Canal System - Porifera

CANAL SYSTEM - PORIFERA

A. Definition. A system of connected cavities found in a sponge body is known as the canal system.

B. Types. There are three main types of canal system. These, in order of increasing complexity, are asconoid, syconoid and leuconoid.

1) Asconoid Canal System

This is the simplest type of canal system. It is found in Leucosolenia. Its important features are enumerated below-

(i) The sponge wall is thin and unfolded.

(ii) The mesenchyme is feebly developed.

(iii) The ostia lead directly into the
spongocoel. An ostium is intracellular, i.e., it is a passage through a tubular cell called the porocyte.

(iv) The spongocoel is large and lined
all over by choanocytes. It opens out by a single terminal osculum.

(v) The route followed by the water
current includes ostia, spongocoel, and
osculum.

The asconoid canal system occurs in
only a few sponges. The sponges with such a canal system are small in size and have a radially symmetrical, vase-like body.

2) Syconoid Canal System

This canal system is relatively complicated. Its important features are given below:

(i) The sponge wall is thick and folded.

(ii) The mesenchyme is well developed.

(iii) The ostia may lead into the radial canals or into the incurrent canals. The radial canals alone are lined by choanocytes.

(iv) The spongocoel is narrow, lined with pinacocytes and opens out by a single apical osculum.

The syconoid canal system can be theoretically derived from the asconoid type. If the wall of an asconoid sponge gives off finger-like outpushings at regular intervals, the outpushings carrychoanocytes inside them to form radial chambers and the spongocoel acquires a lining of pinacocytes, the result will be a syconoid sponge.

There are three grades in the syconoid canal system.

Grade I. In grade I the radial canals are free from one another, are surrounded by water on all sides, receive water directly from outside by ostia and open into the spongocoel by wider apertures, the internal ostia. The route followed by the water current includes - ostia, radial canals, internal ostia, spongocoel and osculum. Sycetta possesses a canal system very similar to the grade I of syconoid type.

Grade II. In grade II of the syconoid canal system the walls of the adjacent radial canals fuse, thereby enclosing tubular passages, the incurrent canals. Since the incurrent canals represent the original outer surface of an asconoid sponge, they are lined by pinacocytes. The incurrent canals open out between the blind outer ends of the radial canals by wide dermal ostia or small dermal pores perforating a pore membrane. The walls between the radial and incurrent canals represent the original wall of an asconoid sponge. These walls thicken and get pierced by long passages called the prosopyles.

The prosopyles correspond to the ostia of an asconoid sponge, but are intercelluar passages and not channels through porocytes. The dermal ostia and dermal pores do not correspond to any structure of an asconoid sponge. The mesenchyme adjacent to the dermal and gastral layers thickens, forming the
dermal and gastral cortex respectively. Formation of gastral cortex produces new chambers between radial canals and the spongocoel. These are lined by pinacocytes and open into the spongocoel by wide gastral ostia. They are named the excurrent or efferent canals. Each radial canal communicates with the excurrent canal internal to it by a pore, the apopyle, in a muscular diaphragm. The route of the water current includes - the dermal ostia or pores, incurrent canals, prosopyles, radial canals, apopyles, excurrent canals, gastral ostia, spongocoel, and osculum. The grade II of the syconoid canal system is found in Scypha (Sycon).

Grade III. In grade III of the syconoid canal system, the dermal cortex becomes extremely thick. Consequently the incurrent canals become very long and narrow, branch and anastomose, and course irregularly through the dermal cortex. Grade III of the syconoid canal system is found in Grantia.

Syconoid canal system occurs in a large number of sponges. The sponges with such a canal system retain the radially symmetrical, vaselike form of asconoid sponges.

3.) Leuconoid Canal System

This canal system in most complicated. It shows the following features -

(i) The sponge wall is very thick.
(ii) The mesenchyme forms extensive dermal and gastral cortex.
(iii) The ostia lead into the incurrent canals directly or through large hypodermal spaces. The incurrent canals are irregular and often branched.
(iv) The spongocoel is very narrow or lacking altogether.
(v) The radial canals are folded to form clusters of small flagellated chambers.
(vi) The excurrent canals lead into the
spongocoel if present or join to form larger excurrent spaces that open out by oscula.

The leuconoid canal system can be theoretically derived from the syconoid type by folding of the radial canals into small flagellated chambers.

