Parasite

This chapter deals with the phylum Protozoa, its general characters and classification, and two representative animals studied in detail: Paramecium (a ciliate) and the malarial parasite Plasmodium vivax (a sporozoan).

General Characters of Phylum Protozoa

1. Unicellular and mostly microscopic.
2. They may be free-living in fresh water, marine water or damp soil. Some are parasites and a few are saprophytes and commensals.
3. The body is naked or covered with a definite pellicle.
4. The protoplasm is divisible into outer ectoplasm and inner endoplasm.
5. Respiration takes place over the general body surface by the process of diffusion.
6. Nutrition may be holozoic (animal-like), holophytic (plant-like), saprophytic or parasitic.

Classification of Protozoa

The phylum is divided on the basis of the locomotory organelle. The most familiar member of class Rhizopoda (Sarcodina), which moves and feeds by pseudopodia, is Amoeba:

Protozoa is classified into four classes on the basis of the locomotory organelles.

Paramecium

Classification

Habit and habitat

Paramecium is a fresh-water organism found in lakes, rivers, streams and ponds, especially in stagnant water where organic matter is plentiful. It feeds upon bacteria, small protozoa and other decaying matter in the water with the help of its oral cilia.

Structure

Paramecium is slipper-shaped, its body resembling the sole of a shoe or a slipper. The body is divisible into an outer distinct part of the cytoplasm called the ectoplasm and an inner granular part called the endoplasm. Its parts are:

• Cilia: longitudinal, hair-like structures distributed evenly over the pellicle except at the posterior end. Cilia are of two types: somatic (body) cilia that help in locomotion, and oral (buccal) cilia that help in feeding and collecting food materials.
• Pellicle: a thin, elastic, layered membrane covering the cytoplasm of the body.
• Trichocyst: oval, elongated, bag-like structures arranged perpendicularly near the inner wall of the pellicle.
• Oral groove: a large depression located on the ventro-lateral side of the body.
• Cytopyge: a temporary aperture on the ventro-lateral side near the posterior end.
• Nucleus: Paramecium shows nuclear dimorphism, i.e. two types of nuclei.
  • Micronucleus: a small, rounded, diploid structure situated in the endoplasm. It helps in reproduction.
  • Macronucleus: generally larger, bean-shaped, situated in the endoplasm (described in the source as a degenerating structure). It controls the metabolic activities.