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Hammerhead Shark
The hammerhead sharks are a group of sharks that form the family Sphyrnidae, named for the unusual and distinctive form of their heads, which are flattened and laterally extended into a Hammerhead shark#Cephalofoil, cephalofoil (a T-shape or "hammer"). The shark's eyes are placed one on each end of this T-shaped structure, with their small mouths directly centered and underneath. Most hammerhead species are placed in the genus ''Sphyrna'', while the winghead shark is placed in its own genus, ''Eusphyra''. Many different—but not necessarily mutually exclusive—functions have been postulated for the cephalofoil, including sensory reception, manoeuvering, and prey manipulation. The cephalofoil gives the shark superior binocular vision and depth perception. Hammerheads are found worldwide, preferring life in warmer waters along coastlines and continental shelves. Unlike most sharks, some hammerhead species will congregate and swim in large Shoaling and schooling, schools during ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Scalloped Hammerhead
The scalloped hammerhead (''Sphyrna lewini'') is a species of hammerhead shark in the family (biology), family Hammerhead shark, Sphyrnidae. It was originally known as ''Zygaena lewini''. The Greek language, Greek word ''sphyrna'' translates into "hammer" in English, referring to the shape of this shark's head, which is its most distinguishing characteristic. The shark's eyes and nostrils are at the tips of the extensions. It is a fairly large hammerhead, but is still smaller than both the Great hammerhead, great and smooth hammerheads. This shark is also known as the bronze, kinky-headed, or southern hammerhead. It primarily lives in warm, temperate, and tropical coastal waters all around the globe between latitudes 46th parallel north, 46°N and 36th parallel south, 36°S, down to a depth of . It is the most common of all hammerheads. Taxonomy The scalloped hammerhead was first named ''Zygaena lewini'' and then renamed ''Sphyrna lewini'' by Edward Griffith (zoologist), Edward G ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Migration Period
The Migration Period ( 300 to 600 AD), also known as the Barbarian Invasions, was a period in European history marked by large-scale migrations that saw the fall of the Western Roman Empire and subsequent settlement of its former territories by various tribes, and the establishment of post-Roman kingdoms there. The term refers to the important role played by the migration, invasion, and settlement of various tribes, notably the Burgundians, Vandals, Goths, Alemanni, Alans, Huns, early Slavs, Pannonian Avars, Bulgars and Magyars within or into the territories of Europe as a whole and of the Western Roman Empire in particular. Historiography traditionally takes the period as beginning in AD 375 (possibly as early as 300) and ending in 568. Various factors contributed to this phenomenon of migration and invasion, and their role and significance are still widely discussed. Historians differ as to the dates for the beginning and ending of the Migration Period. The beginni ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Fertilization
Fertilisation or fertilization (see American and British English spelling differences#-ise, -ize (-isation, -ization), spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a zygote and initiate its development into a new individual organism or offspring. While processes such as insemination or pollination, which happen before the fusion of gametes, are also sometimes informally referred to as fertilisation, these are technically separate processes. The cycle of fertilisation and development of new individuals is called sexual reproduction. During double fertilisation in angiosperms, the haploid male gamete combines with two haploid polar nuclei to form a triploid primary endosperm nucleus by the process of vegetative fertilisation. History In antiquity, Aristotle conceived the formation of new individuals through fusion of male and female fluids, with form and function emerging gradually, in a mode c ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Viviparous
In animals, viviparity is development of the embryo inside the body of the mother, with the maternal circulation providing for the metabolic needs of the embryo's development, until the mother gives birth to a fully or partially developed juvenile that is at least metabolically independent. This is opposed to oviparity, where the embryos develop independently outside the mother in eggs until they are developed enough to break out as hatchlings; and ovoviviparity, where the embryos are developed in eggs that remain carried inside the mother's body until the hatchlings emerge from the mother as juveniles, similar to a live birth. Etymology The term "viviparity" and its adjective form "viviparous" both derive from the Latin ''vivus'', meaning "living"; and ''pario'', meaning "give birth to". Reproductive mode Five modes of reproduction have been differentiated in animals based on relations between zygote and parents. The five include two nonviviparous modes: ovuliparity ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Ampullae Of Lorenzini
