The Fine Structure of the Colonial Colorless Flagellates Rhipidodendron splendidum Stein and Spongomonas uvella Stein with Special Reference to the Flagellar Apparatus

SYNOPSIS. The cell structure of the colorless colonial flagellates Rhipidodendron splendidum Stein and Spongomonas uvella Stein has been examined by electron microscopy to assertain their phylogenetic affinities. The cylindrical cells of R. splendidum have 2 smooth flagella of equal length, an asymmetrical flagellar pocket supported by microtubules, and a curved pit between the latter and an anterior prolongation of the cell. The matrix of the branched tubes comprising the fanshaped colony is composed largely of dense spherules which are produced in special cytoplasmic vesicles some of which contain symbiotic bacteria. The anterior nucleus has a flattened sac pressed closely against its posterior end. The sac has a long tail extending deep into the cytoplasm, a single bounding membrane and homogeneous contents. Several types of vesicle are described but food vacuoles and contractile vacuoles could not be positively identified. A kinetoplast mitochondrion is not present. The various cross-banded, microtubular and amorphous components of the complex and highly asymmetrical flagellar root system are described in detail and a 3-dimensional reconstruction is provided.
The ovoid cells of S. uvella are basically similar to those of R. splendidum ; though a nuclear sac is missing, there are some detailed differences in the structure of the flagellar root system, and bacteria are never present in the vesicles producing the matrix granules. Notwithstanding much similarity, Rhipidodendron is not combined with Spongomonas because of the basic difference in colony structure.
The possible relationships of R. splendidum and S. uvella with other groups are examined and it is concluded that they cannot be considered as colorless chrysomonads as previously thought or considered to be related to any of the other orders comprising the class Phytomastigophorea. They do not, however, appear to be related to any of the orders at present comprising the Zoomastigophorea.