There are four grades in the leuconoid canal system. These are known as the (i) eurypylous, (ii) prosodal, (iii) aphodal and (iv) diplodal.

(i) Eurypylous. In the eurypylous leuconoid canal system the flagellated chambers are wide and thimble-shaped, each opening into the excurrent canal by a large apopyle. The water current takes the following route - dermal ostia, hypodermal spaces, incurrent canals, prosopyles, flagellated chambers, apopyles, excurrent canals, excurrent spaces and oscula. This canal system is found in Leucilia.

(ii) Prosodal. In the prosodal leuconoid canal system, the flagellated chambers are small and rounded. Each flagellated chamber communicates with an incurrent canal by a long, narrow, tubular passage called the prosodus and opens into the excurrent canal by a wide apoyle. The water current takes the following route: dermal ostia, hypodermal spaces, incurrent canals, prosodal canals, flagellated chambers, apopyles, excurrent canals, excurrent
spaces and oscula.

(iii) Aphodal. In the aphodal leuconoid canal system the flagellated chambers are small and rouned as in prosodal grade. But each flagellated chamber communicates with an excurrent canal by a long tubular passage called the aphodus and with the incurrent canals by normal prosopyles. The water current takes the following route: dermal ostia, hypodermal spaces, incurrent canals, prosopyles, flagellated chambers, aphodal canals, excurrent canals, excurrent spaces and oscula.

(iv) Diplodal. In the diplodal leuconoid canal system both prosopyles and apopyles are elongated. This canal system is found in Oscarella. The route followed by the water current includes dermal ostia, hypodermal spaces, incurrent canals, prosodal canals, flagellated chambers, aphodal canals, excurrent canals, excurrent spaces
and oscula. 

The leuconoid canal system is found in the majority of sponges. The sponges with this type of canal system have large, irregular bodies. 

In calcareous sponges, the leuconoid canal system may arise by way of asconoid and syconoid stages or directly from the asconoid stage. In Demospongiae, however, it develops from a simple canal system found in the rhagon larva. For this reason, the leuconoid canal system is also known as the rhagon canal system.

Rhagon has a conical body, tapering from the broad base to the apex. It encloses a spaceous spongocoel that leads out by a single osculum situated at the apex. The basal wall lacks flagellated chambers and is called the hypophare. The vertical walls have rounded flagellated chambers and are termed the spongophare. Each flagellated chamber communicates with the exterior by a dermal pore or prosopyle and opens into the spongocoel by a wide apopyle. The water current takes the following
route - 

Prosopyles, flagellated chambers, apopyles, spongocoel and osculum.

Folding of spongophare gives rise to the
incurrent and excurrent canals.

Formation of dermal membrane outside the incurrent canals produces hypodermal spaces. Thickening of mesenchyme reduces the spongocoel and results in elongation of incurrent and excurrent canals. In this way eurypylous leuconoid canal system is reached. The eurypylous canal system formed in this manner is found in Spongilla. The outer region of the sponge body comprising the dermal membrane and the underlying hyodermal spaces is known as the ectosome, while the inner region with flagellated chambers is called the choanosome.

Functions of Canal System. 

The canal system helps the sponges in nutrition, respiration, excretion and reproduction. The water current flowing through the canal system brings food and oxygen and carries away facces, nitrogenous wastes and carbon dioxide. It also carries spermatozoa from one sponge to another for the fertilization of the ova there. Besides this, the canal system increases the surface area of the sponges in contact with water and this enables the sponge to increase their volume as surface/ volume ratio must remain fixed.

ECONOMIC IMPORTANCE

Sponges are useful as well as harmful to us.

1. Useful Sponges. Sponges are useful in the following respects -

(i) The marine sponges, Spongia and Hippospongia, provide the bath sponge.

(ii) The siliceous skeleton of the deep sea sponge Euplectella - the venus's flower basket, forms a beautiful and significant marriage gift and ornamental piece. It is a cylindrical network of spicules, having the appearance of spun glass.

(iii) Scraps of sponges are used as a packing material and in linoleum.

(iv) Siliceous spicules of sponges form large flint (hard stone) deposits.

2. Harmful Sponges. 

Cliona, the boring sponge, kills useful mullusks, such as oysters and clams, by growing on and boring into their shell. It also damages rocks by penetrating into them and breaking them into pieces.