1. The structure and development of a new quadricapsulate species of Myxosporidian (Agarella gracilis) belonging to the family Chloromyxidae and found in the testis of the Dipnoan fish Lepidosiren paradoxa is described.
2. Attention is drawn to the interrelationships of parasitic Chloromyxidae correlated with interrelationships between their hosts in the groups Dipnoi, Elasmobranchii, and Amphibia.
3. The life-cycle in Myxosporidia is discussed, especially the origin and relationships of the spore-forming nuclei and cells in the pansporoblast, and it is suggested that physiologically the spore of a Myxosporidian is a multicellular unit analogous with the infusoriform embryo of the Mesozoan Dicyema, although the Myxosporidia exhibit Rhizopodan relationships and Mesozoa are probably derived from ciliated ancestors.
4. Although it is not suggested that the Myxosporidia represent a direct link between Protozoa and Metazoa, they may indicate a physiological reason for the origin of a soma, as a protective accessory to germ-cells. The Mesozoa show a similar process occurring in a non-related line of organisms.