Well, obviously they must know that it was prevented if they can travel in time. The Malpighian tubules are a key insect excretory organ. Vertebrate Palaeontology 4th ed. Now, while all of this was transpiring on the surface of the planet, underground there was another extraterrestrial race that had been here - a race that has been here for hundreds of thousands of years. If this is accompanied by a proportionate reabsorption of water, the tubular fluid remains at the same concentration as the blood plasma, but if the reabsorption of water is restricted, as it may be in certain circumstances see below , the tubular fluid becomes more dilute than the blood plasma.
Ammonia is toxic to cells and so must be quickly ejected from the body, however, being water-soluble it is typically ejected in solution, which requires water. The mammalian solution is to convert the ammonia into a less toxic substance called urea. This conversion takes place in the liver: Being less toxic, the urea can be temporarily stored and excreted in a concentrated solution, requiring less water.
Birds and reptiles have a better water-conserving system; they excrete uric acid or urate salts. Uric acid is not readily soluble in water and is of low toxicity and so can be excreted with very little water.
The dry excreta of birds is a mixture of faecal matter and uric acid crystals and when water is scarce birds can produce very dry excreta. Arthropods, including insects, have adopted similar solutions. Woodlice, which are not insects but crustaceans, are only partially adapted to terrestrial conditions, preferring moist habitats, but they do excrete ammonia. Interestingly they can vent off ammonia gas, rather than relying on the wastage of water to remove the ammonia in solution. Insects are better adapted to dry conditions, although aquatic insects and some insect larvae excrete ammonia, most terrestrial forms excrete uric acid or salts of uric acid called urates, such as ammonium urate.
If one considers how small an insect is and how rapidly a small drop of water may evaporate, then one realises that insects have outstanding water-conserving systems.
Bedbugs Rhodnius can survive for weeks without ingesting any water! Some insects can tolerate extremely dry conditions and may excrete uric acid as a dry crystalline powder, along with bone-dry faeces! Insects generally produce only trace amounts of urea. Malpighian tubules The main excretory organ of the insect is the Malpighian tubule. Insects contain anything from 2 to or more Malpighian tubules depending on the genus. Malpighian tubules are tubular outgrowths of the gut.
They typically develop as pouches emerging from the junction between the midgut and the hindgut, though there actual final position varies - they may be attached to the midgut, hindgut or the midgut-hindgut junction as is the case with our ant above.
To learn more about the insect gut and insect nutrition click here. Each Malpighian tubule is a blind-ending tube whose lumen is continuous with the lumen of the gut. Each consists of a single layer of epithelial cells, forming the tubule wall, enclosed by an elastic membrane basement membrane - a fibrous and porous protein mesh. In most insects there is a thin layer of striated muscle around this membrane. Typically muscle cells spiral around the distal end the end furthest from the gut of the tubule, causing it to twist and turn in gentle writhing movements as the muscles contract.
The proximal end near the gut may be coated in circular and longitudinal muscle fibres, giving rise to peristalsis or squeezing movements which empty the contents of the tubule into the gut.
In some cases, such as in caterpillars, the Malpighian tubules on each side 3 on each side in this case empty into a small bladder, which then empties into the gut. In this case only the bladder may be muscular and its lumen is lined by cuticle suggesting that the bladder is an extension of the hindgut.
The tubules do not just hang around in the air! The body cavity of the insect is filled with a fluid, usually colourless, called haemolymph. This fluid bathes the organs and tissues and is circulated around the insect body. The tubules are also typically loosely or firmly anchored in place by the tracheae which attach to them. To learn more about the insect circulatory system click here. The twisting and turning of the Malpighain tubules presumably keeps them in contact with fresh haemolymph perhaps by circulating the heamolymph around the tubule.
Metabolic wastes and other unwanted chemicals that entered the insect system pass into the haemolymph, or are excreted into the haemolymph by the cells.
