Canal System in Sponge – Definition, Types, Structures, Mechanisms, Functions and Examples

What is Canal system in sponges?

Canal system in sponges is a system of water canals present in the body of sponges through which water circulates continuously. It is also referred to as aquiferous system. This system is highly characteristic feature of phylum Porifera and it is essential for their survival.

Sponges are sedentary and filter feeding animals. They do not possess true tissues and organs. Therefore all the physiological functions are performed by the canal system. Water enters the body through numerous minute pores called ostia and it passes through internal canals and chambers. Finally the water is expelled outside through a large opening called osculum.

The movement of water inside the sponge body is driven by the beating of flagella of choanocytes (collar cells). These choanocytes line the internal chambers. The continuous beating of flagella creates a water current which is necessary for food capture and gas exchange.

The canal system performs several important functions. It supplies oxygen for respiration and brings food particles for nutrition. It also removes carbon dioxide nitrogenous wastes and other excretory products. In addition it helps in reproduction by carrying sperm to other sponges for fertilization.

Types of Canal System

Depending upon the complexity of the canals the canal system is of three types–

• Asconoid – It is the simplest type. Water enters through ostia into spongocoel and leaves through osculum.
• Syconoid – In this type the body wall is folded forming radial canals which increase the surface area.
• Leuconoid – It is the most complex type. The spongocoel is reduced and numerous small chambers are formed increasing efficiency of water circulation.

Components of canal system in sponge

Components of canal system in sponge are the different structural parts through which water enters circulates and leaves the body. These components are arranged in a definite sequence and each part performs a specific function in water flow.

The main components are as follows–

1. Ostia (Dermal pores)– These are minute pores present on the external surface of the sponge body. Water enters into the body through these openings. They are numerous in number and allow continuous inflow of water.
2. Incurrent canals– These are passageways which extend from the ostia into the body of sponge. The water that enters through ostia is collected in these canals. These canals are generally lined by flat cells called pinacocytes.
3. Prosopyles– These are small pores which connect the incurrent canals to the radial canals or flagellated chambers. Through prosopyles the water passes into internal chambers.
4. Radial canals (Flagellated chambers)– These are specialized chambers lined with choanocytes (collar cells). The beating of flagella of choanocytes is responsible for movement of water. Food particles are trapped here during circulation.
5. Apopyles (Gastric ostia)– These are internal openings through which water leaves the radial canals. From here the water moves towards central cavity or excurrent canals.
6. Excurrent canals– These are larger canals mainly present in more complex sponges. Water from apopyles is collected in these canals. They transport the water towards spongocoel or directly to osculum.
7. Aphodus and Prosodus– These are narrow connecting tubes found in some complex types. Prosodus connects incurrent canal to flagellated chamber. Aphodus connects flagellated chamber to excurrent canal.
8. Spongocoel (Atrium)– It is the large central cavity of sponge. Water after passing through different canals collects here before leaving the body.
9. Osculum– It is the large opening present at the top of sponge body. Through this opening water along with waste materials is expelled outside.

Types of canal systems in sponge

Types of canal system in sponge are based on the complexity of the water canal arrangement. It is the structural modification of body wall that determines the type of canal system. These types shows progressive increase in complexity and efficiency.

The main types are as follows–

1. Asconoid type (Ascon type)– It is the simplest and most primitive type of canal system. The body is vase like and radially symmetrical. Water enters through ostia directly into the large central cavity called spongocoel. The spongocoel is lined with choanocytes. From here water is expelled through a single osculum. This type is least efficient and is found in small sponges.
2. Syconoid type (Sycon type)– It is more complex than asconoid type. In this type the body wall is folded into finger like projections. Water enters through dermal pores into incurrent canals. From there it passes into radial canals which are lined with choanocytes. The spongocoel is not lined with choanocytes in this type. Finally water leaves through the osculum. This folding increases the surface area for filtration.
3. Leuconoid type (Leucon type)– It is the most complex and most efficient type of canal system. The body is irregular in shape and the spongocoel is highly reduced or absent. Water enters through dermal pores into a network of incurrent canals. It then passes into numerous small flagellated chambers. From these chambers water moves into excurrent canals and finally leaves through one or more oscula. Leuconoid type shows three modifications–
   • Eurypylous type – Flagellated chambers are wide and they open directly into the excurrent canal.
   • Aphodal type – A narrow tube called aphodus connects the flagellated chamber to the excurrent canal.
   • Diplodal type – Narrow tubes are present on both sides. Prosodus connects incurrent canal to chamber and aphodus connects chamber to excurrent canal.
4. Rhagon type– It is mainly observed in the larval or developmental stage of some sponges. The body has broad base called hypophare and a conical upper part called spongophare. The spongocoel is surrounded by oval flagellated chambers which open into it through wide apertures called apopyles.

