Canal system in sponges.

Canal system:

The body surface of the sponges is perforated by numerous apertures for the ingress and egress of water current. Inside the body, the water current flows through a certain system of spaces collectively forming the canal system.In Olynthus, canal system is seen in its simplest type.

In other forms, it may attain a high degree of complexity, but its general evolution can nevertheless be reduced to a simple process of growth on the parts of primitive Olynthus resulting in folding of the walls by a restriction of the collared cells to certain regions.

Function of water current:

The most vital role in the physiology of sponges is played by water current on which their life depends. All exchanges between sponge body and external medium are maintained by means of this current. Food and oxygen are brought into body and excreta and reproductive bodies carry out. This current is caused by beating of flagella of collar cells.

Types of canal systems:

In the gradual and continuous process of differentiation,three distinct types of organisation can be distinguished which connected by numerous transitions may get be considered as three styles of architecture, so to speak under which all existing forms may be classified.There are usually three types of canal system met within sponges, viz-, asconoid type, syconoid type, and leuconoid type.

1. Asconoid type:

Asconoid type of canal system is the simplest type of canal system in which body is thin walled, radially symmetrical and hollow due to the central cavity which is found in asconoid sponges. This cavity is known as spongocoel or para gastric cavity.that opens by means of the circular aperture (known as osculum) at the free distal end of the cylinder.

The wall is composed of an outer and inner epithelium with a mesenchyme between.The outer or dermal epithelium here termed epidermis consists of single layer of flat cells. The inner epithelium, lining the spongocoel, is composed of choanocytes. The mesenchymal contains skeletal spicules and several types of amoebocytes, all embedded in a gelatinous matrix.The wall os asconoid sponges is perforated by numerous microscopic aperture termed incurrent pores or Ostia which extend from the external surfaces to the spongocoel.Each pore is intracellular,i,e it is a canal through a tubular cell called a podocyte.

Surrounding sea water enters the canal system through Ostia. The water current impelled by the flagella of the choanocytes passes through the incurrent pores into the spongocoel and out through the osculum (water from exterior = incurrent pores= spongocoel= osculum=ater out) furnishing in its passage food and oxygen and carries away metabolic wastes. Asconoid type of canal system is found only in few sponges, e,g,.Olynthus, Leucosolenia.
According to Hyman, the important feature of the asconoid structure are the simplest wall and the complete continuous interrupted only by the inner ends of the podocytes. The asconoid type of superficially resembles a typical gastrula.

2.Syconoid Type:

It is a more complex 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 out pushing of the walls of asconoid sponge at regular intervals into the 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 all of the radial canal fuse in such a manner that tubular incurrent canals open to the exterior by dermal Ostia or dermal pores. AS these incurrent canals represent the outer surface of an asconoid type of surface, they are lined by epidermis hile the radial canals which represent the out pushing of scones spongocoel are lined by choanocytes. The interior of the sponge in which radial canals open is a spacious sponge which is lined by flat epithelium derived from the epidermis. The opening of the radial canals into spongocoel are termed as an internal ostium. The spongocoel opens to the exterior by a single osculum. The all between in current and radial is pierced by numerous minute pores called prosophyles.

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

The syconoid sponges differ from the asconoid type into important particulars:

1. In the thick folded walls containing alternating current and radial canals and

2.In the breaking of the choanocyte layer, which no longer lines the hole interior but is limited to certain definite chambers (radial canals).

The syconoid structure occurs in two main stages. The first type illustrated in a few of the heterocoelous calcareous sponges.especially membranes of the genus Sycon. In the second stage, the epidermis and mesenchyme spread over the outer surface forming a thin or thick cortex often containing special cortical spicules. The epidermis becomes pierced by more definite pores than lead into narrowed incurrent canals.

3. Leuconoid type:

This type of canal system is formed from the rhagon type by the out folding of the choanocytes layer. In this type oval or rounded chamber lined by flagellated cells are formed by evagination of radial canals.The surface is perforated by dermal pores. The pores lead into incurrent canals, which are found in mesenchyme. These canals are subdermal spaces, which are larger and provided ith spicules. Incurrent canals open into small, rounded chambers provided ith flagellated cells. The opening of incurrent canals into the flagellated chambers are called prosopyles.The flagellated chamber opens into excurrent canals by small apertures known as apopyles. These excurrent canals are united to form large tubes, which opens into spongocoel. Spongocoel opens to outside by osculum. Leucon type of canal system can be divided into two sub-types.

a)Eurypylous type:It is a simplest and most primitive leucon type of canal system.In this type, the flagellated chambers communicate directly by broad aperture, the apopyles, with excurrent canals.Ex.Plakina.

b) Aphodal type:In this type, the apopyle is drawn out as a narrow canal, called aphodus.This connects the flagellated chamber with excurrent canal.Ex.Geodia.

C) Diplodal type:In some sponges, besides aphodus, another narrow tube, called prosodus, is present between the incurrent canal and flagellated chamber.The pattern is called the diplodal type, Ex Spongilla Oscarella.

Reference:

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L..,Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011.

http://www.biologydiscussion.com/

https://en.wikipedia.org/wiki/Sponge

http://www.biozoomer.com/2014/03/sycon-sponge-scypha-structure-and-its.html