Symbiosis with unicellular algae, most usually with the so-called brown zooxanthellae or green zoochlorellae, is widespread among marine invertebrates. It is also common, and has been more extensively studied, in fresh-water invertebrates including Protozoa such as Paramecium bursaria, Porifera such as Spongilla lacustris or Ephydatia Jluviatilis, and Turbellaria such as Dalyellia viridis. Buchner (1953) has summarized and discussed the literature, largely descriptive, up to 1953, while Yonge (1944) and Fritsch (1952) have reviewed more functional aspects of this type of symbiotic relationship. Reference should be made to these papers for literature dealing with fresh-water animals, many of which are associated with species of Chlorella. ORIGIN OF THE ASSOCIATION As noted (Yonge, 1934), the presence of symbiotic algae is almost always associated with the occurrence of intracellular digestion—i.e., in Protozoa, Porifera Coelenterata, Turbellaria, and Mollusca (other than Cephalopoda). There are isolated cases of algae occurring in the tissues of animals which certainly digest extracellularly—i.e., some Polyzoa, Annelida, Echinodermata, and Ascidia. But the only well-documented cases are those described by Berkeley (1930a) of infection in the Chaetopteridae (Annelida) and by Smith (1935) of association between compound ascidians and algae. In the former case, the algae are green, only from 1 to 3μ in diameter, and are without a cellulose wall, or indeed a cell wall of any kind, have no chromatophore or pyrenoid and apparently no algal pigment. Berkeley regarded them as flagellates belonging to the Chrysocapsinae but was unable to determine whether they were symbionts or parasites. Possibly they...