These include nitrogenous waste and plant toxins such as alkaloids. It is the job of the Malpighian tubules to keep the haemolymph cleansed of these wastes - they remove wastes from the haemolymph and then excrete them into the gut lumen. Outside the muscle layer is a 'peritoneal covering' of cells with embedded tracheoles, which carry oxygen to the Malpighian tubules which their mitochondria use to generate the needed ATP by aerobic respiration. Learn more about insect respiration.
How do Malpighain tubules work? Waste materials and excess water pass from the haemolymph into the Malpighain tubules, by crossing the epithelial wall of these blind-ended tubes. Recent evidence shows that these cells contain pumps, proteins called proton-secreting V-ATPase. These proteins use energy in the form of ATP see respiration to pump protons into the lumen of the Malpighian tubule.
Protons are positively charged and to maintain charge balance the removal of protons from the epithelial cells, into the tubule lumen, is balanced by the inward movement of potassium ions, which move from the haemolymph, into the epithelial cells and then out into the tubule lumen also. The diagram below shows a section through a segment of a Malpighian tubule.
The epithelial cells have microvilli fingerlike projections projecting into the tubule lumen and are rich in mitochondria green stripy rods which produce the ATP required by the pumps. A model of how ion transport across the epithelium is thought to take place is illustrated. The detailed structure of the cell at top right has been simplified to illustrate some of the transport mechanisms.
The influx of these positive ions drags in negative chloride ions to balance the charge. These ions move across the cytoplasm of the cell, the so-called transcellular pathway. Note the potassium-chloride and sodium-chloride symporters, the proton-potassium and proton-sodium antiporters and the ion channels. The flux of ions across the epithelial cell also draws across water, by osmosis.
This probably takes place largely by the paracellular pathway , that is between the epithelial cells. Sugars and amino acids are swept along by the water into the tubule lumen. Since these materials are useful they will be reabsorbed later further downstream. Other small molecules small enough to cross the basement membrane will also move into the tubule through this pathway. The transport of a substance which depends directly on ATP, such as the pumping of the protons in the Malpighian tubule, is called active transport.
The transport of the other ions and water is passive by facilitated diffusion in of itself, but is dependent on proton transport and so indirectly dependent on ATP. The brain is relatively small. The tongue of a snake includes highly sensitive smell sensors. Some researchers speculate that the forked nature of the tongue may offer a stereo sense of smell.
Crocodilians see well in daylight and may even have color vision; additionally, their vertical, cat-like pupil gives them excellent night vision. In crocodilians, the upper and lower jaws also are covered with sensory pits, the crocodile version of the lateral line sensory organ found in fish and many amphibians. These pigmented nodules encase bundles of nerve fibers that respond to the slightest disturbance in surface water, detecting vibrations and small pressure changes in water, making it possible for them to detect prey, danger, and intruders even in total darkness.
While alligators and caimans have the sensory nodules only on their jaws, crocodiles have similar organs on almost every scale on their body. Most reptiles reproduce sexually. This includes many male snakes that rely on scent to find females and that complete fertilization internally. Most reptile species are oviparous egg-laying. Many species of squamates, however, are capable of giving live birth. This is achieved either through ovoviviparity egg retention or viviparity babies born without use of calcified eggs.
Many of the viviparous species feed their fetuses through various forms of placenta, just like mammals Pianka and Vitt They often provide considerable initial care for their hatchlings. Amniotic eggs are covered with leathery or calcareous shells and are compartmentalized by four membranes: Eggs are waterproof, but permeable to gases. Sperm are placed inside the female by internal fertilization prior to the formation of the shell.
In addition to the common pattern of sexual reproduction among reptiles, a pattern of asexual reproduction has been identified in six families of lizards and one snake family. In some species of squamates lizards and snakes , a population of females is able to produce a unisexual diploid clone of the mother. This asexual reproduction, called parthenogenesis , occurs in several species of gecko and is particularly widespread in the teiids especially Aspidocelis and lacertids Lacerta.
