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sharks have eyelids

Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, have two to three layers or more, depending on body size. The jaws of a large great white shark may have up to five p-tsireview.cf: Chondrichthyes. do sharks have eyelids, own observations of mako sharks. “Shortfin mako sharks are less common in for mako sharks are grouped together with blue sharks, tiger sharks, and a. Also unlike humans, shark eyelids serve to protect the eye when attacking prey. Some sharks have a nictating membrane, or a clear membrane that covers and protects the eye when a shark bites its prey. Sharks like the Great White, lacking a nictating membrane, roll their pupils back in their heads for protection when feeding.


Do Sharks Have Eyelids | Popular Science


Sharks are a group of elasmobranch fish characterized by a cartilaginous skeletonfive to seven gill slits on the sides of the head, sharks have eyelids, 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 Etmopterus perryia deep sea species of only 17 centimetres 6. They generally do not live in freshwater although there are a few known exceptions, sharks have eyelids, such as the bull shark and the river sharkwhich can be found in both seawater and freshwater. They have numerous sets of replaceable teeth.

Well-known species such as the tiger sharkblue sharkmako sharkthresher sharksharks have eyelids hammerhead shark are apex predators —organisms at the top of their underwater food chain. Many shark populations are threatened by human activities. Biofluorescence is a characteristic of a few shark species, such as the swell shark and the chain catshark. Until the 16th century, [6] sharks were known to mariners as "sea dogs". The etymology of the word "shark" is uncertain, the most likely etymology states that the original sharks have eyelids of the word was that of "predator, one who preys on others" from the Dutch schurkmeaning "villain, scoundrel" cf.

A now disproven theory is that it derives from the Yucatec Maya word xok pronounced 'shok'meaning "fish". 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, sharks have eyelids.

The majority of modern sharks can be traced 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 sharks have eyelids lifetime, which explains the abundant fossils. The teeth consist of easily fossilized calcium phosphatean 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, [12] which has been found within Paleozoic strata in 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, sharks have eyelids. 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 shape to the great white sharks and the pelagic shortfin and longfin makos.

The presence of 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.

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 the 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: one lineage is of white sharks with coarsely serrated teeth and it probably gave rise to the sharks have eyelids great white shark, and another lineage is of white sharks with finely serrated teeth.

These sharks attained gigantic proportions and include the extinct megatoothed shark, sharks have eyelids, C. Like most extinct sharks, C. Sharks belong to the superorder Selachimorpha in the subclass Elasmobranchii in the class Chondrichthyes. The Elasmobranchii also include rays and skates ; the Chondrichthyes also include Chimaeras. It was thought that the sharks form a polyphyletic group: some sharks are more closely related to rays than they are to some other sharks, [21] but current molecular studies support monophyly of both groups of sharks and batoids.

Lamnoids and Carcharhinoids are usually placed in one cladebut recent studies show the Lamnoids and Orectoloboids are a clade. Some scientists now think that Heterodontoids may be Squalean. The Squaleans 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. It includes the Squatiniformessharks have eyelids, and 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: [24]. Shark teeth are embedded in the gums rather than directly 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 move forward in comparison to a conveyor belt ; some sharks lose 30, or more teeth in their lifetime. The rate of tooth replacement varies from once every 8 sharks have eyelids 10 days to several months, sharks have eyelids. In most species, teeth are replaced one at a time as opposed to the sharks have eyelids replacement of an entire row, which is observed in the cookiecutter shark.

Tooth shape depends on the shark's diet: those that feed on mollusks and crustaceans have dense and flattened teeth used for crushing, those that feed on fish have needle-like teeth for gripping, and those that feed on larger prey such as mammals have pointed lower teeth for gripping and triangular upper teeth with serrated edges for cutting, sharks have eyelids.

The teeth of plankton-feeders such as the basking shark are small and non-functional. Shark skeletons are very different from those 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. This reduces the skeleton's weight, saving energy. Jaws of sharks, like those of rays sharks have eyelids skates, are not attached to the cranium.

The jaw's surface in comparison to the shark's vertebrae and gill arches needs extra support due to its heavy exposure to physical stress and its need for strength. It has a layer of tiny hexagonal plates called " tesserae ", which are crystal blocks of calcium salts arranged as a mosaic. Generally sharks have only one layer of tesserae, but the jaws of large specimens, such as the bull shark, tiger shark, and the great white shark, 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 unsegmented rays named ceratotrichia, filaments of elastic protein resembling the horny keratin in 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.

Unlike bony sharks have eyelids, sharks have a complex dermal corset made of flexible collagenous fibers and arranged 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 acceleration dependent on tail shape. 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 than the ventral portion. This is because the shark's vertebral column extends into that dorsal portion, sharks have eyelids, providing a greater surface area for muscle attachment.

This allows more efficient locomotion among 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.

The tiger shark must be able to twist and turn in the water easily when hunting to support its varied diet, sharks have eyelids 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 sharks have eyelids, sharks do not have gas-filled swim bladders for buoyancy. Instead, 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, sharks have eyelids. Researchers use this condition to handle sharks safely. Like other fish, sharks extract oxygen from seawater as it passes sharks have eyelids their gills.

Unlike other fish, shark gill slits are not covered, but lie in a row behind the head. A modified slit called a spiracle lies just behind the eye, which assists the shark with taking in water during respiration and plays a major role in bottom—dwelling sharks. Spiracles are reduced or missing in active pelagic sharks. While at rest, sharks have eyelids, most sharks pump water over their gills to ensure a constant supply of oxygenated water.

A small number of species have lost the ability to pump water through their gills and must swim without rest.

These species are obligate ram ventilators and would presumably asphyxiate if unable to move. Sharks have eyelids ram ventilation is also true of some pelagic sharks have eyelids fish species. The respiration and circulation process begins when deoxygenated blood travels to the shark's two-chambered heart. Here the shark pumps blood to its gills via the ventral aorta artery sharks have eyelids it branches into afferent brachial arteries.

Reoxygenation takes place in the gills and the reoxygenated blood flows into the efferent brachial arteries, which come together to form the dorsal aorta, sharks have eyelids. 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 there blood enters the heart ventricle and the cycle repeats. Most sharks are "cold-blooded" or, sharks have eyelids, more precisely, poikilothermicmeaning that their internal body temperature matches that of their ambient environment, sharks have eyelids.

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 of 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 have a similar mechanism for maintaining an elevated body temperature. In contrast to bony fish, with the exception of the coelacanth[41] 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.

 

Shark Senses: Sight | HowStuffWorks

 

sharks have eyelids

 

do sharks have eyelids, own observations of mako sharks. “Shortfin mako sharks are less common in for mako sharks are grouped together with blue sharks, tiger sharks, and a. Sharks use their eyelids primarily as a protective measure. When it's feeding time, or when the shark has an encounter with another shark, it will close the eyelids to protect the eyes from abrasion. However, a shark's eyelids don't close all the way. Some sharks have a third lid known as a nictitating membrane, which will fully protect the eye Author: Molly Edmonds. Also unlike humans, shark eyelids serve to protect the eye when attacking prey. Some sharks have a nictating membrane, or a clear membrane that covers and protects the eye when a shark bites its prey. Sharks like the Great White, lacking a nictating membrane, roll their pupils back in their heads for protection when feeding.