The edge Parazoa It is a taxonomic category that includes porifers or sea sponges. These are primarily marine animals, but also freshwater (less than 2% of species), which consist of aggregations of cells that do not form true tissues or organs, organized around a system of water conduits that serves to acquire food and expel waste. metabolic.
Porifers are important components of sedentary animal communities in marine ecosystems. Despite their simple anatomy, they successfully compete with more advanced sessile animals. The bodies of the members of the phylum Parazoa serve as a refuge for a great variety of microorganisms and metazoans..
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One of the fundamental precepts of the International Code of Zoological Nomenclature is the Principle of Priority, according to which the valid scientific name of a group of animals is the oldest that has been applied to it. Sea sponges have received two scientific names with phylum rank, Porifera, coined in 1836, and Parazoa, coined in 1884.
In 1971 the name Placozoa, also with the rank of a phylum, was coined to include a single species., Trichoplax adhaerens. Like porifers, T. adhaerens it has a simple and primitive anatomy. Assuming that this was a reflection of phylogenetic affinity, the name Parazoa was revived, with a higher rank (sub-kingdom), to group Porifera and Placozoa.
Beginning in the 1990s, evidence began to accumulate, provided by molecular phylogenies, indicating that T. adhaerens It is not particularly closely related to porifers, but rather to radiated animals (phylum Cnidaria). Therefore, using the name Parazoa with sub-kingdom rank was no longer justified..
Currently, the name Parazoa has fallen into disuse. Based on the Principle of Priority, it is considered a synonym of Porifera.
The adults of the members of the phylum Parazoa are sessile, with a basal-apical axis, generally asymmetric. When there is radial symmetry, it is not equivalent to that of radiated animals, since it is not organized around a digestive system.
With the exception of a few carnivorous species, they feed by filtering food particles suspended in the surrounding water.
Porifers have sexual reproduction, through which the zygote forms several exclusive types of mobile larvae with cilia or flagella and anteroposterior symmetry..
They also have asexual reproduction, by which adults fragment, multiply by budding, or produce structures with cells and reserve matter called gemmules..
They are organized at the cellular level, which differentiates them from more advanced animals that are organized at the level of tissues or tissues and organs. Their physiology is similar to that of protozoa. They lack mesoderm, nervous tissue, digestive system, musculature, respiratory structures, and gonads.
They have cells more or less independent of each other that, when necessary, can transform into other types of cells and even form new sponges..
These cells are embedded in an extracellular matrix supported by skeletal elements made up of collagen fibers and calcareous or siliceous spicules..
The body of the poriferous consists of:
1) A thin outer layer, which protects from the external environment, called pinacoderm.
2) A thick, fibrous, spicule-reinforced gelatinous middle layer called mesohyl.
3) A thin inner layer, which surrounds the water passages, called coanoderm.
The pinacoderm is made up of a layer of flattened cells called pinacocytes. These are slightly contractile, so they can modify the shape of the sponge. Although mesohyl itself is acellular, it contains three types of amoeboid cells: archeocytes, sclerocytes, and spongocytes..
Archaeocytes are amorphous and mobile. They store reserve matter and remove waste. They can differentiate into other cell types, including eggs and sperm. Sclerocytes produce spicules. On the other hand, spongocytes produce spongin fibers, a protein related to collagen.
The coanoderm is lined by cells called choanocytes, which are distinguished by having a flagellum surrounded by a collar of microvilli. Choanocytes resemble the cells of colonial protozoa called choanoflagellates, pointing to a common evolutionary origin.
The choanocytes generate the currents of water that flow inside the sponges, taking from it small nutritive particles for food, and sperm for fertilization..
Porifers have a basal region attached to a solid substrate. Laterally and apically they are exposed to the surrounding aquatic environment. In order of increasing complexity, defined by increasingly folded body walls, they have three structural types: asconoid, syconoid, leuconoid..
The small asconoid sponges have a sac-like appearance, with an internal cavity lined by choanocytes, called a spongocele. Water enters the spongocele directly from the outside through numerous hollow tubes, each formed by a modified pinacocyte. The water exits through a single large apical hole called the osculum.
Small sycon sponges also have a sac-like appearance. Water enters through invaginations of the body wall called incurring channels. The water then passes through numerous pores to enter radial channels lined by choanocytes that lead to a spongocele without them. Finally, it comes out for a kiss.
The vast majority of sponges are leuconoids. Among them are the largest ones. Water enters through numerous pores, moving through branching incidental channels that lead to chambers lined by choanocytes..
From these chambers the water continues towards excursion channels without them that eventually converge in numerous oscula.
- Exclusively marine and deep sea.
- The entire sponge is made up of a continuous multinucleated syncytium, with some differentiated cells.
- Siliceous spicules, triaxonic or hexaxonic, with square protein axial filaments.
- Viviparous.
- Trichimela larva.
- Marine and freshwater.
- A carnivorous family (Cladorhizidae) (predates crustaceans) with extracellular digestion.
- With or without siliceous spicules. When they do, they are monaxonic or tetraxonic, or in other ways, with triangular protein axial filaments..
- With or without sponge.
- Leuconoids.
- Viviparous or oviparous.
- Parenchymal larva.
- Exclusively marine, shallow and deep waters.
- With or without siliceous or calcareous spicules.
- When they do, they are tetraxonic, almost always without protein axial filaments..
- Without sponge.
- Asconoids, syconoids, or leuconoids.
- Viviparous.
- Larva cinctoblastula, amphiblastula or calciblástula.
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