Parthenogenetic species are also suspected to occur among chameleons , agamids, xantusiids, and typhlopids. Reptiles offer economic, ecological, aesthetic, and symbolic value to humans.
Some species, such as the green turtle, the iguana, and some snakes, are part of the diet, and the giant Galapagos tortoise was so popular as a food among sailors in the nineteenth century that it was nearly exterminated. The skins of crocodilians, snakes , and lizards have been used in leather goods, such as shoes, handbags, gloves, and belts, but international agreements protecting endangered species have prompted a shift of reptile skin sources from hunters of wild species to farmers growing reptiles in captivity.
Reptiles also are very popular pets. In the United States , about 3 percent of households have reptiles as pets with many of the reptiles having been imported into the country either legally or illegally as part of the international trade in live exotic animals.
Ecologically , reptiles are a critical element in the food chains of most ecosystems, and sometimes a keystone species whose removal can drastically alter the populations of other organisms.
The consumption by reptiles of rodents and insect pests aids in control of these animals, which can be serious agricultural pests. Aesthetically, many reptiles can be considered beautiful or awe-inspiring, such as the San Francisco garter snake Thamnophis sirtalis tetrataenia , with its bright orange head, black and red stripes, and turquoise belly, and the chameleons with their color changes. Reptiles appear in designs on apparel and other consumer goods because of their appeal. Symbolically, reptiles appear in literature and religion in a variety of ways.
Perhaps the most famous reference is the Bible reference to the serpent in the Garden of Eden, or Jesus advising his disciples to be "wise as serpents. Some reptiles also present threats to people, whether because they are venomous, like some snakes, or can attack humans, such as some crocodilians. In addition, salmonella, a bacterial disease, is sometimes picked up from a reptile's skin when touching a reptile kept as a pet.
Hylonomus, the oldest-known reptile, was about 8 to 12 inches 20 to 30 cm long. Westlothiana, also suggested as the oldest reptile, is for the moment considered to be related more to amphibians than to amniotes. Other examples of fossil animals considered to be ancient reptiles are those of the genera Petrolacosaurus, Araeoscelis, Paleothyris, Ophiacodontidae, Archaeothyris, and Ophiacodon, and also the family of mesosaurs.
The first true "reptiles" or amniotes are categorized as Anapsids Anapsida , which are vertebrates characterized by solid skulls with the conventional openings for nose, eyes, spinal cord, and so forth, but lacking temporal fenestrae jaw muscle attachment sites at holes in the sides of the skull behind the eyes near the temples.
Turtles are believed by some to be surviving anapsids, indeed the only surviving anapsids, as they also share this skull structure. However, this point has become contentious, with some arguing that turtles reverted to this primitive state in the process of improving their armor. Both sides have marshaled evidence, and the conflict has yet to be resolved. Shortly after the appearance in the fossil record of the first reptiles, a second branch appeared.
The original branch led to the Anapsida, which did not develop the jaw muscle attachment holes in their skulls, and the second led to the Diapsida diapsids , which developed two pairs of jaw muscle attachment holes in their skulls behind the eye holes. Diapsids "two arches" are a group of tetrapod animals that appeared in the fossil record about million years ago during the late Carboniferous period.
Living diapsids are extremely diverse, and are considered to include all birds , crocodiles, lizards, snakes, and tuataras and possibly even turtles. While some lost either one hole lizards , or both holes snakes , they are still classified as diapsids based on their assumed ancestry. During the Permian period million years ago , the Diapsida line of descent split into two lineages: The lepidosaurs modern snakes, lizards, and tuataras, as well as, debatably, the extinct sea reptiles of the Mesozoic era and the archosaurs living crocodilians and birds as well as the extinct pterosaurs and dinosaurs.
The earliest solid-skulled amniotes in addition to giving rise to the anapsids, are also considered to have given rise about million years ago to a separate line, the Synapsida synapsids , which have a pair of holes in their skulls behind and above the eyes; this feature has the advantage of lightening the skull and increasing the space for jaw muscles.
