The nervous system includes a dorsal cerebral ganglion and two pairs of longitudinal nerve cords. The sense organs include eyes, bristles, pits, and antennae. Rotifers are gonochoristic but, depending on the species, males are either rare, unknown, or produced only occasionally. Reproduction is usually parthenogenetic but is sometimes sexual in those species with both sexes. Males, when present, are smaller than females, short-lived, aberrant or degenerate, and usually have no functional gut.
Females have a gonad consisting of a combined ovary and yolk gland.
Descriptions of rotifer anatomy and biology, including this one, typically apply to females unless stated otherwise. Most rotifers are oviparous but a few planktonic species are viviparous. Philodina is a more or less typical rotifer although its mastax does not exhibit the ancestral plan. It is exceedingly abundant and has been called the world's most common metazoan. You will prepare and study wetmounts of living female specimens.