The following points highlight the three main types of canal system encountered in sponges. The types are: 1. Ascon Type 2. Sycon Type 3. Leucon Type and 4. Rhagon Type.
1. Asconoid Type:
The asconoid type of canal system is regarded to be the most simple and primitive grade of canal system. Asconoid type is present in these sponges whose body is vase-like and radially symmetrical. The wall is extremely thin. It encloses a large spongocoel (atrium) opening at the summit by a narrow osculum. The spongocoel is lined by choanocytes.
The wall is pierced by numerous microscopic apertures termed as the incurrent pores or ostia which extend from the external surface to the spongocoel. Each pore is intracellular disposed in a porocyte. The asconoid type of canal system is characterised by the presence of a complete continuous layer of choanocytes lining the spongocoel interrupted only by the porocyte (Fig. 11.12).
Occurrence:
This type of canal system is found in some simple adult calareous sponges (e.g., Leucosolenia, Clathrina). It also occurs in Olynthus stage of some newly settled calcareous sponges (e.g., Clathrina).
Physiology of water current production:
A constant water current is created by the beating of the flagella of choanocytes. The flagella do not beat synchronously, but beat independently. The water current passes directly through the ostia into the spongocoel and goes out through an apical opening, the osculum.
Course of water:
The course of water current is as follows:
2. Syconoid Type:
The syconoid type of canal system represents the transitional grade between the simplest ascon type and more complex ones. The first step above the asconoid type of canal system is represented by the syconoid type of canal system.
The complication is due to the out-pushing of the wall into finger-like projections, called the radial canals, at regular intervals. In this type of canal system, choanocytes are only limited to the radial canals. Their detailed account has already been described with the biology of Sycon.
The syconoid canal system is represented by three grades:
(i) Simple sycon type
(ii) Complex sycon type and
(iii) Sycon type with cortex.
(i) Simple sycon type:
The simple type of canal system is found in the heterocoelous calcareous sponges typified by Sycetta. The radial canals are free projections of the wall without touching at any point and the external surface is composed of the blind outer ends of the radial canals.
The incurrent canals have not definitely formed and the spaces between the radial canals serve as the dermal ostia. The radial tube is lined by flagellated cells but the Spongocoel is lined by flattened pinacocyte cells.
Course of water:
(ii) Complex sycon type:
The complex sycon type canal system is found in Scypha (= Sycon) and there is an opposition. The walls of the adjacent radial canals are arranged in such a way as to leave between them tubular spaces, the incurrent or inhalant canals. Thus radial canals and incurrent canals are arranged alternately and the latter opens to the exterior through the dermal ostia or incurrent pores.
The inner ends of the incurrent canals are blind and the walls are lined with flat ectodermal cells—pinacocytes. The dermal ostia or incurrent pores are intercellular, perforating a pore membrane and are bounded by contractile myocytes. The wall between the incurrent and radial canals is pierced by numerous minute pores, called prosopyles.
Each prosopyle seems to have a intercellular space or channel through a porocyte. The radial canal is lined by choanocytes and opens by a wide aperture, called apopyle, into a short wide ex-current or exhalent canal lined by flat epidermal cells which communicates with the spongocoel by an internal ostium. Finally, the spongocoel opens to the exterior by osculum (Fig. 11.12).
(iii) Sycon type with cortex:
The Syconoid (Stage III) canal system is found in many genera of calcareous sponges, viz., Grantia, Grantiopsis, Heteropia, Ute, etc. The complication is due to the spreading of the dermal membrane (consisting of epidermis plus a thin layer of mesenchyme) over the entire surface of the sponges to form the cortex of variable thickness.
The walls of the radial canals fuse in such a fashion that tubular spaces (incurrent canals) are formed which open to the exterior through dermal ostia or dermal pores. The incurrent canals traverse along irregular course through the cortex before reaching the outer ends of the radial canals. Sometimes large irregular cortical spaces or sub-dermal spaces may be produced.
3. Leuconoid Type:
In this type of canal system the choanocyte lining of radial canals evaginates into many small chambers which repeat the same process to give rise to a cluster of small flagellated chambers. In many cases dermal pores open into subdermal spaces.
The subdermal space and incurrent canals lead into the small rounded flagellated chamber through an opening, termed as prosophyle. The flagellated chambers open by apopyles into excurrent canals which form large tubes. The largest one leads to osculum. Few gradations of leuconoid type of canal system are seen in sponges.
They are:
(i) Eurypylous type:
When apopyles are in direct communication with excurrent canals by broad mouth. This type of canal system is seen in Plakina, Plakortis, Tetilla.
(ii) Aphodal type:
When a slender canal called aphodus connects the chamber to the excurrent canals. This type of canal system occurs in Geodia, Stellata.
(iii) Diplodal type:
When a narrow tube, the prosodus is present between the incurrent canals and the flagellated chambers and is found in Oscarella, Chondrosia, Spongilla.
The main advancement of leuconoid canal system over that of syconoid type of canal system is due to:
(i) The limitation of choanocytes only to the small chambers,
(ii) The gradual development of the mesenchyme and
(iii) The elaboration and complication of the incurrent and excurrent water passages.
4. Rhagon Type:
The body is conical with osculum situated at the summit. The spongocoel is bordered by oval flagellated chambers, opening into it by wide apopyles. Between the epidermis and the chambers lies a considerably thickened mesenchyme which is traversed by incurrent canals and subdermal spaces. The rhagon type is a little complex than the complex syconoid type and is found in the larval stage of Demospongiae.
Mechanism of water circulation:
The planar beating of the flagella of choanocytes lining in the radial canals produces a water current which is on one hand draws the water inside through the ostia and on the other hand forces it to go out through the osculum. The choanocytes are arranged in such a way that their collars are oriented towards the apopyles.
The flagella of the choanocytes are flexible with slight stiffness at the base. As a result the flagella beat from base to tip creating a water current in the same direction and drive the water from the flagellated chambers to the spongocoel. To fill up the void within the flagellated chambers the water is sucked into the radial chambers from the incurrent canals through the small prosopyles.
Due to the larger size of the apopyles than the prosopyles, the water seeks larger outlets—the apopyles and water leaves the spongocoel through a large elevated opening, the osculum.