The nucleus is pear-shaped, with its narrow end located anteriorly beneath the roots
of the flagellae. The root of one flagellum (sometimes both flagellae) attaches to a
spray of striated fibers, the rhizoplast, which extends posteriorly into the cytoplasm
around the nucleus. Rhizoplast fibers can contract, pulling intermittently on the root to
generate the wave-like movements of the flagella. To one side of the nucleus and root
structure of the flagellae sits a contractile vacuole. Located anteriorly is the Golgi
body, a system of flattened membranous cisternae involved in secretory activity. In
many forms it produces the scales that cover the cell externally.
(^) There may be one or two chloroplasts. If there is just one, then it is cup-shaped and
fills the posterior end of the cell. If two, they wrap the sides of the cell, one often
extending farther posteriorly. Thylakoids, the layers of membrane inside the
chloroplast bearing the photosynthetic pigment systems, are arranged parallel to the
external surface of the cell. There may be two to many thylakoids, and thylakoids
vary among groups in the number of membranes constituting them. In the center of
the chloroplast cup is a pyrenoid, which organizes the formation of storage product,
variously lipid or starch, in a mass below the nucleus. The anterior-most extension of
the chloroplast often bears an eye-spot consisting of several layers of pigmented
granules. This probably enables phototaxis or other responses to light and, thus, slight
vertical migration. Mitochondria either scatter about between the chloroplast and the