Model Answer
0 min readIntroduction
Porifera, meaning “pore-bearing”, represents the simplest multicellular animals and belongs to the subkingdom Parazoa. These are primarily marine, sessile, and filter-feeding animals exhibiting cellular level of organization. Their body is perforated by numerous pores called ostia, through which water enters, and a single large opening called osculum, through which water exits. Understanding their characteristics and classification is fundamental to comprehending the evolution of multicellularity and animal diversity. They are considered the earliest evolved metazoans, providing insights into the origins of animal life.
General Characters of Porifera
Porifera exhibit several unique characteristics:
- Level of Organization: Cellular level. Cells are loosely aggregated and do not form true tissues or organs.
- Body Plan: Typically asymmetrical or radially symmetrical. The body wall is composed of three layers: pinacoderm (outer layer), mesohyl (middle layer), and choanoderm (inner layer).
- Canal System: A complex system of canals and chambers facilitates water circulation for feeding, respiration, and excretion. The canal system varies in complexity from asconoid to syconoid to leuconoid.
- Skeletal Structure: Possess an internal skeleton composed of spicules (made of calcium carbonate or silica) and/or spongin fibers (a protein).
- Feeding Mechanism: Filter feeders. Choanocytes (collar cells) line the internal chambers and use flagella to create a water current, trapping food particles.
- Reproduction: Reproduce both asexually (budding, fragmentation, gemmules) and sexually (gamete formation and fertilization).
- Absence of True Tissues: Lack true tissues like epithelium, muscles, and nerves.
Classification of Porifera
Porifera is classified into three classes based on the complexity of their canal system, spicule composition, and body organization:
Class 1: Calcarea (Calcareous Sponges)
These are the most primitive sponges.
- Spicules: Composed of calcium carbonate (calcite).
- Canal System: Asconoid or syconoid type.
- Body Shape: Generally small and vase-like.
- Examples: Sycetta, Leuconia.
Class 2: Hexactinellida (Glass Sponges)
These sponges are characterized by their glassy skeleton.
- Spicules: Composed of silica, having six rays.
- Canal System: Syconoid or leuconoid type.
- Body Shape: Cup-shaped or cylindrical.
- Habitat: Deep sea.
- Examples: Euplectella (Venus’ Flower Basket), Hyalonema.
Class 3: Demospongiae (Common Sponges)
This is the largest class of sponges, containing about 90% of all sponge species.
- Spicules: Composed of silica, but may be absent. If present, they have various shapes and sizes.
- Skeleton: May also contain spongin fibers.
- Canal System: Leuconoid type, allowing for a large surface area for filtration.
- Habitat: Marine and freshwater.
- Examples: Spongilla (freshwater sponge), Bath sponge (Spongia), Cliona.
| Class | Spicules | Canal System | Skeleton | Habitat |
|---|---|---|---|---|
| Calcarea | Calcium Carbonate | Asconoid/Syconoid | Calcareous spicules | Marine |
| Hexactinellida | Silica (6-rayed) | Syconoid/Leuconoid | Siliceous spicules | Deep Sea |
| Demospongiae | Silica or Absent | Leuconoid | Siliceous spicules & Spongin fibers | Marine & Freshwater |
Conclusion
Porifera, as the simplest multicellular animals, provide crucial insights into the evolutionary history of the animal kingdom. Their unique characteristics, including the canal system and skeletal structures, are adaptations for a sessile, filter-feeding lifestyle. The classification into Calcarea, Hexactinellida, and Demospongiae reflects the diversity within this phylum, based on variations in spicule composition, canal system complexity, and habitat. Further research into their genomic and developmental biology will continue to illuminate their evolutionary significance.
Answer Length
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