Reptile Classification

Respiratory system
Aquatic worms, for example, lengthen and flatten their bodies to refresh the external medium at their surfaces. A polypeptide is a string of three or more amino acids. They possess only poison glands. Lack of oxygen is termed asphyxiation, and thus carbon monoxide is an asphyxiant. Examples include aspirin and acetaminophen.

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The detached tail continue to wiggle which makes the predator think that the lizard is still struggling, keeping their attention from the escaping prey. The shed tail can be regenerated within a few weeks, but it is smaller than the original one and looks quite different from the rest of the body.

The principal defense mechanism in various snake species is their ability to deliver poison through their fangs. Poison glands located inside their mouth produce deadly venom that helps the creatures to protect themselves from enemies. These animals originated around million to million years ago with the first reptiles evolving from the advanced reptiliomorph labyrinthodonts.

Animals from the Casineria genus are the earliest animals suspected to be amniotes rather than advanced amphibians. The origin of reptiles occurred in steaming swamplands of late Carboniferous period.

Fossils found in Nova Scotia show footprints with imprints of scales and typical reptilian toes. The prints have been attributed to Hylonomus which is the earliest recognized reptile. Hylonomus was a lizard-like creature 8 to 12 in long with sharp teeth that indicates an insectivorous diet. Earlier, the bigger labyrinthodont amphibians like Cochleosaurus largely overshadowed the reptiles, who remained inconspicuous until the CRC or Carboniferous Rainforest Collapses.

This significant extinction event affected the existence of many large animals with the Amphibians being among the worst affected populations. But, the reptiles coped better with the drier conditions post CRC. One of the biggest problems the Amphibians faced was the lack of water bodies which prevented them from reproducing. This did not affect reptiles as they were able to lay eggs on dry lands.

Gradually, the reptiles became a dominating class with new diet strategies including both carnivory and herbivory. This established the foundation for the Mesozoic stage, also referred to as the Age of Reptiles. The genus Mesosaurus from the Permian period is counted among the most recognized early reptiles.

Most Reptiles are unable to see properly during nighttime as their vision is mainly adapted to the daylight conditions. They have color vision with the visual depth perception being much more advanced than Amphibians and many Mammals.

The vision is reduced in species like the Blind Snake, while some snakes have extra visual or sensory organs that make them sensitive to heat and infrared radiation.

The horny epidermis layer makes their skin watertight, allowing these animals to inhabit dry land. Reptiles have thinner skin compared to mammals and it also lacks the dermal layer present in mammal skin. The exposed skin areas are covered in scutes or scales which may have a bony base, creating their armors.

In turtles, a hard shell made up of fused scutes covers the entire body. Reptiles use their lungs for breathing. The skin of the aquatic turtles is more permeable for allowing them to respire while the cloaca is modified in various species to increase the gas exchange area. Despite these adaptations, lungs remain a very important part of their respiratory system. The main Reptile groups accomplish lung ventilation in different procedures. Squamates are known to ventilate the lugs mainly by their axial musculature.

Certain lizard species are capable of buccal pumping apart from the normal axial breathing. The proto-diaphragm in Tegu lizards separates their pulmonary cavity from visceral cavity, helping with their respiration by allowing greater lung inflation.

The muscular structure of the diaphragm in the Crocodilians species resembles that of various mammals. However, there are some differences in their diaphragmatic setup. They also have two aortas playing a major role in their systemic circulation. The oxygenated and deoxygenated blood may get mixed with each other in their three-chambered heart with the level of mixing depending on the species and the physiological state of the animal.

Their circulatory system is capable of shunting back the deoxygenated blood to the body and the oxygenated blood to the lungs if necessary. Unlike other Reptiles, animals in the crocodilian subgroup have four-chambered hearts. But, their two systemic aortas are only capable of bypassing their pulmonary circulation. On the other hand, the three-chambered hearts in various lizard and snake species can function as the four-chambered ones during contraction. Majority of these animals have short digestive tracts because their diet mainly consists of meat, which is very simple to digest.

Their digestion process is slower than that in mammals due to their inability of mastication and their low metabolism rate while resting. The energy requirements for their poikilotherm metabolism are very low which allows large animals from this class such as various constrictors and crocodiles to survive for months from one large meal, digesting it slowly.

Herbivorous reptiles are also unable to masticate their food, which slows down the digestive process. Some species are known to swallow pebbles and rocks that help in grinding up plant matters within the stomach, assisting their digestion. The basic nervous system in the Reptiles is similar to that in the Amphibians. But, Reptiles have slightly larger cerebrum and cerebellum. Most of the important sensory organs are properly developed in these creatures.

However, there are certain exceptions such as the absence of external ears in snakes they have the inner and middle ears.

