But the body wall is so folded as to from regularly arranged alternating invaginations and evaginations, establishing sycon type of canal system. Various components of canal system are as follows: (i) Ostia or dermal pores: The external grooves of body surface are stretched over by a thin pore membrane. It bears two or more openings for the ingress of outside water into the body of sponge. These pores are known as ostia (L., Ostium, door) or dermal pores.
(ii) Incurrent canals: These canals are the invaginated folds of body wall and are also called incurrent canals. These communicate with outside through ostia but end blindly at their inner ends. Pinacocytes line these canals throughout. (iii) Prosopyles: In current canals communicate with radial canals through intracellular spaces called prosopyles (Gr.
, Pros, near + pyle, gate). (iv) Radial canals: These canals are lined by flagellated cells, choanocytes and formed by evagination of body wall. Therefore these chambers are called flagellated or radial canals.
These canals are parallel and alternate with incurrent canal, both vertically and radially. Both the canals in the body wall are arranged in such a manner that each raidal canal is surrounded by incurrent canals and likewise each incurrent canal is surrounded by radial canals. Radial canals, in turn, open into spongcoel by the inner ends. (v) Apopyles: Apopyles or inner ostia are the specific openings by which radial canals communicate into the spongocoels. Having contractile myocytes they act as a sphincter. (vi) Spongocel: Centrally inside the body, a cavity forms its vertical axis is called spongocoel. Spongocoel is lined by epidermal pinacocytes. (vii) Osculum: Osculum is the exit point through which spongocoel opens outside.
It is provided with sphincters to regulate the rate of water flow in the body. Sphincters are lined by special contractile pinacocytes, called myocytes. (viii) Flow path of water current: In the canal system water flow in the form of current and maintained by continuous beating of flagella of choanocytes lining the radial canals.