Unit 1: Protozoa, Porifera and Cnidaria

Table of Contents

1. Phylum Protozoa

Protozoa (from Greek: protos = first, zoon = animal) are microscopic, unicellular eukaryotic organisms. They represent the simplest form of animal life, exhibiting a protoplasmic grade of organization, where all life functions are performed within the confines of a single cell.

1.1 General Characters and Classification

General Characters

Classification of Protozoa (Up to Classes)

The primary basis for classifying Protozoa is their locomotory organelle.
Subphylum Superclass / Class Key Characteristics Examples
Sarcomastigophora
(Locomotion by flagella, pseudopodia, or both)		 Superclass Mastigophora (Flagellata) One or more flagella for locomotion. Euglena, Trypanosoma, Volvox
- Class Phytomastigophorea Plant-like; contain chromatophores. Euglena, Chlamydomonas
- Class Zoomastigophorea Animal-like; lack chromatophores. Trypanosoma, Leishmania
Superclass Sarcodina (Rhizopoda) Locomotion by pseudopodia. Amoeba, Entamoeba, Radiolaria
Sporozoa All parasitic; no specialized locomotory organelles. Plasmodium (malarial parasite), Monocystis
Ciliophora Numerous cilia for locomotion and feeding. Two types of nuclei (macro and micro). Paramecium, Vorticella

1.2 Locomotion in Protozoa

Protozoans exhibit four main types of movement, which are also the basis for their classification:

  • Pseudopodia (Amoeboid movement): Temporary protoplasmic outgrowths ("false feet") used for movement and capturing food. The cytoplasm flows into these extensions. Found in Sarcodina (e.g., Amoeba). Types include lobopodia (blunt), filopodia (thread-like), and axopodia (with axial filament).
  • Flagella (Flagellar movement): Long, whip-like structures that propel the organism. The flagellum beats in an undulating or whip-like motion. Found in Mastigophora (e.g., Euglena).
  • Cilia (Ciliary movement): Short, hair-like structures that cover the cell surface. They beat in a coordinated rhythm (metachronal rhythm) to facilitate movement and create feeding currents. Found in Ciliophora (e.g., Paramecium).
  • Gliding/Wriggling: A slow gliding or wriggling motion seen in some parasitic forms like Sporozoa that lack specialized locomotory organelles.

2. Phylum Porifera (Sponges)

Porifera (from Latin: porus = pore, ferre = to bear) are multicellular organisms with a cellular grade of organization. They are the simplest metazoans, characterized by a unique water canal system.

2.1 General Characters and Classification

General Characters

  • Multicellular: Simplest multicellular animals with no true tissues or organs.
  • Habitat: Mostly marine, with a few freshwater species. They are sessile (attached to a substrate).
  • Body Wall: The body is perforated by numerous pores called ostia (incurrent) and one or more large openings called oscula (excurrent).
  • Choanocytes: The internal cavity (spongocoel) or canals are lined with flagellated collar cells (choanocytes), which create water currents and capture food.
  • Skeleton: An internal skeleton made of calcareous or siliceous spicules, or proteinaceous spongin fibres, provides support.
  • Reproduction: Both asexual (budding, gemmules) and sexual.

Classification of Porifera (Up to Classes)

Classification is based on the type of skeletal material.
Class Skeletal Material Canal System Examples
Calcarea Calcareous spicules (calcium carbonate). Asconoid, Syconoid, or Leuconoid. Sycon, Leucosolenia
Hexactinellida Six-rayed siliceous spicules (silica). "Glass sponges." Syconoid or Leuconoid. Euplectella (Venus's flower basket)
Demospongiae Siliceous spicules (not six-rayed) and/or spongin fibres. Leuconoid only. This is the largest class. Spongilla (freshwater sponge), Euspongia (bath sponge)

2.2 Canal System in Sycon

Sycon (or Scypha) exhibits the Syconoid type of canal system, which is more complex than the simple Asconoid type but less complex than the Leuconoid type.

