Unit 1: Protozoa and Porifera

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 Nutrition, Locomotion and Reproduction in Protozoa

Nutrition

  1. Holozoic (Animal-like): Ingesting solid food particles. This involves phagocytosis, where food is engulfed to form a food vacuole. Digestion is intracellular. Example: Amoeba.
  2. Holophytic (Plant-like): Synthesizing food through photosynthesis, using sunlight and chlorophyll. Example: Euglena.
  3. Saprozoic: Absorbing soluble organic nutrients from the environment through the body surface. Common in parasitic forms.
  4. Parasitic: Deriving nutrients from a living host, causing harm. Example: Plasmodium.

Locomotion

  • Pseudopodia: Temporary protoplasmic outgrowths ("false feet"). Used for movement and capturing food in Sarcodina. Types include lobopodia (blunt), filopodia (thread-like), and axopodia (with axial filament).
  • Flagella: Long, whip-like structures that propel the organism. Found in Mastigophora.
  • Cilia: Short, hair-like structures that beat in a coordinated rhythm (metachronal rhythm) to facilitate movement and create feeding currents. Found in Ciliophora.

Reproduction

  • Asexual Reproduction:
    • Binary Fission: The parent cell divides into two identical daughter cells. It can be longitudinal (Euglena) or transverse (Paramecium).
    • Multiple Fission (Schizogony): The nucleus divides multiple times, followed by cytoplasmic division, producing many daughter cells simultaneously. Common in parasitic Sporozoa like Plasmodium.
  • Sexual Reproduction:
    • Syngamy: Complete fusion of two gametes to form a zygote.
    • Conjugation: A complex process seen in ciliates like Paramecium. Two individuals temporarily unite to exchange micronuclear material, leading to genetic recombination. They separate after exchange and then undergo fission.
Exam Tip: Conjugation in Paramecium is a key topic. Remember that it's a method of nuclear reorganization and genetic recombination, not reproduction itself, as the number of individuals does not increase at the end of the process.

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) is lined with flagellated collar cells called choanocytes, which create water currents and capture food.
  • Skeleton: An internal skeleton made of calcareous or siliceous spicules, or proteinaceous spongin fibres, provides support.
  • Digestion: Intracellular, taking place within choanocytes and amoebocytes.
  • Reproduction: Both asexual (budding, gemmules) and sexual. Sponges are mostly hermaphroditic.

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). Known as "glass sponges." Syconoid or Leuconoid. Euplectella (Venus's flower basket), Hyalonema
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 Sponges

The canal system is the hallmark of Phylum Porifera. It serves for food collection, respiration, and excretion.

  1. Asconoid Type:
    • The simplest type, found in small sponges like Leucosolenia.
    • The body wall is thin and unfolded.
    • Path of water: Ostia → Spongocoel (lined by choanocytes) → Osculum.
  2. Syconoid Type:
    • More complex, with a folded body wall forming incurrent and radial canals. Found in Sycon.
    • The spongocoel is lined by simple epithelial cells, not choanocytes. Choanocytes are restricted to the radial canals.
    • Path of water: Dermal Ostia → Incurrent Canals → Prosopyles → Radial Canals → Apopyles → Spongocoel → Osculum.
  3. Leuconoid Type:
    • The most complex and efficient type, found in the largest sponges like Spongilla.
    • The body wall is very thick. The spongocoel is often absent or reduced to excurrent canals.
    • Choanocytes are confined to small, spherical flagellated chambers.
    • Path of water: Dermal Ostia → Incurrent Canals → Flagellated Chambers → Excurrent Canals → Osculum.
Common Mistake: Students often forget the small openings connecting the canals. Remember: Prosopyle connects the incurrent canal to the radial canal, and Apopyle connects the radial canal to the spongocoel in the Syconoid type.

3. Metazoan Body Plans

3.1 Body Symmetry of Metazoa

Symmetry refers to the arrangement of body parts around a central point or axis.

  • Asymmetry: The body cannot be divided into two equal halves through any plane. Examples: Amoeba, most sponges.
  • Radial Symmetry: The body can be divided into two equal halves by any vertical plane passing through the central axis. This is advantageous for sessile or slow-moving animals as it allows them to respond to stimuli from all directions. Examples: Cnidarians (Hydra), adult Echinoderms (starfish).
  • Bilateral Symmetry: The body can be divided into two identical right and left halves by only one plane (the sagittal plane). This symmetry is associated with cephalization (development of a distinct head), which allows for directed movement and active lifestyle. Examples: Platyhelminthes to Chordates.

3.2 Segmentation of Metazoa

Segmentation, or metamerism, is the serial repetition of similar body segments along the antero-posterior axis.

Metamere: Each individual segment of a segmented animal.
  • True Metamerism:
    • Found in Annelida, Arthropoda, and Chordata.
    • Segmentation is both external and internal, affecting muscles, nerves, and blood vessels.
    • It allows for greater mobility and complexity, as different segments can become specialized for different functions (a phenomenon called tagmosis in arthropods).
  • Pseudometamerism (Strobilization):
    • Found in tapeworms (Cestoda).
    • It is a superficial segmentation where new segments (proglottids) are continuously added from the neck region.
    • These segments are not true metameres; they are essentially reproductive units that are not coordinated for locomotion.
Exam Tip: Understand the evolutionary advantage of segmentation. It allows for independent movement of different body parts and specialization of segments, which has been a major driver in the evolution of complex animals.