phytoplankton alter certain im- 

 portant parameters of the environment. First, a suspension of unicellular 

 phytoplankton polarizes a vertical beam of light so the light which is 

 scattered horizontally is strongly polarized in the horizontal plane. We 

 presume the mechanism operating to polarize the light is a simple reflec- 

 tion from the many spherical or cylindrical surfaces presented by the 

 suspended phytoplankton or bacteria. Clearly, the sizes of particles are 

 wrong to produce polarization by Rayleigh scattering. Second, the pig- 

 ments present in many such plant cells absorb relatively more of the short 

 end of the visible spectrum than the long end. Third, the respiration and 

 photosynthesis of such plant cells cause a change in the pH of the water 

 suspending them. With this information, we are in a position to consider 

 the behavior patterns of those animals orienting to such suspensions as 

 food objects. 



Polarized Light Orientation 



Daphnids orient at right angles to a vertical beam of plane polarized light 

 by swimming back and forth in the beam at right angles to the plane of 

 vibration of the light. When the geometry of the light beam and of the 

 compound eye are examined, one sees that a vertical beam of light polarized 

 in the transverse plane of the animal enters only those facets of the com- 

 pound eye which are directed upward, forward, or backward. Fig. 1 shows 

 a highly diagrammatic geometrical analysis of this optical situation. Only 

 a fraction of the normal complement of cone lenses is shown and these 

 are directed upward in the anterior-posterior, forward in the dorsal- 

 ventral, and to the right in the lateral planes of the animal. The pigment 

 mass in which the cone lenses are embedded is not shown. Polarized light 

 vibrating in the transverse plane of the animal finds parallel internal re- 

 flecting surfaces only in those cone lenses ( 1 and 3 of Fig. 1 ) lying in or 

 near the plane defined by the dorsal-ventral-anterior-posterior lines. Note 

 that in this case the lateral lenses (2 of Fig. 1) do not permit internal re- 

 flection, since there is no surface parallel to the plane of vibration of the 

 light. Since polarized light can penetrate to the inner tip of only those cone 

 lenses presenting an internal surface parallel to the plane of vibration of 