Reptiles have twelve cranial nerve pairs. They have to use electrical tuning for expanding the range of their audible frequencies because they have short cochlea. These animals are believed to be less intelligent compared to mammals and birds because the relative size of their brain and body is much smaller than that of the latter.

However, the brain development can be more complex in some larger Reptiles. Modern species also have pineal glands in their brains. Most of these animals are tetrapods, meaning they have four legs. Snakes are examples of legless Reptiles. Their skeletal system is similar to other tetrapods with a spinal column supporting their bodies.

Their excretory system consists of two small kidneys. The diapsid species excrete uric acid as the principal nitrogenous waste product. But, turtles excrete mainly urea. Some of these species use their colons for reabsorbing water, while some are able to absorb the water stored in their bladders. Certain Reptiles excrete the excess salts in their bodies through the lingual and nasal salt glands. Reptiles have certain characteristic features that help in distinguishing them from Amphibians, Mammals and Aves:.

They are capable of adapting to almost all kinds of habitats and environmental conditions, except for extremely cold regions. These animals can inhabit dry deserts, forests, grasslands, wet meadows, shrub lands and even marine habitats.

Reptiles are capable of adapting to extremely high temperatures because they are cold blooded. Various snakes including the Rattle Snakes and King Snakes as well as different lizards like the Gila Monsters live in desert habitats. Grassland is another common type of habitat for various snakes and lizards e.

Garter Snakes, Fox Snakes. The vegetation in this habitat attracts many insects and rodents, making it easier for the Reptiles to catch prey. Swamps and large water bodies are inhabited by different Reptiles such as crocodiles, alligators, certain turtles and snakes. Animals like the Saltwater Crocodile and Marine Iguana inhabit seaside, travelling in and out of ocean as necessary. Some species, such as the Sea Snakes and Sea Turtles, live in the ocean. They leave the waters only during the breeding season for laying eggs.

These animals typically practice sexual reproduction with some specific species using asexual reproduction. Majority of these animals are amniotes, laying eggs covered with calcareous or leathery shells. The eggs are generally laid in underground burrows dug by the females.

The viviparity and ovoviviparity modes of reproduction are used by many species such as all boas and many vipers. However, the level of viviparity may vary with some species retaining their eggs until shortly before hatching while others nourish the eggs for supplementing the yolks. In some Reptile species, the eggs do not have any yolk with the adults providing all the necessary nourishment through a structure resembling the mammalian placenta.

Six lizard families and one snake family from the Squamata sub-group are known to be capable of agamogenesis or asexual reproduction.

In some squamate species, the females are capable of giving birth to unisexual diploid clones of themselves. This type of asexual reproduction is known as parthenogenesis, occurring in various teiid lizards, geckos and lacertid lizards. Komodo dragons have reproduced through parthenogeny in captivitiy.

Like many mammals, birds and Amphibians, their embryonic life consists of an amnion, chorion, as well as an allantois. The incubation period may vary depending on the species and other factors like the temperature of the surroundings. Usually, hatchlings are able to take care of themselves almost immediately after coming out of the eggs.

But, the females of some species are known to protect their eggs and hatchlings. For example, female Pythons coil themselves around the eggs in order to protect them and regulate their temperature. Similarly, crocodiles are known to guard their young after the eggs hatch. TDSD or temperature-dependent sex determination can be observed in many Reptiles. In TDSD, the incubation temperature determines the sex of the offspring. This characteristic is most commonly seen in crocodiles and turtles, but can also occur in tuataras and lizards.

Different food habits can be observed between the four sub-groups. So when the sphincter contracts, it not only constricts the walls of the esophagus, it also pulls the sides of the limiting ridge's "U" together, thus hiding and tightly closing the esophageal opening Montedonico et al.

Diagram of the limiting ridge and the esophageal opening in the rat's stomach when the esophageal spincter is a open and b closed. Anatomical textbooks on rats usually mention in passing that rats can't vomit. They tend to implicate the limiting ridge or the lack of striated muscle in the rat's esophagus, and sometimes both Fox et al. Looking deeper into the scientific literature, I found a complex story about why a rat is unable to vomit:.

Rats have a powerful and effective gastroesophageal barrier , consisting of the crural sling, the esophageal sphincter, and the centimeters of intraabdominal esophagus see above. The pressure at the two ends of this barrier is much higher than the pressure found in the thorax or abdomen during any phase of the the breathing cycle Montedonico et al.

The strength and pressure of this barrier make reflux in rats nearly impossible under normal conditions Montedonico et al. In order to vomit, the rat would have to overcome this powerful barrier.

