Statocysts

STATOCYSTS, or auditory vesicles, are organs of equilibrium, consisting of an open (Fig. 1) or closed vesicle (Fig. 2) of ectodermal origin. The vesicle is filled with fluid, in which there is one or more statoliths or otoliths (auditory stones). In open statocysts, sand grains may serve this role. The statocyst is lined with epithelium and is innervated by special nerves. The epithelium is either homogeneous or differentiates into intermediate, ciliated, and sensory cells (Fig. 2). With different body positions, the otolith presses on different parts of the statocyst wall, which is perceived by the animals as a sensation of normal or abnormal body orientation.

Longitudinal section through a statocyst.

Arenicala marina: 1 - entrance opening; 2 - cilia of the epithelium; 3 - statoliths (sand grains); 4 - connective tissue. (From Büchli.)

Statocysts also participate in the reception of auditory stimuli, which is why they are often called auditory vesicles. Statocysts are found in cnidarians, worms, mollusks, crustaceans, and some tunicates. In vertebrates, the labyrinth corresponds to them functionally.

Figure 2. Statocyst of Pterotrachea; the left half of the statocyst is shown intact, the right half is in section. The bundles of cilia are shown in a state of rest: 1 and 7 - ciliated cells; 2 and 6 - intermediate cells; 3 - sensory cells; 4 - central sensory cell; 5 - nerve fibers; 8 - auditory nerve. (From Büchli.)

oboloch.