The synapsids eventually evolved into mammals and the early synapsids have been referred to as mammal-like reptiles by some specialists, while others argue that even the early synapsids were no longer reptiles. As noted above, from the classical standpoint, reptiles included all the amniotes except birds and mammals. Thus, reptiles were defined as the set of animals that includes crocodiles , alligators , tuatara, lizards , snakes , amphisbaenians , and turtles , grouped together as the class Reptilia Latin repere, "to creep".
This is still the usual definition of the term. However, in recent years, many taxonomists have begun to insist that for clear identification of the ancestor-descendant relations of all organisms each defined taxon should be monophyletic, that is, each taxon should include all the descendants from the originating stock. The reptiles as defined are clearly not monophyletic but rather are paraphyletic , since they exclude both birds and mammals, although these also are considered to be descendant from the original reptile.
Colin Tudge writes:. Mammals are a clade [a monophyletic taxon], and therefore the cladists are happy to acknowledge the traditional taxon Mammalia ; and birds, too, are a clade, universally ascribed to the formal taxon Aves. Mammalia and Aves are, in fact, subclades within the grand clade of the Amniota. But the traditional class Reptilia is not a clade.
It is just a section of the clade Amniota: The section that is left after the Mammalia and Aves have been hived off. It cannot be defined by synamorphies, as is the proper way. It is instead defined by a combination of the features it has and the features it lacks: Reptiles are the amniotes that lack fur or feathers.
Some cladists thus redefine Reptilia as a monophyletic group, including the classic reptiles as well as the birds and perhaps the mammals depending on ideas about their relationships. Others abandon it as a formal taxon altogether, dividing it into several different classes. However, other biologists believe that the common characters of the standard four orders Crocodilia crocodiles , Rhynchocephalia tuataras , Squamata snakes and lizards , and Testudines turtles are more important than the exact relationships, or feel that redefining the Reptilia to include birds and mammals would be a confusing break with tradition.
A number of biologists have adopted a compromise system, marking paraphyletic groups with an asterisk, for example, class Reptilia. Colin Tudge notes other uses of this compromise system:. College-level references, such as Benton , offer another compromise by applying traditional ranks to accepted phylogenetic relationships. In this case, reptiles belong to the class Sauropsida, and mammal-like reptiles to the class Synapsida, with birds and mammals separated into their own traditional classes.
The terms Sauropsida "Lizard Faces" and Theropsida "Beast Faces" were coined to distinguish between lizards, birds, and their relatives on one hand Sauropsida and mammal-like reptiles and mammals Theropsida on the other. This classification supplemented, but was never as popular as the classification of the reptiles according to the positioning of temporal fenestrae mentioned above under evolution of the reptiles Anapsida, Diapsida, Synapsida, and so on. A diverse group of egg-laying vertebrate animals, the Sauropsida includes all modern and most extinct "reptiles" excluding synapsids.
Living sauropsids include lizards, snakes, turtles, crocodiles, and birds. Extinct sauropsids include dinosaurs except birds , pterosaurs , plesiosaurs , ichthyosaurs , and many others. The synapsids were originally defined, at the turn of the twentieth Century, as one of the five main subclasses of reptiles on the basis of their distinctive temporal openings. The synapsids represented the reptilian lineage that led to the mammals, and gradually evolved increasingly mammalian features, hence, "mammal-like reptiles.
In the current cladistic based system, the Linnean classification of the class Reptilia in terms of four sub-classes has been replaced. The term "Theropsida" is replaced by Synapsida, which now refers to both the old subclass Synapsida and the mammals.
In the new edition of his textbook , Michael Benton uses the term "Class Sauropsida" to refer to all non-synapsid reptiles. Because synapsids evolved into mammals, the mammals therefore are included under the clade Synapsida. That is, "synapsids" are now also known as "theropsids. The following is a very abbreviated classification of the extensive classification system presented by Benton The following classification of living reptiles was given by Uetz , which was modified from the overall taxonomy of Zug et al.
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