All the activities of their body of the sponges depend on the current of water entering through ostia and passing out through osculum or oscula. Inside the body, water current flows through system of spaces which collectively constitute the canal system. The entire physiological activities of the animal depend on the water current and the exchanges between the body and the exterior arc maintained through the water current.The food and oxygen are brought through this current while excreta and reproductive bodies are excluded through this current. The perforations of the body of the sponge by a large number of ostia is characteristic of phylum Porifera.

All the living tissues of the sponges are soft and it is a direct need of the animal to maintain a constant shape with different canals or passage-ways. Therefore, deeper layers of the body are provided with supporting spicules. Around the osculum, spicules are long and straight, while spicules situated around ostia are short and straight; around spongocoel they are T-shaped, while triradiate in the body-wall.

Thus, all the cavities of sponges with intricate passages of canals, traversed by currents of water entering by pores and passing out by osculum are collectively termed as a canal system of sponges.

A typical canal system is composed by following components :

(a) Incurrent canal - It opens externally to the outside by a small pore known as incurrent pore or ostium, but internally it ends blindly.

(b) Radial canal or excurrent canal- It is closed externally but opens internally by minute pores or apopyles into a central cavity or cloacal cavity or gastral cavity or spongocoel, which cannot be compared in any way with the stomach or intestine of other animals.

The incurrent canals are lined by flat squamous cells and their functions are only to form water conduits and to form a smooth and firm surface.

The radial canals are lined by collar cells opening at the surface and are provided with flagella or whips. The lashing movements of flagellum procure the food particles and push them into the cell-mouth. Thus, this is food-capturing arrangement of sponges.

Spongocoel or cavity is lined by a thin gastric epithelium. It opens to the outside by an aperture, called osculum.

The arrangement, and complexity of the canal system varies considerably in different sponges and has been divided into four types :

1. Ascon type

2. Sycon type

3. Rhagon type

4. Leucon type

Ascon type:

It is the simplest type of canal system in which the body is thin-walled, radially symmetrical and hollow due to the central cavity. This cavity is known as the spongocoel or paragastric cavity that opens to the outside by means of a circular aperture (known as osculum) at the free distal end of the cylinder. Numerous minute pores the ostia are also present in the thin wall of the cylinder. These minutes pores are regularly disposed intracellular apertures each of which is a canal like structure situated within a tubular porocyte. They extend from the outer surface of the body wall to the spongocoel. The water current reaches the spongocoel after passing through the ostia and goes out through the osculum. The body wall of ascon type of sponges is formed of two layers. The outer layer is called ectoderm and the inner layer is known as endoderm. The ectoderm is formed of thin and flat pinacocytes. The endoderm is formed by the choanocytes and lines the spongocoel. Between these two layers is a thin mesenchyme formed of non-living gelatinous substance. It contains different kinds of amoebocytes and triradiate spicules formed of calcium carbonate.

The ascon type of canal system found in some calcareous sponges like Clalhrina. Besides this, it is found in Leucosoleniaand some other simple sponges.

The course of the water current is as follows

Sycon type

It is more complcx system as it is folded version of the asconoid body. It is found in Scypha and the embryonic development of Scypha clearly shows the asconoid type by the outpushings of the wall of an asconoid sponge at regular intervals into finger-like projections called radial canals. At first, these canals are in direct contact with the outside water but in most of the sponges, the wall of the radial canal fuse in such a manner that tubular incurrent canals are formed in between. These incurrent canals open to the exterior by dermal ostia or dermal pores. As these incurrent canals represent the outer-surface of asconoid type of surface, they are lined by epidermis while the radial canals which represent the outpushings of asconoid spongocoel

arc lined by choanocytes. The interior of the sponge in which radial canals open is a spacious spongocoel which is lined by the flat epithelium derived from epidermis. The openings of the radial canals into spongocoel are termed internal ostia. The spongocoel opens to the exterior by a large single osculum. The wall between incurrent and radial canals is pierced by numerous minute pores called prosopyles.

