Haeckel’s three kingdom classification system

Haeckel’s three kingdom classification system

The Early classification system was put forward by various Linnaeus was two kingdom system. All plants were included in the kingdom Plantae and all animals were included in the kingdom Animalia. But this classification system could not classify all the living beings.There are some organisms which are neither plant like nor animal like. There was no place for the living being in the two-kingdom system of classification of various Linnaeus , so in 1866, Haeckel purposed three kingdom system of classification for living beings.Three kingdoms of this system are:

1. Kingdom Animalia
2. Kingdom Plantae
3. Kingdom Protista

All the plants are placed in kingdom Plantae and animals are placed in the kingdom Animalia. Those organisms which are neither plant like nor animal like are placed on kingdom Protista.

Kingdom Protista

Definition of Protista: The kingdom Protista is defined as the primitive, unicellular, microscopic, eukaryotic organisms that die in an intermediate form between the plants and animals.

Introduction: The Protists vary in their size from 0.1 to 200in diameter.They are single celled organisms. Their body is never partitioned into separate cells at any stage in their life. Among these organisms some are commonly called plants because they possess hard cell wall, perform photosynthesis and are generally plant like. Other are commonly called animals as they are motile , eat plants or animals as food and are generally animal like.However, some of these unicellular organisms act like plants or some parts of the time and like the animal for some part of time. Some look intermediate between a plant and animal. In fact, sometimes the same organisms may be classified as a plant and sometimes the organisms may be classified as an animal. Some organisms look intermediate between plants and animals. These organisms are neither the plant nor the animals but are called protists.

Characteristics

The organisms include in kingdom Protista show the following characteristics

• They are mostly aquatic, unicellular, primitive and microscopic organisms.
• They possess eukaryotic cell with organized nucleus.
• The cell contains a number of membrane-bound organelles like mitochondria, plasmid, endoplasmic reticulum, Golgi bodies etc.
• Cell wall, if present, contains cellulose.
• They possess diverse modes of nutrition, viz, photosynthetic, saprotrophic, parasitic, holozoic etc.
• These organisms mostly from planktons. Planktons are of types phytoplankton and Zooplanktons . The nonphotosynthetic organisms are called zooplanktons which have holozoic nutrition. They do not possess cell wall.
• They reproduce by both asexual and sexual methods.
• Their development does not show much cellular differentiation, hence tissue system is absent.
• In these organisms there is no physiological division of labour, so all the vital activities of the body are performed by the single cell.

Classification of kingdom Protista: It is as follows:

• Photosynthetic protists
• Slime moulds
• Protozoan Protista
• Protozoa

Definition: Protozoa is defined as microscopic, unicellular, animalcules existing singly or in colonies without tissue and organs having one or more nuclei.

General characters:

• Habit and habitat: Protozoans may be aquatic, terrestrial, free living commensal or parasites. Parasitic forms cause diseases in humans and other animals. They are solitary or colonial.
• Body form: They are small, microscopic, simplest and primitive of all animals. There is no physiological division of labour. All the vital activities of life are performed by a single cell. The body may be covered by a delicate membrane or firm pellicle or calcareous or siliceous shell or without pellicle. The shape of the body is variable. It may be spherical, oval, elongated, flattened or irregular.
• Symmetry: They show radial or bilateral symmetry or the symmetry may be irregular.
• The level of body organization: Protozoans have molecular r protoplasmic level of body organization. Protoplasm is differentiated into ectoplasm and endoplasm.
• Nuclei: One to several nuclei are present in protozoans. If one (monomorphic), two (dimorphic) and more than two (polymorphic).
• Locomotory organelles: These are pseudopodia, flagella, cilia or none.
• Nutrition: The mode of nutrition is holozoic (animal-like), holophytic (plant-like) saprozoic or parasitic .Digestion is intracellular and occurs inside the food vacuoles.
• Respiration: Respiration takes place through the general body surface by diffusion.
• Excretion: Excretion also takes place through the general body surface or in some forms through a temporary opening in the ectoplasm or through the permanent pore, the Cytopyge. The contractile vacuole also takes part in the removal of soluble waste matter.
• Reproduction: They reproduce asexually as well as sexually. Asexually reproduction occurs by syngamy (fusion of gametes) or conjugation (temporary union of two individuals).
• Encystment: During unfavourable condition, some protozoans from cysts which help in dispersal and dispersal and survival during adverse conditions.

Classification: Protozoa consists of about 50,000 known species. The phylum protozoa are classified into four classes on the basis of locomotory organelles like pseudopodia flagella or cilia.

Phylum protozoa

1. Class 1. Rhizopoda
2. Class 2. Ciliata
3. Class 3. Flagellata
4. Class 4. Sporozoa

References

Arvind, Keshari K. and Kamal K Adhikari. A Textbook of Biology. Vidyarthi Pustak Bhander.

Michael J.Pleczar JR, Chan E.C.S. and Noel R. Krieg. Microbiology. Tata Mc GrawHill, 1993.

Powar. and Daginawala. General Microbiology.

Rangaswami and Bagyaraj D.J. Agricultural Microbiology.