What is parazoa ?
Ans : Parazoa is a Sub-Kingdom under the Kingdom Animalia. The only surviving Parazoans are the sponges, which belong to the phylum Porifera, and the Trichoplax in the phylum Placozoa.
INTRODUCTION TO THE PARAZOA : The parazoan level of organization is a loose association of cells and structural elements that behave almost as a cellular aggregate rather than a multicellular organism. Still, the two phyla likely share only primitive characters. Furthermore, Trichoplax, the sole genus in the Phylum Placozoa, likely is secondarily simplified. Either way, this subkingdom is a paraphyletic group. The affinities between the Porifera and choanoflagellates are more clear. Please consult The Major Clades of the Animal Kingdom for some views on the relationships of the parazoan phyla with each other and with the other phyla of the animal kingdom.
Parazoa is the animal sub-kingdom that includes organisms of the phyla Porifera and Placozoa. Sponges are the most well-known parazoa. They are aquatic organisms classified under the phylum Porifera with about 15,000 species worldwide. Although multicellular, sponges only have a few different types of cells, some of which may migrate within the organism to perform different functions.
The three main classes of sponges include glass sponges (Hexactinellida), calcareous sponges (Calcarea), and demosponges (Demospongiae). Parazoa from the phylum Placozoa include the single species Trichoplax adhaerens. These tiny aquatic animals are flat, round, and transparent. They are composed of only four types of cells and have a simple body plan with just three cell layers.
Glass sponges of the class Hexactinellida typically live in deep sea environments and may also be found in Antarctic regions. Most hexactinellids exhibit radial symmetry and commonly appear pale with regard to color and cylindrical in form. Most are vase-shaped, tube-shaped, or basket-shaped with leuconoid body structure. Glass sponges range in size from a few centimeters in length to 3 meters (almost 10 feet) in length.
Calcareous sponges of the class Calcarea commonly reside in tropical marine environments at more shallow regions than glass sponges. This class of sponges has fewer known species than Hexactinellida or Demospongiae with around 400 identified species. Calcareous sponges have varied shapes including tube-like, vaselike, and irregular shapes. These sponges are usually small (a few inches in height) and some are brightly colored. Calcareous sponges are characterized by a skeleton formed from calcium carbonate spicules. They are the only class to have species with asconoid, syconoid, and leuconoid forms.
Demosponges of the class Demospongiae are the most numerous of the sponges containing 90 to 95 percent of Porifera species. They are typically brightly colored and range in size from a few millimeters to several meters. Demosponges are asymmetrical forming a variety of shapes including tube-like, cup-like, and branched shapes. Like glass sponges, they have leuconoid body forms. Demosponges are characterized by skeletons with spicules composed of collagen fibers called spongin. It is the spongin that gives sponges of this class their flexibility. Some species have spicules that are composed of silicates or both spongin and silicates.
phylum Placozoa contains only one known living species Trichoplax adhaerens. A second species, Treptoplax reptans, has not been observed in more than 100 years. Placozoans are very tiny animals, about 0.5 mm in diameter. T. adhaerens was first discovered creeping along the sides of an aquarium in an amoeba-like fashion. It is asymmetrical, flat, covered with cilia, and able to adhere to surfaces. T. adhaerens has a very simple body structure that is organized into three layers. An upper cell layer provides protection for the organism, a middle meshwork of connected cells enable movement and shape change, and a lower cell layer functions in nutrient acquisition and digestion. Placozoans are capable of both sexual and asexual reproduction. They reproduce primarily by asexual reproduction through binary fission or budding. Sexual reproduction occurs typically during times of stress, such as during extreme temperature changes and low food supply.
0 Comments