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This study examines the photobehaviors of the acoel Convolutriloba retrogemma, the factors affecting these behaviors, their regulatory functions, and how they affect or are affected by the acoel’s algal endosymbiont. The first behavior detailed is a step-up, photophobic response to sudden increases in light. The variable response is blue-light-mediated and triggered by visual, photic stimuli. The second behavior detailed is a phototactic-photoaccumulative behavior responsible for observed mass basking formations. The photoaccumulative behavior is regulated by the photosynthetic activity of the algal endosymbiont. It is not exhibited by aposymbiotic animals. The effects of holozoic starvation are examined, particularly as they apply to host phototactic and photoaccumulative behavior. The data show that contrary to expected behavior, acoels denied prey for over 20 days do not seek out areas of high intensity light, but instead retreat to areas of lower intensity or to shadows. The results of this study support a hypothesis that the basking behaviors of these acoels serve as methods of photoregulating their algal endosymbionts. The results further suggest that starved acoels have diminished capabilities for processing translocated algal photosynthates and that under high-light conditions a build-up of these compounds results in hyposmotic stress in the animals. It is suggested that the photoregulatory basking behaviors of C. retrogemma may function as a method of host osmoregulation and that intercellular change in osmotic pressure resulting from photosynthesis is the photoregulatory stimulus. The term “photosmoregulation” is offered to describe the process. Presented is a novel method for detecting the movements of photosynthate in vivo in C. retrogemma utilizing differential weight change in animals subjected to light and dark treatments without holozoic feeding. Though successful in yielding desired results, the method was decidedly labor-intensive and an alternative method is suggested. Also presented is a refined method for the separation of algal symbionts from host tissue, and a method for determining accurate wet-weight of this and other soft-bodied, invertebrate species. Lastly, contained herein is a monograph describing a new species of acoel, Convolutriloba macropyga.