The canal system of sponges includes a large central cavity called the spongocoel or atrium that connects all branches within the sponge body and opens to the outside through an osculum. The spongocoel is lined by flat polygonal pinacocytes and elongated contractile mycocytes surround the osculum to act as a sphincter and close it when needed.

1. Canal System of Scypha
There isan elaborate systemof intercommunicatingcavitiesin the sponge body. This is called canal
system. It includes in each branch a large central cavity the spongocoel sometimes called atrium,
paragastric cavity or cloaca. The spongocoels of all the branches are continuous at base. The
spongocoel opensoutthe osculumalreadyreferredto.Itislinedbylarge flatpolygonal cells termed
the pinacocytes. The cells surrounding the osculum are elongated and contractile. They are called
the mycocytes. They function as a sort of sphincter to close the osculum when required. (Figure
Scypha showing various canals in it)