Bryozoan zooids have a U-shaped digestive system within
the body cavity. Both the mouth and the anus face upward
toward the surface; the mouth opens within the lophophore
and the anus opens outside the lophophore and just beneath
it. A water hydraulic system extends from the body cavity into
the ring of hollow tentacles; body-wall muscles contract against
the incompressible fluid in the body cavity, forcing the tentacles
to protrude. Cilia (and possibly touch receptors) stud the tentacles
on the lophophore. With a coordinated beat, cilia waft
currents bearing fine particles toward the mouth; selected food
particles pass through an esophagus and a stomach, which leads
downward to a large cavity called the cecum. Food is digested
by intracellular modes in the large stomach. The intestine leads
back up to the anus, which discharges solid waste. Gas exchange
takes place directly from all soft tissues of the zooid to water;
adults lack respiratory, circulatory, and excretory organs.