The ultrastructure of Peridinium cinctum, was examined by serial sectioning with particular emphasis on the detailed construction of the
flagellar apparatus. The pusular system of P. cinctum included two sac pusules in open connection with the flagellar canals; disorganized
material was found inside the pusules and near the flagellar pores, indicating excretory activity of the pusules. Several cells had replicated
basal bodies, which were oriented toward the cell apex. The functional basal bodies formed an angle of about 65° and were linked by a
layered connective, a structure so far seen only in peridinioids. A transverse microtubular root looped around the transverse flagellar canal
and nucleated several rows of microtubular extensions that lined most of the transverse sac pusule. A single-stranded root was associated
with the surface of the longitudinal basal body almost at a right angle to the basal body triplets. A fibrous connective between the
transverse striated root and the longitudinal microtubular root was absent. The striated collars encircling the flagellar pores had a complex
structure and were linked by a large connective; a smaller fibrous connective was also found in one cell. An extension of the longitudinal
striated collar lined part of the longitudinal sac pusule; it showed a tubular substructure and was perhaps contractile. Bacteria were present
in the peripheral cytoplasm of the cell, enveloped by several unit membranes and surrounded by vacuoles; near the sac pusules they
appeared disorganized with aggregated ribosomes, possibly the result of digestion.