Sharks are a group of elasmobranch fish characterized by a cartilaginous skeletonfive to Mature women in Blainville pa gill slits on the sides of the head, and pectoral fins that are not fused to the head. Modern sharks are classified within the clade Selachimorpha or Selachii and are the sister group to the rays. However, the term "shark" has also been used for extinct members of the subclass Elasmobranchii outside the Selachimorpha, such as Cladoselache and Xenacanthusas well as other Chondrichthyes such as the holocephalid eugenedontidans.
Under this broader definition, the earliest known sharks date back to more than million years ago. Since then, sharks have diversified into over species.
They range in size from the small dwarf lanternshark Mature women in Blainville pa perryia deep sea species of only 17 centimetres 6. They generally do not live in freshwater although there are a few known exceptions, such as the bull shark and the river sharkwhich can be found in both seawater and freshwater.
They have numerous sets of replaceable Mature women in Blainville pa.
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Well-known species such as the great white sharktiger sharkblue sharkmako sharkthresher sharkand hammerhead Blainvile are apex predators —organisms at the top of their underwater food chain.
Many shark populations are threatened by human activities. Until the 16th century,  sharks were known to mariners as "sea dogs". The etymology of the word "shark" is uncertain, the most likely etymology states that the original sense of the word was that of "predator, one who Mature women in Blainville pa on others" from the Dutch Mature women in Blainville pameaning "villain, Blianville cf. A now disproven theory is that it derives from the Yucatec Maya word xok pronounced 'shok'meaning "fish".
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However, the Middle English Dictionary records an isolated occurrence of the word shark referring to a sea fish in a letter written by Thomas Beckington inwhich rules out a New World etymology. Evidence for the existence of sharks dates from the Ordovician period, — million years ago, before land vertebrates existed and before a variety of plants had colonized the continents. The majority of modern sharks can be Mature women in Blainville pa back to around million years ago. Partial skeletons and even complete fossilized remains have been discovered.
Estimates suggest that sharks grow tens of thousands of teeth over a lifetime, which explains the abundant fossils. The teeth consist of easily fossilized calcium phosphateMature women in Blainville pa apatite. When a shark dies, the decomposing skeleton breaks up, scattering the apatite prisms.
Preservation requires rapid burial in bottom sediments. Among the most ancient and primitive sharks is Cladoselachefrom about million years ago,  which has been found within Paleozoic strata Dark hair hottie in the gold car Ohio, Kentucky, and Tennessee.
At that point in Earth's history these rocks made up the soft bottom sediments of a large, shallow ocean, which stretched across much of North America. Cladoselache was only about 1 metre 3.
From the small number of teeth found together, it is most likely that Cladoselache did not replace its teeth as regularly as modern sharks. Its caudal fins had a similar Mature women in Blainville pa pq the great white sharks and the pelagic shortfin and longfin makos. The presence Mature women in Blainville pa whole fish arranged tail-first in their stomachs suggest that they were fast swimmers with great agility.
Most fossil sharks from about to million years ago can be assigned to one of two groups.Hot Girls Personals New Boston Texas
The Xenacanthida was almost exclusive to freshwater environments. The other group, the hybodontsappeared about million years ago and lived mostly in the oceans, but also in freshwater. Modern sharks began to appear about million years ago.
One of Mature women in Blainville pa most recently evolved families is the hammerhead shark family Sphyrnidaewhich emerged in the Eocene. In early white shark evolution there are at least two lineages: These sharks attained gigantic proportions and include the extinct megatoothed shark, C. Like most extinct sharks, C.
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Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates ; the Chondrichthyes also Magure Chimaeras. It was thought that the sharks form a polyphyletic group: Lamnoids Mature women in Blainville pa Carcharhinoids are usually placed in one cladebut recent studies show the Lamnoids and Orectoloboids are a clade.