1. Ascon type of Canal system

Ascon type of canal system is the simplest and most primitive type found in sponges. It shows direct passage of water into central cavity without folding of body wall.

The important points are as follows–

• It is the simplest and least common type of canal system.
• The body is radially symmetrical and vase like or cylindrical in shape. The body wall is thin.
• Water enters through numerous minute pores called ostia present on the outer surface.
• Each ostium is formed by a tubular cell called porocyte which forms a direct channel.
• Water passes directly into a large central cavity called spongocoel (atrium).
• The spongocoel is lined with choanocytes (collar cells). The beating of flagella creates water current and traps food particles.
• Water is expelled through a single terminal opening called osculum.
• The route of water flow is–
  Exterior water → Ostia → Spongocoel → Osculum → Outside.
• Since choanocytes line only the spongocoel the efficiency is low and size of sponge is small.
• It is found in calcareous sponges like Leucosolenia and Clathrina and also seen in Olynthus stage.

Structure of Ascon type of Canal system

Structure of Ascon type of canal system shows the simplest organization of sponge body wall. It consists of thin body wall with central cavity and direct water passage.

The structural components are as follows–

• Body shape– The body is radially symmetrical and vase like in appearance. The wall is very thin and delicate.
• Body wall– The body wall is composed of outer epithelium and inner epithelium with a gelatinous layer in between. This middle layer is called mesenchyme.
• Outer epithelium (Epidermis)– The outer layer is made up of flat cells called pinacocytes. These cells form a protective covering of the sponge body.
• Mesenchyme (Middle layer)– It is a gelatinous matrix present between outer and inner layers. It contains skeletal spicules and different types of amoebocytes. These cells are involved in transport and support.
• Inner epithelium– The inner surface of the spongocoel is completely lined by choanocytes (collar cells). These cells possess flagella which helps in water current formation.
• Ostia (Incurrent pores)– Numerous minute pores are present on the outer wall called ostia. Each ostium is formed by a tubular cell known as porocyte. It forms a direct intracellular canal into spongocoel.
• Spongocoel– It is the large and spacious central cavity. Water after entering through ostia collects here.
• Osculum– A single narrow opening is present at the distal free end. Through this opening water is expelled outside.
• Water flow route– The pathway of water is– Exterior → Ostia → Spongocoel → Osculum → Outside.

2. Sycon type of canal system

Sycon type of canal system is more complex than ascon type. It is formed due to folding of body wall which increases the surface area for water filtration. This type shows better efficiency in comparison to asconoid type.

The important points are as follows–

• It is formed by horizontal folding of the body wall into finger like projections. This folding forms two types of canals arranged alternately.
• Two types of canals are present– incurrent canals and radial canals.
• Water enters from outside through dermal ostia (dermal pores) into the incurrent canals.
• Incurrent canals are lined by flat non flagellated cells called pinacocytes. These canals end blindly on the inner side.
• Water passes from incurrent canals into radial canals through small pores called prosopyles.
• Radial canals are lined by choanocytes (collar cells). The beating of flagella produces water current and food particles are trapped here.
• Radial canals open into the central cavity through openings called apopyles.
• The spongocoel is lined by pinacocytes and not by choanocytes in this type.
• Water collects in spongocoel and then is expelled through a single large opening called osculum.
• The route of water flow is–
  Dermal pores → Incurrent canals → Prosopyles → Radial canals → Apopyles → Spongocoel → Osculum → Outside.
• In some advanced forms dermal membrane becomes thick forming cortex and incurrent canals become irregular and branched.
• It is found in sponges like Sycon (Scypha) and Grantia.

Structure of Sycon type of canal system

Structure of Sycon type of canal system is more complex than ascon type. It is formed by horizontal folding of body wall into finger like projections. These foldings results in formation of different types of canals arranged in definite order.

The structural parts are as follows–

• Dermal ostia (Dermal pores)– Minute openings are present on the outer surface of body. Through these pores surrounding water enters into sponge body.
• Incurrent canals– These are tubular spaces present between radial canals. They open outside through dermal ostia but their inner ends are blind. These canals are lined by flat cells called pinacocytes.
• Prosopyles– These are small connecting pores present between incurrent canals and radial canals. Through these pores water passes into radial canals.
• Radial canals (Flagellated canals)– These are finger like chambers formed by out pushing of original body wall. They end blindly towards outer side. These canals are lined with choanocytes (collar cells). The beating of flagella generates water current.
• Apopyles (Internal ostia)– These are internal openings through which water leaves radial canals. They open into the central cavity. These openings are surrounded by contractile cells which regulate the diameter.
• Spongocoel– It is the central hollow cavity. In sycon type it is narrow and lined by pinacocytes instead of choanocytes.
• Osculum– A single large opening is present at the top of spongocoel. Through this opening water is finally expelled outside.
• Water route– The pathway of water is– Exterior → Dermal pores → Incurrent canals → Prosopyles → Radial canals → Apopyles → Spongocoel → Osculum → Outside.