Ampullae of Lorenzini (: ''ampulla'') are electroreceptors, sense organs able to detect electric fields. They form a network of mucus-filled pores in the skin of Chondrichthyes, cartilaginous fish (sharks, Ray (fish), rays, and chimaeras) and of basal Osteichthyes, bony fishes such as reedfish, sturgeon, and lungfish. They are associated with and evolved from the mechanosensory lateral line organs of early vertebrates. Most bony fishes and terrestrial vertebrates have lost their ampullae of Lorenzini. History Ampullae were initially described by Marcello Malpighi and later given an exact description by the Italian physician and ichthyologist Stefano Lorenzini in 1679, though their function was unknown. Electrophysiology, Electrophysiological experiments in the 20th century suggested a sensibility to temperature, mechanical pressure, and possibly salinity. In 1960 the ampullae were identified as specialized receptor organs for sensing electric fields. One of the first descriptio ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Electroreception
Electroreception and electrogenesis are the closely related biological abilities to perceive electrical stimuli and to generate electric fields. Both are used to locate prey; stronger electric discharges are used in a few groups of fishes, such as the electric eel, to stun prey. The capabilities are found almost exclusively in aquatic or amphibious animals, since water is a much better Conductor (material), conductor of electricity than air. In passive electrolocation, objects such as prey are detected by sensing the electric fields they create. In active electrolocation, fish generate a weak electric field and sense the different distortions of that field created by objects that conduct or resist electricity. Active electrolocation is practised by two groups of weakly electric fish, the order Gymnotiformes (knifefishes) and family Mormyridae (elephantfishes), and by the monotypic genus ''Gymnarchus'' (African knifefish). An electric fish generates an electric field using an Elect ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Vertebra
Each vertebra (: vertebrae) is an irregular bone with a complex structure composed of bone and some hyaline cartilage, that make up the vertebral column or spine, of vertebrates. The proportions of the vertebrae differ according to their spinal segment and the particular species. The basic configuration of a vertebra varies; the vertebral body (also ''centrum'') is of bone and bears the load of the vertebral column. The upper and lower surfaces of the vertebra body give attachment to the intervertebral discs. The posterior part of a vertebra forms a vertebral arch, in eleven parts, consisting of two pedicles (pedicle of vertebral arch), two laminae, and seven processes. The laminae give attachment to the ligamenta flava (ligaments of the spine). There are vertebral notches formed from the shape of the pedicles, which form the intervertebral foramina when the vertebrae articulate. These foramina are the entry and exit conduits for the spinal nerves. The body of the vertebr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Sister Group
In phylogenetics, a sister group or sister taxon, also called an adelphotaxon, comprises the closest relative(s) of another given unit in an evolutionary tree. Definition The expression is most easily illustrated by a cladogram: Taxon A and taxon B are sister groups to each other. Taxa A and B, together with any other extant or extinct descendants of their most recent common ancestor (MRCA), form a monophyletic group, the clade AB. Clade AB and taxon C are also sister groups. Taxa A, B, and C, together with all other descendants of their MRCA form the clade ABC. The whole clade ABC is itself a subtree of a larger tree which offers yet more sister group relationships, both among the leaves and among larger, more deeply rooted clades. The tree structure shown connects through its root to the rest of the universal tree of life. In cladistic standards, taxa A, B, and C may represent specimens, species, genera, or any other taxonomic units. If A and B are at the same taxono ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Phylogenetic Tree
A phylogenetic tree or phylogeny is a graphical representation which shows the evolutionary history between a set of species or taxa during a specific time.Felsenstein J. (2004). ''Inferring Phylogenies'' Sinauer Associates: Sunderland, MA. In other words, it is a branching diagram or a tree showing the evolutionary relationships among various biological species or other entities based upon similarities and differences in their physical or genetic characteristics. In evolutionary biology, all life on Earth is theoretically part of a single phylogenetic tree, indicating common ancestry. Phylogenetics is the study of phylogenetic trees. The main challenge is to find a phylogenetic tree representing optimal evolutionary ancestry between a set of species or taxa. Computational phylogenetics (also phylogeny inference) focuses on the algorithms involved in finding optimal phylogenetic tree in the phylogenetic landscape. Phylogenetic trees may be rooted or unrooted. In a ''rooted'' p ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mitochondrial DNA
Mitochondrial DNA (mtDNA and mDNA) is the DNA located in the mitochondrion, mitochondria organelles in a eukaryotic cell that converts chemical energy from food into adenosine triphosphate (ATP). Mitochondrial DNA is a small portion of the DNA contained in a eukaryotic cell; most of the DNA is in the cell nucleus, and, in plants and algae, the DNA also is found in plastids, such as chloroplasts. Mitochondrial DNA is responsible for coding of 13 essential subunits of the complex oxidative phosphorylation (OXPHOS) system which has a role in cellular energy conversion. Human mitochondrial DNA was the first significant part of the human genome to be sequenced. This sequencing revealed that human mtDNA has 16,569 base pairs and encodes 13 proteins. As in other vertebrates, the human mitochondrial genetic code differs slightly from nuclear DNA. Since animal mtDNA evolves faster than nuclear genetic markers, it represents a mainstay of phylogenetics and evolutionary biology. It als ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
