The Porifera are sponges. They are primarily marine animals. There are nine thousand species of sponges. There are following characteristics of the phylum Porifera:
- Their bodies consist of loosely organized cells.
- They vary in size from less than a centimeter to a mass that can fill your arm.
- They are asymmetrical or radially symmetrical.
- They have three cell types: pinacocytes, mesenchyma cells, and choanocytes.
- They have central cavity or spongocoel. This cavity may be divided into series of branching chambers. Water circulates through these chambers for feeding.
- Numerous pores are present in the body wall. i.e. ostia and osculum
- They have no tissue or organ.
- Skeleton is composed of spicules.
- Nervous system is absent but neurosensory cells are present.
- Asexual reproduction takes place by budding.
- They are hermaphrodites and larvae are produced during development.
CELL TYPES, BODY WALL AND SKELETONS
Sponges have simple bodies. But still sponges are more than colonies of independent cells. The sponges also have specialized cells. Therefore, division of labour is present in them. Following types of cells are present in phylum porifera.
1. Pinacocytes: Pinacocytes are thin walled and flat cells. They line the outer surface of a sponge. Pinacocytes are slightly contractile. Their contraction can change the shape of some sponges. Some pinacocytes forms tube like contractile porocytes. Porocytes regulate water circulation. The openings of the porocytes are pathways tiw water movement of water through the body wall.
2. Mesohyl: Mesohyl is a jelly like layer present below the pinacocytes. Amoeboid cells are present in it. These cells are called mesenchyma cells. The meseuchyma cells freely move in the mesohyl. These cells are specialized for reproduction, secreting, skeletal elements, transporting and storing food and forming contractile rings around openings in the sponge wall.
3. Choanocytes: Choanocytes or collar cells are present below the mesohyl. They form the lining of the inner chamber. Choanocytes are flagellated cells. They have a collar like ring of microvilli surrounding a flagellum. Microfilaments connect the microvilli. It forms a netlike structure within the collar. The flagellum creates water currents through the sponge. The collar filters microscopic lbod particles from the water. Collar cells arc also present in a group of protists called choanollagellates. Choanocytes are present in sponges and choantlagellaes. It suggests an evolutionary link between these groups.
4. Skeleton: The nature of the skeleton is an important characteristic in sponge taxonomy. There are two types of skeleton in sponges:
(a) Spit lest The spicules consist of microscopic needlelike spikes Spic les are formed by amoeboid cells. They are made of calcium carbonate or silica. They have different shapes.
(I)) Spongin fibers: The spongin fibers are made up of .spongin. Spongin is a fibrous protein made of collagen. It is dried, beaten and washed and all cells are removed. This washed and dried spongin produce a commercial sponge.
WATER CURRENTS AND BODY FORMS (Water Canal System)
The lit of a sponge depends on the water currents. Thechoanocytes organized to form water canal system. Water currents bring food and oxygen for a sponge. It also carries away metabolic and digestive wastes. Water canal system is used for circulation and filtration of food. There are three principal types of water canal systems:
1. Ascon type
It is simplest and least common sponge body form. Ascon sponges are vase like. Ostia are the outer openings of porocytes. The porocytes open directly into spongocoel. Choanocytes line the spongocoel. The movements of flagella of choanocytes draw water into the spongocoel through the ostia. Water leaves the sponge through the osculum. Osculum is a single large opening at the top of the sponge.
2. Sycon type
The sponge wall is folded in the sycon body form. Following canals are formed by the folding of its wall:
(a) Incurrent canal: The invaginations of the body wall of sycon form incurrent canals. Water enters into incurrent canal through dermal pores.
(b) Radial canal: Pores in the wall of incurrent wall connect incurrent canals with radial canals. Choanoeytes line the radial canals. The beating of flagella of choanocytes moves water through incurrent radial canals and spongocoel. It finally moves out the osculum.
(c) Spongocoel: The radial canals lead to spongocoel.
Water path through sycon type: Dermal pore — incurrent canal — pore — radial canal — spongocoel — osculum.
3. Leucon type
Leucon sponges have an extensively branched canal system. There are following chambers in leucon type:
(a) Branched incurrent canal: Water enters the branched incurrent canals through ostia
(b Choanocytes chamber: Incurrent canal lead into choanocytes-lined chambers.
