Concept of Protozoa and Metazoa

Concept of Protozoa and Metazoa

Protozoa may be defined as microscopic and a cellular animalcules ,without tissues and organs , having one o r more nuclei, but no nucleus ever in charge of a specialized part of cytoplasm. They exit either single or in colonies which differ from a metazoan in having all the individuals alike except when engaged in reproductive activities.

General characters

1. Small, usually microscopic animalcules, ordinarily not visible without a microscope.
2. Simplest and most primitive of all animals, with the protoplasmic grade of organization.
3. Body unicellular , containing one or more nuclei which are monomorphic or dimorphic.
4. Body symmetry none, bilateral, radial or spherical.
5. Solitary or forming loose colonies in which individuals remain alike and independent.
6. Naked body or bounded by pellicle and often provided with simple to elaborate shells or exoskeletons.
7. Body from usually constant , varied in some while changing with environment or age in many.
8. The single-celled body performs all the essential and vital activities which characterize the animals body; hence the only sub-cellular physiological division of labor.
9. Locomotor organelles are finger- like pseudopodia or whip o- like flagella or hair-like cilia or absent.
10. Nutrition holozoic ( animal – like ), holophytic ( plant – like ), saprozoic or With or without definite oral and anal apertures. Digestion occurs intracellularly inside food vacuoles.
11. Respiration and excretion through general surface or through contractile vacuoles, which serve mainly for osmoregulation.
12. Reproduction asexual by binary or multiple fission and budding and sexual by the conjugate of adult (hologamy) or by fusion of gametes(syngamy) .
13. Life history often complicated with alternation of asexual and sexual phases.
14. Encystmentcommonly occurs to help in dispersal as well as to resist unfavorable conditions of food, temperature and moisture.
15. Free –living protozoa mostly aquatic , inhabiting fresh and sea waters and dam places. Parasitic and commensal protozoa live over or inside the bodies of animals and plants. Sufficient moisture is essential in their environment.
16. The single – celled individual not differentiated into soma to plasm and germplasm; therefore , exempt from natural death which is the price paid for the body.
17. About 50,000 known species .

Concept of metazoan

They are multicellular animals composed of many cells, usually arranged in distinct layers , tissues and organs. It may be defined as holozoic multicellular organisms which are developed from embryos. Their body cells are generally differentiated into tissues and organs which are specialized for the different function. Their games are never formed within unicellular structures but are produced within multicellular sex organs or at least within surrounding somatic cells. The definition holds good to differentiated metazoan form protozoa.

General characters

1. They are multicellular animals composed of many cells, usually arranged in distinct layers, tissues and organs.
2. They have tissue, organ and system grade of organization.
3. They have diploblastic and triploblastic body layer.
4. Body symmetry is bilateral, radial.
5. They are solitary, colonial, sedentary, sessile , free swimming in habitat.
6. Body form usually constant, varied in some while changing with environment or age in many.
7. The tissues and organs perform specialized activities.
8. Locomotororganellsare state , parapodia, appendages, muscular limbs etc.
9. Nutrition may be holozoic, saprozoic or parasitic.
10. With or without definite oral and anal apertures. Digestion occurs intracellular or extracellular.

Origin of metazoan

Theory of origin of Metazoa

Metazoan is the multicellular animals which were originated from unicellular animalcules. The cells of metazoans are potentially capable of performing all essential activities but these cells are dependent on each other. In lower metazoans , cells are the primitive and simple layer of cells but in the higher metazoan tissues level as well as organ level of organization are found.

• Colonial theory: This theory was explained by Hyman and approved by Butschli, Lankester, and Haeckel.

Preposition: According to him, ancestral metazoan originated from a colonial flagellated unicellular animalcules. At first, flagellated unicellular animalcules make the colony and when they become completely matured then fragment their part and act as independent metazoans. This theory was supported by

1. Plastic group: Animals which have the capacity to change their body parts rapidly.
2. Compact colonies: Flagellates show the tendency of forming compact colonies which help in the embryonic development of metazoan.
3. Flagellated spermatozoa: Metazoan tailed sperm resembles with advanced flagellates.
4. Flagellated body cell: Flagellated body cell is common in lower metazoan (sponges and coelenterates).

Criticism

1. He never explained other unicellular animalcules like cilia possessing animalcules, pseudopodia containing unicellular animalcules etc.
2. This theory didn’t explain how flagellated appearances were lost during evolution of metazoans.

• Syncytical theory: It states that ancestral metazoans arose from multinucleated syncytical which became cellularized by the appearance of call boundaries. It was explained by Hanson and Hadzi. The main points of Hadzi theory are given as;

1. The ancestral metazoan was at 1^st syncytical in structure and bilaterally symmetrical. It gave rise to acoelomate flatworm by the process of cellularization. Acoelomates and bilaterally symmetrical animals considered being primitive metazoans.
2. Coelenterates especially anthozoans were derived from Rhacdocoel turbellarians.
3. Ctenophores evolve from polyclade by retention of several structural features of Mullers’s larva.
4. The theory of germs cell is rejected by Hadzi.
5. Conjugation in ciliates resembles with sexual reproduction of metazoan.

And this theory is supported by turbellarians, conjugation, and partial cellularization. He compares turbellarians with multinucleate ciliates where both are similar in size, have the anterior and posterior end and lacks digestive cavity and trichocyst of ciliates os similar saggitocyst.

Criticism:

1. He didn’t explain about germ cell.
2. HBe only explained that metazoans are formed by bilaterally symmetrical.
3. Anthozoans are not hermaphrodite althoi\ugh turbellarians are.
4. Mscronucleus is present in ciliates but absent in acoelomates.

• Polypohyletical theory: More recently , Greenberg (1959) and Preston (1976) held that probably all metazoan have not originated from any single ancestor. They have suggested a polypohyletic origin for the metazoans. It is fairly certain that sponges developed by wayu of colonial flagellates ( e.g. Proterospongis ) whereas other multicellular groups originated from the cellularization of syncytial protociliates or perhaps the mesozoans.

Reference:

Aggarwal Sarita. A Text Book of Biology,New Delhi.: Madhuban Educational Books, 2011.

Bhamrah, H.S., and Kavita, Juneja. A Text Book of Invertebrates, New Delhi: Anmol Publications Pvt Ltd, 2011.

Jordan E.L. and P. S., Verma. Invertebrate Zoology, New Delhi,: S. Chand and Company Pvt. Ltd., 2011.

Kotpal, R. L., Modern Text Book of Zoology: Invertebrates, New Delhi, India: Rastogi Publications,2011

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