Unit 1: Protozoa, Porifera and Cnidaria
Table of Contents
1. Protozoa: General Characters, Classification, and Locomotion
Protozoa are microscopic, acellular (unicellular) eukaryotic organisms that exist singly or in colonies.
General Characters
- They are unicellular organisms where all biological activities are performed by a single cell.
- Locomotion occurs through specialized organelles like pseudopodia, flagella, or cilia.
- Nutrition can be holophytic, holozoic, or saprozoic.
Classification (up to Classes)
Protozoa are classified primarily based on their locomotory organelles:
- Sarcodina (Rhizopoda): Move via pseudopodia (e.g., Amoeba).
- Mastigophora (Flagellata): Move via flagella (e.g., Euglena).
- Ciliata: Move via cilia (e.g., Paramecium).
- Sporozoa: Parasitic forms lacking locomotory organelles (e.g., Plasmodium).
Locomotion in Protozoa
Locomotion is essential for finding food and escaping predators. The primary mechanisms include:
- Amoeboid movement: Streaming of cytoplasm and formation of pseudopodia.
- Flagellar movement: Whip-like action of flagella.
- Ciliary movement: Oar-like rhythmic beating of cilia.
2. Porifera: General Characters, Classification, and Canal System
Porifera, commonly known as sponges, are the simplest multicellular animals with a cellular level of organization.
General Characters
- Bodies are porous with multiple inhalant pores (Ostia) and a single exhalant opening (Osculum).
- Skeleton consists of calcareous or siliceous spicules or spongin fibers.
- Possess unique flagellated cells called choanocytes.
Classification (up to Classes)
- Calcarea: Spicules are made of calcium carbonate (e.g., Sycon).
- Hexactinellida: Six-rayed siliceous spicules (e.g., Euplectella).
- Demospongiae: Siliceous spicules or spongin fibers (e.g., Spongilla).
Canal System in Sycon
The canal system is a specialized water circulatory system in sponges used for nutrition, respiration, and excretion.
Syconoid Canal System: Water flows through dermal ostia → Incurrent canals → Prosopyles → Radial canals (lined with choanocytes) → Apopyles → Spongocoel → Osculum.
3. Cnidaria: General Characters, Classification, and Corals
Cnidarians are diploblastic, tissue-grade animals characterized by the presence of stinging cells called cnidoblasts.
General Characters
- Radially symmetrical body with a central gastrovascular cavity.
- Exists in two basic body forms: Polyp (sessile) and Medusa (free-swimming).
- Metagenesis (alternation of generations) is often observed.
Classification (up to Classes)
- Hydrozoa: Both polyp and medusa present (e.g., Hydra, Obelia).
- Scyphozoa: Medusa stage is dominant (e.g., Aurelia - Jellyfish).
- Anthozoa: Only polyp stage present; includes corals and sea anemones.
Coral and Coral Reef Formation
Corals are skeletal structures composed of calcium carbonate secreted by polyps of the class Anthozoa.
- Fringing Reefs: Formed near the shore of a mainland or island.
- Barrier Reefs: Separated from the shore by a deep lagoon.
- Atolls: Circular reefs enclosing a central lagoon, usually formed around submerged volcanic islands.
4. Exam Focus Enhancements
Exam Tips
- Choanocytes: Always mention these as the "collar cells" unique to Porifera that drive the canal system.
- Locomotion: Be ready to compare flagellar and ciliary movement; flagella are long and few, while cilia are short and numerous.
- Reef Types: Barriers reefs are the largest; Atolls are horseshoe or circular in shape.
Common Mistakes
- Confusing Ostia with Osculum. Ostia are for water entry (many), Osculum is for water exit (usually one).
- Thinking sponges are plants. Sponges are sessile animals with a cellular level of organization.
Frequently Asked Questions
- Classify Protozoa based on locomotory organelles with examples.
- Describe the syconoid type of canal system in Sycon.
- What are the different types of coral reefs? Explain their formation.