Cytoplasmic organization and symbiotic associations of Didymocyrtis tetrathalamus (Haeckel) (Spumellaria, Radiolaria)

Cytoplasmic organization and symbiotic associations of Didymocyrtis tetrath alamus (Haeckel) were examined. The most distinctive cytological feature is the presence of an extraordinary, non-living wall structure (termed endoca psular wall) that lies immediately inside the outer medullary shell. It clo sely resembles the capsular wall, which is the most diagnostic feature for radiolarian cytoplasmic organization, but is completely enclosed by the lat ter wall. The capsular wall lies inside the cortical shell, except in the p olar region where the wall is always outside the latticed skeleton of the b ilocular cortex. Based on these observations, and previously reported skele tal changes during maturation, five ontogenetic stages are described. In th e first stage, the test consists of the double medullary shell and the endo capsular 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 thi rd stage, forming the nearly completed hour-glass-shaped shell. In the four th and fifth stages, the cortical shell is completed and polar caps lying a bove each pole are deposited, associated with the expansion of the central capsule to partially encompass the bilocular shell. Two ultrastructually di fferent symbiotic dinoflagellates were observed in the extracapsular region around the cortical shell. One is identified as Amphidinium sp. and the ot her remains unnamed. They were never observed in the same host and appear t o mutually exclude one another. In addition to dinoflagellates, there were also bacterial endobionts within vacuoles.