Types of Syconoid Type

Types of Syconoid type shows different grades of complexity in canal arrangement. It is formed by folding of body wall and gradual fusion of radial canals. These types represents progressive development within syconoid canal system.

The types are as follows–

1. Simple sycon type – It is the most basic grade of syconoid canal system. The radial canals are free projections of the body wall and they do not touch each other. Definite incurrent canals are absent in this type. The spaces between radial canals act as dermal ostia and water directly enters through these gaps. Radial canals are lined with choanocytes while the spongocoel is lined with pinacocytes. Example– Sycetta.
2. Complex sycon type– It is more advanced than simple sycon type. The walls of adjacent radial canals fuse with each other. Due to this fusion definite tubular incurrent canals are formed between radial canals. Incurrent canals open outside through dermal ostia and their inner ends are blind. Prosopyles connect incurrent canals with radial canals. Radial canals are lined with choanocytes and they open into spongocoel through apopyles. The spongocoel opens outside through osculum. Example– Scypha (Sycon).
3. Sycon type with cortex– It is the most advanced grade of syconoid canal system. The dermal membrane spreads over the entire outer surface forming a cortex of variable thickness. Incurrent canals pass irregularly through this cortex before reaching radial canals. Sometimes large sub dermal spaces are formed in the cortex. Radial canals fuse and form definite incurrent canals as in complex type. Examples– Grantia, Heteropia and Ute.

3. Leucon type of canal system

Leucon type of canal system is the most complex and most efficient type found in sponges. It is characteristic feature of leuconoid sponges and present in majority of species. This type shows highest degree of folding and branching of canal system.

The important points are as follows–

• It is the most complex and highly efficient type of canal system. It is found in more than 90% of sponges.
• Due to complexity the original radial symmetry is lost and the body becomes irregular in shape.
• The central cavity (spongocoel) is highly reduced or completely absent in this type.
• Radial canals are replaced by numerous small rounded or oval flagellated chambers.
• Choanocytes (collar cells) are restricted only to these flagellated chambers.
• Water enters through dermal pores (ostia) into irregular branching incurrent canals.
• From incurrent canals water passes into flagellated chambers through minute pores called prosopyles.
• Water leaves flagellated chambers through apopyles into excurrent canals.
• Excurrent canals carry water towards one or more oscula through which it is expelled outside.
• The route of water flow is–
  Dermal pores → Incurrent canals → Prosopyles → Flagellated chambers → Apopyles → Excurrent canals → Osculum → Outside.
• Leucon type shows three evolutionary stages–
  • Eurypylous type – Flagellated chambers are wide and open directly into excurrent canals through broad apopyles.
  • Aphodal type – A narrow tube called aphodus connects the flagellated chamber to the excurrent canal.
  • Diplodal type – Two narrow tubes are present. Prosodus connects incurrent canal to chamber and aphodus connects chamber to excurrent canal.

Structure of Leucon type of canal system

Structure of Leucon type of canal system shows highest degree of complexity in sponges. It is formed by excessive folding of choanocyte layer and thickening of body wall. Due to this modification the internal organization becomes highly branched.

The structural parts are as follows–

• Body shape and symmetry– The body becomes irregular in shape and radial symmetry is lost. The wall is thick and extensively folded.
• Dermal ostia (Pores)– The outer surface is covered by epidermal epithelium. Numerous dermal pores are present through which water enters into body.
• Incurrent canals– Dermal pores lead into highly branched and irregular incurrent canals. These canals pass through mesenchyme tissue.
• Prosopyles– These are minute pores connecting incurrent canals to flagellated chambers. Through these openings water enters the chambers.
• Flagellated chambers– Choanocytes are restricted only to small rounded or oval chambers. These chambers replace the radial canals of simpler types.
• Apopyles– Water leaves the flagellated chambers through internal openings called apopyles.
• Excurrent canals– Water from apopyles enters excurrent canals. These canals unite to form larger channels. These canals are lined with flat cells called pinacocytes.
• Spongocoel– The central cavity is highly reduced or completely absent in this type.
• Oscula– Large excurrent canals open outside through one or more oscula.
• Water flow route– The pathway of water is– Dermal ostia → Incurrent canals → Prosopyles (prosodus if present) → Flagellated chambers → Apopyles (aphodus if present) → Excurrent canals → Larger channels → Oscula → Outside.