Structure and Path of Water

The body wall of Sycon is folded, forming alternating incurrent and radial canals.

  1. Incurrent Canals: Lined by simple epithelial cells (pinacocytes) and open to the outside via dermal ostia. They end blindly on the inside.
  2. Radial Canals: Lined by flagellated choanocytes. These are the feeding chambers. They end blindly on the outside.
  3. Prosopyles: Minute pores that connect the incurrent canals to the radial canals.
  4. Apopyles: Larger openings that connect the radial canals to the central spongocoel.
  5. Spongocoel: The central cavity, which is lined by pinacocytes (not choanocytes).
  6. Osculum: The single large excurrent opening at the top of the sponge.

Path of Water:

Water (Outside) → Dermal Ostia → Incurrent Canals → Prosopyles → Radial Canals (Food capture) → Apopyles → Spongocoel → Osculum → Water (Outside)

Key Difference: In the Syconoid type, choanocytes are restricted *only* to the radial canals, unlike the Asconoid type where they line the entire spongocoel.

3. Phylum Cnidaria

Cnidaria (from Greek: knide = nettle) are radially symmetrical, diploblastic animals. They are characterized by the presence of stinging cells called cnidocytes or nematocysts. They exhibit a tissue grade of organization.

3.1 General Characters and Classification

General Characters

  • Habitat: Mostly marine, a few are freshwater (e.g., Hydra).
  • Symmetry: Radial symmetry.
  • Body Organization: Diploblastic (two germ layers: outer epidermis and inner gastrodermis) with a gelatinous, non-cellular layer called mesoglea.
  • Body Cavity: A central gastrovascular cavity (coelenteron) with a single opening (mouth/anus).
  • Cnidocytes: Specialized stinging cells, especially on the tentacles, used for defense and prey capture.
  • Body Forms: Exhibit two main body forms: a sessile, cylindrical polyp and a free-swimming, umbrella-shaped medusa.

Classification of Cnidaria (Up to Classes)

Class Dominant Form Key Characteristics Examples
Hydrozoa Both Polyp and Medusa Polyp stage is often colonial. Medusae have a velum. Mesoglea is non-cellular. Hydra, Obelia, Physalia
Scyphozoa Medusa is dominant "True jellyfish." The polyp stage is reduced. Medusae are large and lack a velum. Aurelia (moon jelly)
Anthozoa Polyp only No medusa stage. The gastrovascular cavity is partitioned by septa or mesenteries. All marine. Metridium (sea anemone), Corals

3.2 Coral and Coral Reef Formation

Corals

Corals are marine invertebrates belonging to the Class Anthozoa. A coral "head" is a colony of numerous tiny, genetically identical polyps. Each polyp secretes an exoskeleton of calcium carbonate. Over generations, these exoskeletons build up to form the massive structures known as coral reefs.

Many corals (reef-building corals) have a symbiotic relationship with single-celled algae called zooxanthellae that live within their tissues. The algae provide the coral with food via photosynthesis, and in return, the coral provides a protected environment. This is why most reef-building corals are found in clear, shallow, warm tropical waters.

Coral Reef Formation

Coral reefs are vast underwater ecosystems built by corals. They are often called "rainforests of the sea" due to their immense biodiversity.

  1. Fringing Reefs: Grow directly from the shore, forming a border along the coastline.
  2. Barrier Reefs: Located further offshore, separated from the land by a wide, deep lagoon. The Great Barrier Reef in Australia is the most famous example.
  3. Atolls: Ring-shaped reefs that enclose a central lagoon. They typically form on submerged volcanic islands.

Coral Bleaching: This is a major threat to reefs. When corals are stressed (e.g., by high water temperatures), they expel their symbiotic zooxanthellae. This causes them to lose their color and turn white, and if the stress is prolonged, the coral will die.