Unit 1: Protozoa, Porifera and Cnidaria

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