Cytoplasmic organization and symbiotic associations of Didymocyrtis tetrathalamus (Haeckel) were examined. The most distinctive cytological feature is the presence of an extraordinary, non-living wall structure (termed endocapsular wall) that lies immediately inside the outer medullary shell. It closely resembles the capsular wall, which is the most diagnostic feature for radiolarian cytoplasmic organization, but is completely enclosed by the latter wall. The capsular wall lies inside the cortical shell, except in the polar region where the wall is always outside the latticed skeleton of the bilocular cortex. Based on these observations, and previously reported skeletal changes during maturation, five ontogenetic stages are described. In the first stage, the test consists of the double medullary shell and the endocapsular wall may serve as the "first capsular wall." In the second stage, the equatorial girdle surrounding the medullary shell and the capsular wall is formed. A major portion of the cortical shell is constructed in the third stage, forming the nearly completed hour-glass-shaped shell. In the fourth and fifth stages, the cortical shell is completed and polar caps lying above each pole are deposited, associated with the expansion of the central capsule to partially encompass the bilocular shell. Two ultrastructually different symbiotic dinoflagellates were observed in the extracapsular region around the cortical shell. One is identified as Amphidinium sp. and the other remains unnamed. They were never observed in the same host and appear to mutually exclude one another. In addition to dinoflagellates, there were also bacterial endobionts within vacuoles.
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