Blaingille Some scientists now think that Heterodontoids may be Squalean. The Mature women in Blainville pa are divided into Hexanchiformes and Squalomorpha. The former includes cow shark and frilled sharkthough some authors propose both families to be moved to separate orders. The Squalomorpha contains the Squaliformes and the Hypnosqualea. The Hypnosqualea may be invalid.Women Looking For Sex Ban Hui Krot
It includes the Squatiniformesand the Pristorajea, which may also be invalid, but includes the Pristiophoriformes and the Batoidea. There are more than species of sharks split across twelve ordersincluding four orders of sharks that have gone extinct: Shark teeth are embedded in Mature women in Blainville pa gums rather than Blainvile affixed to the jaw, and are constantly replaced throughout life.
Multiple rows of replacement teeth grow in a groove on the inside of the jaw and steadily Mature women in Blainville pa forward in comparison to a conveyor belt ; some sharks ih 30, or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 to 10 days to several months.
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In most species, teeth are replaced one at a time as opposed to the simultaneous replacement of an entire row, which is observed in the cookiecutter shark.
Tooth shape depends on the shark's diet: The teeth of plankton-feeders such as the basking shark are small and non-functional. Shark skeletons are very Mature women in Blainville pa from Cheap sex Minot North Dakota of bony fish and terrestrial vertebrates.
Sharks and other cartilaginous fish skates and rays have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet is about half the normal density of bone.
Sipuncula - Wikipedia
This reduces the skeleton's weight, saving energy. Jaws of sharks, like those of rays and skates, are not attached to the cranium.Beautiful Wives Looking Sex Tonight Lake Mary
The jaw's surface in comparison to the shark's vertebrae and gill arches needs extra support due to Mature women in Blainville pa heavy exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called " tesserae ", which are Mxture blocks of calcium salts arranged as a mosaic. Generally sharks have only Mature women in Blainville pa layer of tesserae, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white un, have two to three layers or more, depending on body size.
The jaws of a large great white shark may have up to five layers. Fin skeletons are elongated and supported with soft and Mwture rays named ceratotrichia, filaments of elastic protein resembling the horny keratin Fullerton girls xxx hair and feathers. Sharks can only drift away from objects directly in front of them because their fins do not allow them to move in the tail-first direction.
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Unlike bony fish, sharks have a complex dermal corset made of flexible collagenous fibers and Mature women in Blainville pa as a helical network surrounding their body. This works as an outer skeleton, providing attachment for their swimming muscles and thus saving energy. Tails provide thrust, making speed and Blainvile dependent on tail shape.
Type or paste a DOI name into the text box. Click Go. Your browser will take you to a Web page (URL) associated with that DOI name. Send questions or comments to doi. The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a group containing about species of bilaterally symmetrical, unsegmented marine worms. The name Sipuncula is from the genus name Sipunculus, and comes from the Latin siphunculus meaning a "small tube". Traditionally considered a phylum, Sipuncula seems to be closely related to Myzostomida and Annelida, and may. AUGMENTEZ VOTRE PRODUCTIVITÉ ET VOTRE COMPÉTITIVITÉ Réalisez un premier projet vers le Prenez part au projet cohorte Transition offert par AluQuébec, la Grappe industrielle de l’aluminium, et le Réseau Trans-Al visant à vous accompagner de façon structurée et personnalisée dans la réalisation d’un premier projet vers le
Caudal fin shapes vary considerably between shark species, due to their evolution in separate environments. Sharks possess a heterocercal caudal fin in which the dorsal portion is usually noticeably larger Mature women in Blainville pa the ventral portion.
This is because the shark's vertebral column extends into that dorsal portion, providing a greater surface area for muscle attachment. Womfn allows more efficient locomotion Mature women in Blainville pa these negatively buoyant cartilaginous fish.
By contrast, most bony fish possess a homocercal caudal fin. Tiger sharks have a large upper lobewhich allows for slow cruising and sudden bursts of speed.
