In this article we will discuss about the role and evolutionary significance of canal system in sponges.
Role of Canal System:
The canal system which draws water current inside the sponge’s body and maintains a continuous uninterrupted flow of water, plays a vital role in the physiology of sponges, because it serves the various kinds of functions.
The sponges are holozoic and bring various kinds of microscopic organisms as food such as bacteria, diatoms, protozoans and other organic particles with the water current inside the body. The selected food are digested by choanocytes of the flagellated chambers and serves the purpose of nutrition.
A continuous flow of water brings oxygen inside, and exchange of gases takes place between the dissolved oxygen of the flowing water and the cells of the sponge along the course of water flow.
The outgoing flow of water current removes the various kinds of metabolic waste materials such as ammonia, urea, uric acid and other nitrogenous excretory products.
The incoming water current brings sperms which are captured by the choanocytes and help in fertilization.
Evolutionary Significance of Canal System:
1. Ascon Type:
1. Simplest type.
2. Hollow and gastrula-like construction.
3. Thin body wall.
4. Central large (spacious) spongocoel lined by a cell layer of choanocytes.
5. Unfolded layer of choanoderm.
6. Canals run straight through the body wall.
7. Straight entry of water into the spongocoel through the tubular porocytes.
8. With the increase of the size of spongocoel, the surface area of choanocyte layer is not increased sufficiently for the movement of water from the spongocoel and water is not pushed out readily through an apical narrow opening, the osculum.
9. These structures impose very definite size limitations.
10. Thus asconoid sponges become always small vase-like and grow in groups attached to the rocks in shallow seas.
2. Sycon Type:
1. Slightly complex type than ascon type.
2. Thick folded walls. Despite the folding of the body wall, the syconoid sponges still retain the radial symmetry.
3. Central large spongocoel is not lined by choanocytes.
4. Rather complex water passage (a twisted route).
5. Restriction of choanocytes to the evaginated radial canals only.
6. These structures help to increase the surface area of the choanocyte layer and allow sponges to increase in size and helps more water flow efficiently through the body. They do not live in groups as asconoid sponges.
7. The syconoid sponges pass through an asconoid stage in their development suggesting that they have evolved from some ancestral asconoid.
3. Leucon Type:
1. Most complex type.
2. No spongocoel.
3. Highest degree of folding takes place.
4. Numerous incurrent canals lead into the small, rounded, choanocyte chambers, necessary to propel more water through the body.
5. Canals are large and branched.
6. These structures help to increase significantly in the surface area and the system seen among the largest-sized sponges.
7. The complexity of structure of the leuconoid canal system helps to increase the most efficiency rather than other canal systems.
8. The leuconoid canal system may have evolved through asconoid and syconoid stages.
The differences between four basic types of canal system in sponges are shown in Table 11.2—Porifera.
Canal system in sponge furnishes a peculiar kind of pump, driving in and out the current of water. A sponge, called Callyspongia, having a body of 10 cm long and 4 cm diameter, pumps 78 litres of water a day. Another sponge, Leuconia aspera having almost an identical body, drains 22.5 litres.
A flow of water through the body is essential because there is no other fluid circulating in the body. It supplies food and oxygen and helps to remove waste products. Water current is caused by the beating of the flagella of the choanocytes. But, as the flagella do not bear in co-ordination, how such a synchronous flow of water current is produced is not clearly understood.