(c Excurrent canal: Choanocyte chamber open into the chambers of excurrent canals A large number of chambers and canals are present in leucon type. Therefore, spongocoel is absent in them. They have many oseulums for water leaving the sponge. Luecon type canal system is formed by the evolution of simple canal system.
Advantages of complex canal system
Complex sponges have an increased surface area for choanocytes. Therefore, a large amount of water through the sponge. It increases the filtering capability of sponges.
Food: Sponges feed on particles with size from 0.1 to 50 pm. Their food consists of bacteria, microscopic algae, protists and other suspended organic matter. The preys are slowly drawn into the sponge and digested. Large populations of sponges play an important role in reducing the turbidity of coastal waters. A single leucon with size 1 cm in diameter and 10cm high can fitter 20 liters of water everyday.
A few species of sponges are carnivores. The deep-water sponges like Asbestopduma have spicules covered filaments. It captures small crustaceans with the help of these spiculces.
Mechanism of feeding: Choanocytes filter small suspended food particles. Water passes through their collar near the base of the cell. It then moves out of choanocytes into the sponge chamber through the open end of the collar. Suspended food is trapped on the collar. It passes through microvilli and reaches at the base of the collar. It forms a food vacuole in collar. The pH of food is changed. Digestion in food vacuole takes place by lysosomal enzymes. Partially digested food is passed into amoeboid cells. Amoeboid cells distribute it to other cells.
Pinacocyts lines the incurrent canal. Large food particles (up to 50 pm) can enter into pinacocytcs byphagocytosis. Sponges also absorb nutrients dissolved in seawater by active transport.
2. Excretion and respiration
There is an extensive canal system in sponges. A large volume of water circulates of through these canals. All sponge cells are in close contact with water. Thus, nitrogenous waste (principally ammonia) is removed and gas exchange occurs by diffusion.
There are two forms of coordination in sponges:
(a) Coordination by external factors: Sponges do not have nerve cells to coordinate body functions. Mostly individual cells show response to a stimulus. For example, Light inhibits the constriction of porocytes and other cells surrounding the ostia. It keeps the incurrent canals open. Therefore, water circulation through some sponges is minimum at sunrise. But it is maximum just before sunset.
(b) Coordination by internal factors: Some other reactions suggest that some communication is present among cells. For example, the rate of water circulation through a sponge can drop suddenly without external cause. This reaction takes place due to choanocytes. It stops the activities simultaneously. These reaction shows that some form of internal communication is present in the sponges. The nature of this communication is unknown. Amoeboid cells may transmit chemical messages and ion movement over cell surfaces. It is a possible control mechanism.
1. Sexual Reproduction
Most sponges are monoecious. But individual sponges produce eggs and sperm at different times. Therefore, they do not self-fertilized.
(a) Gametogensis: Certain choanocytes lose their collars and flagella. They undergo and form flagellated sperm. Other choanocytes (and amoeboid cells) undergo meiosis and form eggs.
(b) Fertilization: Eggs are retained in the mesohyl of the parent. Sperm cells passes from one sponge through the osculum and enter another sponge with the incurrent water. Sperm are trapped by choanocytes. Sperm is covered into a vacuole in choanocytes. The choanocytes lose their collar and flagellum and become amoeboid cells. It transports sperm into the eggs and zygote is formed.
(c) Development: Early development occurs in the mesohyl. Cleavage occurs in zygote. A flagellated larval stage is formed. These larvae may be parenchymula larva or amphiblastula larva. The larva becomes free. The water currents carry the larva out of the parent sponge. It freely swims for two days. Then the larva settles on the substrate and develops into the adult body form.
2. Asexual reproduction
(a) Gemmule formation: Asexual reproduction takes place by gemmules formation. Gemmule is a resistant capsule containing masses of amoeboid cells. The parent sponge dies in the winter and it releases gemmules. Gemmules can survive both in freezing and drying condition. The conditions become favorable in the spring. Now the ameboid cells come out through a tiny opening called the micropyle. It develops into a sponge.
(b) Regeneration: Some sponges possess great powers of regeneration. Portions of a sponge are cut or broken. The broken piece forms new sponge.