Evidence suggests that rats cannot do this, because 1 they can't open the crural sling at the right time, and 2 they can't wrench open the esophageal sphincter. In addition, 3 rats lack the necessary neural connections to coordinate the muscles involved in vomiting. The diaphragm is has two muscles: The esophagus passes through the crural sling, so when the crural diaphragm contracts the esophagus is pinched closed.

During the expulsive phase of vomiting in humans, the activity of these two diaphragm muscles diverges. The costal section contracts, putting pressure on the stomach, while the crural section relaxes, allowing stomach contents to pass through the esophagus reviewed in Pickering and Jones Rats, however, do not dissociate the activity of these two parts of their diaphragm: Instead, both muscles contract or relax together Pollard et al. The rat's inability to separately and selectively control its two diaphragmatic muscles therefore plays an important role in its inability to vomit: In humans, the esophageal sphincter is opened during vomiting with the help of the longitudinal muscle of the esophagus Lang and Sarna This allows the expulsion of stomach contents during vomiting.

Rats, however, have only a thin, weak longitudinal muscle which is unstriated where it joins the stomach. It is too weak to wrench open the sphincter and permit the evacuation of stomach contents Steinnon Animal species that vomit have a "vomiting center" in the brainstem, consisting of several interconnected nuclei that coordinate all the many muscles involved in vomiting see Borison and Wang Animals that don't vomit, like rats and rabbits, have the brainstem nuclei and the muscle systems used in vomiting, but they don't have the complex connections between the nuclei or between the brainstem and the viscera that are required for such a coordinated behavior King As of yet, no empirical research has been done on whether the inability to vomit benefits the rat in some way.

Remember that Davis et al. The rat uses its senses of smell and taste to avoid foods that made it feel ill in the past Garcia et al. In fact, rats avoid foods in response to cues that cause vomiting in other species Coil and Norgren So the rat who avoids foods that made it feel ill should not ingest lethal amounts of that food in the future.

Rats can, in fact, detect toxins in the stomach Clarke and Davison , and in the circulation Coil and Norgren but they don't respond by vomiting, instead they avoid that food in the future. So, the theory goes, rats have lost the ability to vomit because they no longer need it: However, an alternative theory is that rats developed their hyper-sensitive food avoidance to compensate for the inability to vomit.

It makes sense for a rat to scrupulously avoid ingesting toxic food if it can't get rid of it later. So, it might indeed benefit the rat to be able to vomit, but as vomiting isn't an anatomical option, the rat has developed other methods of protecting itself, including food avoidance. Also, rats do still need a strategy to cope with ingested toxins. Rat food avoidance isn't foolproof. Rats do experience nausea and have evolved an alternative to vomiting: When rats feel nauseous they eat things like clay, kaolin a type of clay , dirt and even hardwood bedding eating clay and dirt is a type of pica called geophagia.

Their consumption isn't random, though: Rats engage in pica in response to motion-sickness Mitchell et al. The incidence of pica decreases in response to anti-emetics Takeda et al. Pica in rats is therefore analogous to vomiting in other species. The consumption of non-nutritive substances may be an adaptive response to nausea. Nausea is frequently caused by a toxin, and non-nutritive substances may help dilute the toxin's effect on the body.

Clay in particular binds and inactivates many types of chemicals and is therefore good at deactivating toxins e. Pica may therefore be part of the rat's second line of defense against toxins. The evolution of behavior is studied by examining and comparing the behavior of living species see Martins for more.

The behavior is then mapped onto the phylogeny, or evolutionary "family tree" of those species, and deductions can then be made about when a particular behavioral trait appeared in the past.

For example, if a group of related species exhibit the same behavior, then their common ancestor probably did, too. If just one species in a group has a particular behavior, then that behavior probably wasn't present in the common ancestor, but evolved later just in that species. Such broad comparative studies involving dozens of species have not yet been done for vomiting. In fact, it is a bit difficult to determine how common vomiting is in the animal kingdom: Hatcher says that the ability to vomit is a primitive, common trait and many species do it.

Harding , however, states that very few species are capable of vomiting. Until a survey of many different species is done, we won't know the answer for sure.

A survey of the literature shows that information on vomiting does exist for a few species Table 1. Tentatively, from this table, it looks like the ability to vomit is widespread among vertebrates, and hence is an evolutionarily old trait that appeared in a distant common vertebrate ancestor and was passed down to its many descendants.

The ability to vomit may then have been lost in the common ancestor of rodents and rabbits and later regained in the woodchuck. Further inquiry into the exact mechanisms of vomiting in different species would cast more light on whether vomiting has evolved multiple times.

Note that these deductions are purely speculative and casual at this point, as they are based on just a few scattered species. A broad, comprehensive survey of many different species is needed before the evolution of vomiting is fully understood.

European storm petrel Hydrobates pelagicus.

Types of poison