Protozoa and Porifera Characteristics and Classification

Posted on Jan 17, 2026 in Biology

Phylum Protozoa: Primitive Unicellular Animals

Phylum Protozoa represents the most primitive group of animals, consisting of microscopic, unicellular organisms. Below is an examination of their general characteristics, classification, and a detailed look at the malaria parasite, Plasmodium.

1. General Characters of Phylum Protozoa

• Habit and Habitat: Mostly aquatic (freshwater or marine), but can also be found in damp soil. Many are commensals or parasites.
• Body Organization: Unicellular or acellular; they exhibit a protoplasmic grade of organization where all life functions are performed within a single cell.
• Symmetry: May be asymmetrical, spherical, radial, or bilateral.
• Locomotion: Carried out by specialized organelles: Pseudopodia (Amoeba), Flagella (Euglena), Cilia (Paramecium), or absent in some parasitic forms.
• Nutrition: Can be Holozoic (animal-like), Holophytic (plant-like), Saprozoic (absorbing dissolved nutrients), or Parasitic.
• Reproduction:
  • Asexual: Binary fission, multiple fission, or budding.
  • Sexual: Syngamy (fusion of gametes) or Conjugation (exchange of nuclear material).

2. Classification Up to Class Level

The classification is primarily based on the mode of locomotion and type of locomotory organelles.

3. Type Study: Plasmodium (Malaria Parasite)

Plasmodium is a digenetic endoparasite, meaning it requires two hosts to complete its life cycle: Humans (Intermediate host) and the Female Anopheles mosquito (Definitive host).

Life Cycle in Humans (Asexual Cycle)

• Inoculation: Sporozoites (infective stage) are injected into the human bloodstream via a mosquito bite.
• Exo-erythrocytic Schizogony: Sporozoites enter liver cells, multiply asexually, and form Merozoites.
• Erythrocytic Schizogony: Merozoites enter Red Blood Cells (RBCs). They pass through the Ring stage, Trophozoite stage, and finally the Schizont stage.
• Haemozoin: When RBCs rupture to release new merozoites, they also release a toxin called Haemozoin, which causes the characteristic chills and high fever of malaria.
• Gametogony: Some merozoites transform into sexual stages called Gametocytes (Microgametes and Macrogametes).

Life Cycle in Mosquitoes (Sexual Cycle)

• Gametes Formation: After a mosquito sucks infected blood, gametes fuse in its stomach to form a Zygote.
• Ookinete & Oocyst: The zygote becomes a motile Ookinete, penetrates the stomach wall, and forms an Oocyst.
• Sporogony: Inside the oocyst, thousands of Sporozoites are formed. When the oocyst ruptures, these sporozoites migrate to the mosquito’s salivary glands, ready for the next infection.

Pathogenicity

• Symptoms: High fever with shivering (paroxysms), anemia (due to RBC destruction), and splenomegaly (enlarged spleen).
• Incubation Period: Usually 10 to 14 days depending on the species (e.g., P. vivax, P. falciparum).

Phylum Porifera: The Simplest Multicellular Animals (Sponges)

Phylum Porifera, commonly known as sponges, represents the most primitive group of multicellular animals (Metazoans). They are unique for their “pore-bearing” bodies and lack of true tissues.

1. General Characters of Phylum Porifera

• Level of Organization: They exhibit a cellular grade of organization, where cells are loosely arranged and not organized into true tissues or organs.
• Habitat: Mostly marine; only one family (Spongillidae) is found in freshwater.
• Body Form: Typically sedentary (sessile) adults attached to a substratum. Their shapes vary from vase-like to cylindrical or irregular.
• Water Transport System: The most distinguishing feature is the canal system. Water enters through minute pores called Ostia, circulates through a central cavity called the Spongocoel, and exits through a large opening called the Osculum.
• Choanocytes: The internal cavities are lined with unique “collar cells” or Choanocytes, which have flagella that create water currents and capture food.
• Skeleton: Supported by an internal skeleton made of calcareous spicules, siliceous spicules, or spongin fibers.
• Reproduction:
  • Asexual: By fragmentation, budding, or formation of internal buds called Gemmules.
  • Sexual: They are mostly hermaphroditic. Fertilization is internal, and development includes a free-swimming larval stage (e.g., Amphiblastula or Parenchymula).

2. Classification (Up to Class Level)

The classification is primarily based on the chemical nature and shape of the skeleton.

3. Type Study: Sycon (Scypha)

Sycon is a small, marine, calcareous sponge commonly found attached to rocks and corals in shallow coastal waters.

External Morphology

• Shape: It looks like a group of small, branched cylinders joined at the base.
• Body Wall: The wall is thick and folded to form a complex system of canals and chambers.
• Pores: The surface is covered in Ostia for water intake, and each cylinder has a single Osculum at the top, surrounded by a crown of spicules.

The Canal System (Syconoid Type)

The water current in Sycon follows a specific pathway driven by the beating flagella of choanocytes:

1. Ingressive Water → Ostia
2. Incurrent Canals
3. Prosopyles (small openings)
4. Radial Canals (lined with Choanocytes)
5. Apopyles (openings into the spongocoel)
6. Spongocoel (central cavity)
7. Osculum → Outside

Physiology

• Nutrition: Filter feeders; choanocytes trap microscopic plankton. Digestion is intracellular.
• Respiration and Excretion: Occurs through simple diffusion between the cells and the water current flowing through the canals.
• Reproduction: Sycon reproduces both asexually (budding) and sexually (internal fertilization). The resulting larva is an Amphiblastula.