But there are organisms that blur the two: for instance, the algae Dinobryon has chloroplasts, but it can also move around and feed on organic matter. Many ostensibly unicellular organisms also arrange themselves into thread-like or spherical colonies, such as cyanobacteria or the chrolophyte Volvox. The term "protozoan" basically covers any unicellular organism that is not an animal, plant, bacteria, or fungi.
By definition, every member of kingdom Animalia is multicellular, and another name for the kingdom is in fact Metazoa. Stationary photosynthetic unicellular organisms are not protozoans in part because the suffix -zoa means animal, and these are clearly not at all animal-like. Protozoans are always motile, as there are no protozoans that can survive by stationary filter-feeding, in contrast to the numerous animals such as sponges, corals, barnacles, limpets, and brachiopods that do so.
Protozoans range in size from 10—50 microns millionths of a meter , but a few species are as large as 1 mm. These organelles show great functional differentiations for the purposes of food intake, locomotion, sensory reception, response, protection etc. Protozoans are usually microscopic and show an extraordinary diversity in shape, structure, symmetry and adaptations to numerous environments.
Protozoans are considered as the protoplasmic level of organization. Protozoans can be classified into several subgroups mainly based on their morphology , namely; Flagelletes, Amoeboids, Sporozoans, and Ciliates.
However, their classification has been a problematic area of taxonomy. Some examples for protozoa include; Entamoeba sp, Plasmodium sp, Paramecium s p , etc. Metazoa includes all the multicellular animals of Kingdom Animalia. The metazoans have organized group of cells, which are defined as tissues or organs systems. These animals show much higher differentiation and specialization of their parts and also higher complexity of morphology than the protozoans.
All the metazoans are visible by the naked eye except a very few, which include Daphnia and Cyclops, which are microscopic. Metazoans can be further divided into two major subgroups; 1 Invertebrates ; those who lack backbone and internal skeleton such as earthworms, insects , snails etc. They are a primitive form of metazoans evolved from choanozoans that are unicellular aquatic protists sometimes forming colonies.
Sponges show a single cell-level of organization and do not develop nerves or muscles as found in a regular metazoan. Also, metazoans are monophyletic as they are developed from a single ancestor. One of the main characteristic features of metazoans is their bilateral body symmetry. Here, animals with a bilateral symmetry belong to a distinct clade known as Bilateria. The two main types of bilaterians are protostomes and deuterostomes. Protostomes include invertebrates such as nematodes, arthropods, and mollusks while deuterostomes include chordates and echinoderms.
Furthermore, the other significant features to metazoans are the consumption of organic material, breathing oxygen, ability to move, sexual reproduction, and the development from a hollow sphere of cells known as the blastula during embryonic development.
Protozoa refers to a group of phyla which comprises the single-celled microscopic organisms, belonging to the kingdom Protista while Metazoa refers to a major division of the animal kingdom that comprises all animals other than protozoans and sponges. Thus, this is the fundamental difference between Protozoa and Metazoa. Protozoans make up the kingdom Protista while metazoans make up the kingdom Animalia. Amoebae, flagellates, ciliates, and Sporozoa are the main forms of protozoans while the two main forms of metazoans are vertebrates and invertebrates.
Moreover, their cellular organization is the main difference between Protozoa and Metazoa. Protozoans are unicellular while metazoans are multicellular. In addition, protozoans are microscopic while metazoans can be either microscopic or macroscopic. Besides, one other difference between Protozoa and Metazoa is the division of labor. Protozoans have no division of labor while metazoans show the division of labor. The single cell of protozoa undergoes all biological activities inside the cytoplasm including digestion, excretion, reproduction, etc.
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