Types of Leucon type of canal system

Types of Leucon type of canal system shows different evolutionary stages in leuconoid sponges. It is based on the manner in which flagellated chambers are connected with incurrent and excurrent canals. These types represents progressive structural modification.

The types are as follows–

1. Eurypylous type– It is the simplest and most primitive stage of leuconoid canal system. The flagellated chambers are wide and thimble shaped. These chambers open directly into excurrent canals through broad openings called apopyles. Water enters the chamber directly from incurrent canal through prosopyle. Example– Plakina, Leucilla, Plakortis and Tetilla.
2. Aphodal type– In this type the flagellated chambers are small and rounded. The apopyle is drawn out into a narrow tube called aphodus. Aphodus connects the flagellated chamber to excurrent canal. Water from incurrent canal still enters directly into chamber. Example– Geodia and Stelleta.
3. Diplodal type– It is the most complex variation of leucon type. A narrow tube called prosodus connects the incurrent canal to the flagellated chamber. Another narrow tube called aphodus connects the chamber to excurrent canal. Thus the chamber is connected by narrow tubes on both incoming and outgoing sides. Example– Spongilla, Oscarella and Chondrosia.

4. Rhagon Type

Rhagon type is a primitive type of canal system mainly seen in developmental stage of demosponges. It represents an intermediate condition before the formation of fully developed leuconoid system.

The main points are as follows–

• It is found in larval or young stage of Demospongiae such as Spongilla.
• In adult stage it is modified into leucon type of canal system.
• The body has a broad basal part and a conical upper part.
• A single osculum is present at the summit of the conical region.
• The broad basal wall is called hypophare and it is devoid of flagellated chambers.
• The upper conical wall is called spongophare and it bears small oval flagellated chambers.
• The mesenchyme is thickened and contains subdermal spaces and branching incurrent canals.
• The flagellated chambers lie around the spongocoel and are lined with choanocytes.
• Water from chambers passes into spongocoel through wide openings called apopyles.
• The route of water flow is–
  Ostia → Subdermal spaces → Incurrent canals → Prosopyles → Flagellated chambers → Apopyles → Excurrent canals → Spongocoel → Osculum → Outside.

Structure of Rhagon Type of canal system

Structure of Rhagon type of canal system shows a primitive organization mainly in developmental stage of demosponges. It represents an intermediate condition before formation of complete leuconoid system.

The structural features are as follows–

• Occurrence– It is found in larval or developmental stage of Demospongiae such as Spongilla. In adult stage it is transformed into leucon type of canal system.
• Body shape– The body has a broad basal region and a conical or tent like upper region. A single terminal osculum is present at the summit.
• Hypophare (Basal wall)– The broad basal wall is called hypophare. It is completely devoid of flagellated chambers.
• Spongophare (Upper wall)– The conical upper wall is called spongophare. It contains small oval flagellated chambers lined with choanocytes.
• Mesenchyme– The middle gelatinous layer between epidermis and chambers is considerably thickened.
• Ostia and subdermal spaces– Dermal pores (ostia) are present on the surface. These pores open into subdermal spaces which extend beneath the outer surface.
• Incurrent canals– From subdermal spaces branching incurrent canals arise. These canals carry water directly into small flagellated chambers.
• Spongocoel and apopyles– The central cavity (spongocoel) is large and bordered by oval flagellated chambers. Water leaves the chambers through wide openings called apopyles and enters the spongocoel.
• Water flow route– The pathway of water is– Ostia → Subdermal spaces → Incurrent canals → Prosopyles → Flagellated chambers → Apopyles → Spongocoel → Osculum → Outside.

Functions of Canal System

Functions of canal system in sponges are performed by continuous circulation of water. Since sponges do not possess true organs all essential activities are carried out by this system.

The functions are as follows–

• It brings microscopic food particles such as bacteria and diatoms into the body.
• Food particles are trapped by choanocytes and digested intracellularly.
• It supplies oxygen to the body cells for respiration.
• Carbon dioxide produced in cells is removed through outgoing water.
• Metabolic wastes such as ammonia and other nitrogenous substances are carried away.
• Fecal matter is expelled through the osculum with water current.
• It transports sperms from one sponge to another helping in fertilization.
• It helps in maintaining internal water balance by preventing excess accumulation of water.
• The branching canals increases the surface area and maintains surface area to volume ratio.
• The internal chambers and passages provide shelter to small aquatic organisms.

FAQ

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