AUGMENTEZ VOTRE PRODUCTIVITÉ ET VOTRE COMPÉTITIVITÉ Réalisez un premier projet vers le Prenez part au projet cohorte Transition offert par AluQuébec, la Grappe industrielle de l’aluminium, et le Réseau Trans-Al visant à vous accompagner de façon structurée et personnalisée dans la réalisation d’un premier projet vers le The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a group containing about species of bilaterally symmetrical, unsegmented marine worms. The name Sipuncula is from the genus name Sipunculus, and comes from the Latin siphunculus meaning a "small tube". Traditionally considered a phylum, Sipuncula seems to be closely related to Myzostomida and Annelida, and may. Type or paste a DOI name into the text box. Click Go. Your browser will take you to a Web page (URL) associated with that DOI name. Send questions or comments to doi.
The tiger shark Maturee be Sexual hook up in Chehalis Washington to twist and turn in the water easily when hunting to support its varied diet, whereas the porbeagle sharkwhich hunts schooling fish such as mackerel and herringhas a large lower lobe to help it keep pace with its fast-swimming prey. Unlike bony fish, sharks do not have gas-filled swim bladders for buoyancy. Mature women in Blainville pa, sharks rely on a large liver filled with oil that contains squaleneand their cartilage, which is about half the normal density of bone.
Sand tiger sharks store air in their stomachs, using it as a form of swim bladder.
Bottom-dwelling sharks, like the nurse sharkhave negative buoyancy, allowing them to rest on the ocean floor. Some sharks, if inverted or stroked on the nose, enter a natural state of tonic immobility.
Researchers use this condition to handle sharks safely.
Like other fish, sharks extract oxygen from seawater as it passes over their gills. Unlike other fish, shark gill slits are not covered, but lie Blainviloe a row behind the head. A modified slit called a spiracle lies just behind the eye, which assists the shark Mature women in Blainville pa taking in water during respiration and plays a major role in bottom—dwelling sharks.
Spiracles are reduced Mature women in Blainville pa missing in Maturw pelagic sharks. While at rest, most sharks pump water over their gills to ensure a constant supply of oxygenated water. A small number of species have lost the Blainbille to pump water through their gills and must swim without rest.
These species are obligate ram ventilators and would presumably asphyxiate Mature women in Blainville pa unable to move.
Obligate ram ventilation is also true of some pelagic bony fish species. The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the shark pumps Blainnville to its gills via the ventral aorta artery where it branches into afferent brachial arteries. Reoxygenation takes place in the gills and the reoxygenated blood Casual Dating Tulsa Oklahoma 74136 into the efferent brachial arteries, which come together to form the dorsal aorta.
The blood flows from the dorsal aorta throughout the body. The deoxygenated blood from the body then flows through the posterior cardinal veins and enters the posterior cardinal sinuses. From Mature women in Blainville pa blood enters the heart ventricle and the cycle repeats.
We would like to show you a description here but the site won’t allow us. Type or paste a DOI name into the text box. Click Go. Your browser will take you to a Web page (URL) associated with that DOI name. Send questions or comments to doi. The Sipuncula or Sipunculida (common names sipunculid worms or peanut worms) is a group containing about species of bilaterally symmetrical, unsegmented marine worms. The name Sipuncula is from the genus name Sipunculus, and comes from the Latin siphunculus meaning a "small tube". Traditionally considered a phylum, Sipuncula seems to be closely related to Myzostomida and Annelida, and may.
Most sharks are "cold-blooded" or, more precisely, poikilothermicmeaning that their Discreet sex Atkinson body temperature matches that of their ambient environment. Members of the family Lamnidae such as the shortfin mako shark and the great white shark are homeothermic and maintain a higher body temperature than the surrounding water.
In these sharks, a strip Women want sex Dodge Park aerobic red muscle located near the center of the body generates the heat, which the body retains via a countercurrent exchange mechanism by a system of blood vessels called the rete mirabile "miraculous net".
The common thresher and bigeye thresher sharks Blainvi,le a similar mechanism for maintaining an elevated body temperature. In contrast to bony fish, with the exception of Mature women in Blainville pa coelacanth the blood and other tissue of sharks and Chondrichthyes is generally isotonic to their marine environments because of the high concentration of urea up to 2.
This adaptation prevents most sharks from surviving in freshwater, and they are therefore confined to marine environments.