Porifera: Canal System in Sponges

 

• The arrangement of passage way in the leading from incurrent dermal pores to the spongocoel then out through the osculum is known as canal system.
• There are three types of canal system are found in sponges-

1. Ascon type- Leucosolenia
2. Sycon type- Scypha
3. Leucon type- Spongilla 

Ascon type-

• This type of canal system is found in simple sponge like- Leucosolenia. The body is simple vas like. Dermal pores are found on the body surface. Incurrent pores are intracellular pores leading through tubular porocyte. Porocyte opens directly into the spongocoel. The entire spongocoel lined by flagellated cells or choanocytes. Spongocoel opens at the tip of sponge is called osculum.
• Water current from the outer side enters through ostia or dermal pores into the spongocoel. Lastly water goes out through osculum.

             Water from exterior--- Ostia --- Spongocoel ---Osculum --- Exterior 

Histology

Body wall consists of outer layer of flattened cells called pinacocytes. Inner layer is of flagellated cells. Between outer layer (ectoderm) and inner layer (endoderm), mesenchyme present. Mesenchyme contains- Amoebocytes, Archeocytes and skeletal spicules.

 

 

 

 Sycon type-

• This type of canal system is found in sycon or schypha. Sycon type is advanced over ascon type of canal system. Sycon type of canal system has been evolved from ascon type of canal system.
• The body of sycon is thick and massive. Thick body wall is due to the folding of simple ascon type. Due to the out pushing of the body wall radial canals or flagellated chambers are formed. These out pushing are at regular intervals. Between two out pushing incurrent canals are present. Incurrent canals are lined by flat ectodermal cells or pinacocytes. Flagellated chambers or radial canals are lined by choanocytes cells.
• Between the radial canals and incurrent canals, small pores are present called prosopyle. Prosopyle is intracellular with porocyte.
• Incurrent canals opens outside on the surface of body by minute pores called ostia.
• The flagellated chambers opens towards paragastric cavity or spongocoel by an aperture is called apopyle. Apopyle opens into excurrent canal and excurrent canal opens to the spongocoel through gastric ostium.
• Spongocoel opens outside through terminal aperture called osculum.
• Incurrent canals lined by dermal flat pinacocytes. Flagellated chambers lined choanocytes. The spongocoel of sycon or scypha is lined by the pinacocytes cells.
• The course of water current is given below in this type of canal system.

Exterior water --- ostia --- incurrent canal --- prosopyle --- radial canal --- apopyle --- excurrent canal --- gastric cavity --- spongocoel --- osculum --- exterior water

In this way water current passes through a number of canal systems. Flagellated canals and incurrent canals are radially arranged around the spongocoel.

 

 

 

 

Leucon type

• This type of canal system is complex than the sycon type. In this type of canal system, single flagellated chamber or radial canals are subdivided into many small rounded or oral flagellated chambers. This is due to the out folding of single flagellated chambers of sycon type. Mesenchyme becomes thick, flagellated chambers irregularly arranged in incurrent canals. Excurrent canals develop between radial canals and spongocoel.
• The excurrent canals from different radial chambers opens into wider channel and finally opens to the outside through osculum. Similarly the incurrent canals also get branched and irregularly arranged. The dermal ostia may lead directly into the incurrent canals or opens into the subdermal spaces crossed by the spicules.
• Leucon type of canal system has three grades of canal systems-

1. Eurypylous canal system
2. Aphodal canal system
3. Diplodal canal system

Eurypylous canal system- 

This is the simplest type of leucon type of canal system. It bears the structure like above description. Following in the course of water current in eurypylous type.

Outside --- Dermal Ostia ---Subdermal space & Incurrent canal ---Prosopyles ---Radial canal --- Apopyles --- Excurrent canals --- Large channels --- Osculum --- Exterior

Aphodal canal system-

•  In certain leuconoid sponges such as Stelleta and Geodia has aphodal canal system is found. 
• In this case apopyles are not a small opening but it is a narrow canal. It is called as aphodus. Aphodus opens into excurrent canals. Excurrent canals opens into spongocoel of sponge then out through osculum.

Outside --- Dermal pores --- Incurrent canals --- Prosopyle --- Radial canals --- Aphodus --- Excurrent canals --- Spongocoel --- Osculum --- Exterior  

Diplodal canal system- 

This type of canal system is found in Spongilla and Oscarella. In this case, both apopyles and prosopyles are in the form of tubular canals. In this way incurrent canals opens into radial canals through prosodus. Radial canals opens into incurrent canals through aphodus.

Euryphalous and Aphodal respectively 

Diplodal 

Rhagon type

In non-calcareous desmospongia leuconoid type of canal system is derived from larval stage. This is called as Rhagon. The canal system of rhagon is called as rhagon type. It has flattened pyramid like form with broad base (hyposphere) and a spacious spongocoel. The upper lateral walls passing a row of small oral flagellated chambers. The dermal pores leading into the flagellated chambers called as prosopyles. The openings of flagellated chambers into the spongocoel is called as apopyles. The spongocoel opens to the exterior by osculum.