The course of water current through the canal system can be represented as follows :

Rhagon type

This type of canal system is found in the larva of Demospongiae called rhagon which has a broad base and is conical in shape. Due to excessive growth of mesenchyme sub-dermal spaces are formed in its body wall. The ostia open in these spaces which lead into incurrent canals. The incurrent canals open by prosopyles into flagellated canals which are lined with choanocytes. The flagellated canals open by apopyles into excurrent canals which lead into paragastric cavity. The paragastric cavity opens to the outside by the osculum which is present at the apex. The incurrent and excurrent canals may be complex and branched in it, The source of water through this system is as follows:

Leuconoid type

This type of canal system is formed from the rhagon type by the outfolding of the choanocyte layer. In this type, oval or rounded chambers lined by flagellated cells are formed by evagination of the radial canals. The surface is perforated by dermal pores These pores lead into incurrent canals, which are found in mesenchyme. These canals are usually branched. In many cases, dermal pores open into subdermal spaces, which are large and provided with spicules. Incurrent canals open into small, rounded chambers provided with flagellated cells. The openings of the incurrent canals into the flagellated chambers are called prosopyles. The flagellated chambers open into the excurrent canals by small apertures, known as apopvles. These excurrent canals are united to form large tubes, which open into spongocoel. This cavity is largely obliterated. Spongocoel opens to the outside by the osculum. Leuconoid type of canal system can be divided into 3 sub-types

(a) Euryphylous type

In this case the sponge has a flat broad base having an opening at the apex and looks like a pyramid. There are a number of flagellated chambers {radial canals) in the upper wall into which the prosopyles open. The lower basal wall is without flagellated chambers and is known as hypophore whereas the upper wall with flagellated chambers is called spon

gophore. The folded spongophore gives rise to incurrent canals. Due to this folding the flagellated chambers do not open into the gastral cavity but into the diverticula of it forming the excurrent canals.

(b) Aphodal type

In this type of canal system the flagellated chambers do not open into the excurrent canals directly but are removed from the excurrent canals by prolongations of apopyles into small canals called aphodus which are lined by the prolongation of the epithelium of excurrent canals into which they open. There is only one prosopyle to each chamber.

In this type incurrent canals do not open directly open into flagellated canals but open by narrow canals called prosodus which are prolongations of prosopyles. Thus each flagellated chamber has a prosodus leading to incurrent canals and an aphodus leading from it to excurrent canals.

Mechanism of current production

To produce an incurrent or cxcurrent condition there are two factors which are essential:

(i) For entering water through ostia into the body there must be a pressure within it less than that in the incurrent canals.

(ii) For escaping water through osculum there must be a pressure within chambers higher than that in the excurrent canals.

But as the pressure in the incurrent and excurrent canals is the same, there must be a difference of pressure within the chamber itself and the lower pressure must be towards the periphery. Such a distribution of pressure is set up when each flagellum causes a flow of water towards the centre of the chamber.

Functions of the Canal System

1. The canal system serves the purpose of nutrition. It is regarded as a highway for the food through the body cells in the radial canal with flagella, which capture the food particles. Water-currents are produced by flagella. thus, waters flows into the central cavity or spongocoel. Smaller food-particles e.g. diatoms, protozoa and particles of organic debris are ingested into the cells protoplasm and digested. The digestion is intracellular. Robert Grant first of all observed the flow of water in the body-wall by adding powdered carmine to the water. Thus, canal system here does the same functions as circulatory system in higher animals.

2. In sponges, as a result of development of elaborate canal system, massive growth is found.

3. Streaming currents of water have dissolved air, therefore, gaseous exchange or respiration takes place in the cells. Oxygen is taken in by simple process of diffusion and carbon-dioxide is given out. The respiration is also intracellular.

4. The function of the canal system is also excretory. Currents of water, which pass outside the osculum remove the carbonic acid and other nitrogenous waste substances, which are the excretory products of the body.

5. The purpose of the canal system is also to increase the surface area of the animal in water. This is a characteristic point by which increase of volume is allowed by keeping the ratio of the surface to the volume.

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