Phylum Porifera is the simplest and most primitive group of multicellular animals in the Animal Kingdom. The members of this phylum are commonly known as sponges and are characterised by the presence of numerous pores on their body surface through which water circulates continuously. Poriferans are mainly aquatic, sessile animals that are predominantly found in marine environments, although some species inhabit freshwater habitats. They exhibit a cellular level of organisation and represent an early stage in the evolution of multicellular animals, making them important for understanding animal evolution.
General Characteristics of Phylum Porifera
The members of Phylum Porifera possess several characteristic features that distinguish them from other animal groups.
- They are multicellular, eukaryotic, heterotrophic animals.
- Most poriferans are marine, while a few inhabit freshwater bodies.
- They are generally sessile and remain attached to a substratum.
- The body contains numerous minute pores called ostia.
- Water enters through ostia and leaves through a larger opening called the osculum.
- They exhibit a cellular level of organisation and lack true tissues and organs.
- Most sponges are asymmetrical, although some may exhibit radial symmetry.
- The body contains a central cavity called the spongocoel.
- Digestion is intracellular.
- They do not possess specialised digestive, circulatory, respiratory, nervous, or excretory systems.
- The body is supported by skeletal elements called spicules or spongin fibres.
- They reproduce both sexually and asexually.
- Most sponges are hermaphrodites.
- They possess remarkable regenerative power.
Habitat of Porifera
- Poriferans are found in a wide variety of aquatic habitats. Most species inhabit seas and oceans, where they remain attached to rocks, coral reefs, shells, or submerged objects.
- A few species, such as Spongilla, occur in freshwater ponds, lakes, and streams.
- They are commonly found from shallow coastal waters to deep oceanic regions.
- Their distribution depends upon factors such as water temperature, salinity, oxygen concentration, and the availability of suitable surfaces for attachment.
Body Organisation of Porifera
The body of a sponge is usually cylindrical, vase-shaped, tubular, or irregular in form. Numerous tiny pores called ostia are present on the body wall. These pores open into a system of canals through which water circulates continuously. The body encloses a central cavity called the spongocoel, which opens to the exterior through the osculum. The body wall consists of the following layers:
- Outer Layer (Pinacoderm): The outermost layer consists of flattened cells known as pinacocytes. These cells provide protection and maintain the shape of the body.
- Inner Layer (Choanoderm): The inner layer is lined by specialised flagellated cells called choanocytes or collar cells. These cells generate water currents and capture food particles from the surrounding water.
- Mesohyl: Between the outer and inner layers lies a gelatinous matrix called mesohyl. It contains amoebocytes, reproductive cells, spicules, and spongin fibres.
Water Canal System in Porifera
The most characteristic feature of Porifera is the water canal system. Water continuously flows through the sponge body, carrying food particles and oxygen while removing waste materials and carbon dioxide. This system is essential for nutrition, respiration, excretion, and reproduction.
Skeleton of Porifera
The skeleton provides support and protection to the sponge body. It is formed by spicules or spongin fibres.
- Spicules: Spicules are microscopic skeletal structures that may be composed of Calcium carbonate (calcareous spicules) and Silica (siliceous spicules)
- Spongin Fibres: Spongin is a proteinaceous substance that forms flexible skeletal fibres. The type of skeleton forms the basis of sponge classification.
Nutrition in Porifera
- Poriferans are filter feeders and exhibit holozoic nutrition.
- Water entering through the ostia carries microscopic food particles such as bacteria, protozoans, algae, and organic debris. Choanocytes trap and ingest these food particles.
- Digestion occurs inside food vacuoles and is therefore intracellular.
- Amoebocytes distribute the digested nutrients to various body cells.
Respiration in Porifera
- Poriferans do not possess specialised respiratory organs.
- Respiration occurs by simple diffusion through the body surface.
- Oxygen dissolved in water diffuses directly into body cells, while carbon dioxide diffuses outward into the surrounding water.
- The continuous water current ensures an adequate supply of oxygen.
Reproduction in Porifera
Poriferans reproduce by both asexual and sexual methods.
Asexual Reproduction
- Budding: Small outgrowths called buds arise from the parent body. These buds may detach and develop into new individuals.
- Fragmentation: Broken pieces of the sponge body can regenerate into complete organisms.
- Gemmule Formation: Freshwater sponges form resistant structures called gemmules during unfavourable conditions. These gemmules germinate when favourable conditions return.
Sexual Reproduction
Most sponges are hermaphrodites, possessing both male and female reproductive cells. Process of Sexual Reproduction are given below:
- Sperms are released into the water.
- Sperms enter another sponge through the water current.
- Fertilisation occurs internally.
- A zygote is formed.
- The zygote develops into a free-swimming larva.
- The larva settles on a suitable substratum and develops into an adult sponge.
Economic Importance of Porifera
Beneficial Importance
- Bath sponges are obtained from certain demosponges.
- They help maintain water purity by filtering large quantities of water.
- They provide shelter to many marine organisms.
- Some species produce biologically active compounds used in medicine.
- They contribute to nutrient cycling in aquatic ecosystems.
Harmful Importance
- Boring sponges such as Cliona damage oyster shells and coral reefs.
- Some species may negatively affect aquaculture operations.