Body of all sponges is the perforated by large number of apertures through which water enters Inside body and flows through a system of criss-crossing canals collectively forming the canal system which is a characteristic feature of poriferans. Following types of canal systems are found in sponges:
ASCON TYPE
This is the simplest type of canal system and is found in Leucosolenia and other homocoela. Ostia are present on the surface of body and lead directly into the spongocoel, which is lined by flagellated choanocyte cells. Spongocoel opens to the outside through a narrow circular opening, the osculum located at the distal free end of the sponge body. Water enters through ostia into spongocoel and goes out of body through the osculum.
SYCON TYPE
This type of canal system is a characteristic of syconoid sponges, e.g. Scypha and Grantia. Body wall is secondarily folded to form incurrent and radial canals, which open into the spongocoel by an opening called apopyle. Both types of canals are interconnected by minute pores called prosopyles. Incurrent pores or ostia are found on the outer surface of body and open into the incurrent canals, which lead into adjacent radial canals through minute openings called prospyles. Radial canals are the flagellated chambers that open into central spongocoel by internal openings called apopyles. Spongocoel is a narrow, without flagellated cells but is lined by pinacocytes and opens to exterior through the osculum.
In more complex sycon type, as found in Grantia, the incurrent canals travel along an irregular course through the tissue and connect to the radial canals, thus forming large sub-dermal spaces.
LEUCON TYPE
In this case, the radial canals get divided into small rounded or oval flagellated chambers by further folding of the body wall. This is a characteristic feature of the leuconoid sponges such as Spongilla. Incurrent canals open into flagellated chambers through prosopyles. Flagellated chambers, in their turn, communicate with excurrent canals through apopyles. Excurrent canals are formed as a result of division of spongocoel which has almost disappeared in these sponges. Thus excurrent canals communicate with the outside through a small spongocoel and an osculum.
This type of canal system has varying degree of complexity of canals and based on that it can be classified into the following three types:
RHAGON TYPE
In Demospongiae, leuconoid condition is derived from the larval stage, called rhagon as found in Spongilla. The body is conical and tent like in shape, tapering towards the osculum. The spongocoel is bordered by oval flagellated chambers opening into it by apopyles. Mesenchyme is considerably thick and is traversed by incurrent canals and subdermal cavity. Water enters into the subdermal cavity through ostium and then enters the incurrent canal or it can be called prosodus. Flagellate chambers are connected to the spongocoel through the excurrent canal or it can be called aphodus. This canal system is primitive as compared to diplodal type and when the larva grows transformed to diplodal type.
Significance of Canal System
The flagella of choanocytes beat to produce a water current, which enters the sponogocoel through ostia. It carries food particles and oxygen and sweeps away the metabolic wastes through osculum. Therefore, the canal system serves the function of food collection, respiration and excretion. In simple type of canal system, there is lesser number of cells and thin body wall but as the canal system becomes more complex, the number of flagellated cells increases and the force to draw water current is increased. The syconoid canal system is therefore more efficient than the asconoid type and the leuconoid type is the most efficient.