Trachylepis is a skink genus in the subfamily Lygosominae found mainly in Africa. Its members were included in the wastebin taxon Mabuya. As defined today, Trachylepis contains the clade of Afro-Malagasy mabuyas, the ancestors of T. atlantica are believed to have rafted across the Atlantic from Africa during the last 9 million years. The following species are recognized as being valid, the species Mabouya punctatissima belongs in this genus, but its exact identity remains unclear
The placenta provides oxygen and nutrients to growing babies and removes waste products from the babys blood. The placenta attaches to the wall of the uterus, and the umbilical cord develops from the placenta. The umbilical cord is what connects the mother and the baby, Placentas are a defining characteristic of placental mammals, but are found in some non-mammals with varying levels of development. The homology of structures in various viviparous organisms is debatable. The classical plural is placentae, but the form placentas is common in modern English and metatherial mammals produce a choriovitelline placenta that, while connected to the uterine wall, provides nutrients mainly derived from the egg sac. In humans, the placenta averages 22 cm in length and 2–2.5 cm in thickness, with the center being the thickest, and it typically weighs approximately 500 grams. It has a dark reddish-blue or crimson color and it connects to the fetus by an umbilical cord of approximately 55–60 cm in length, which contains two umbilical arteries and one umbilical vein.
The umbilical cord inserts into the chorionic plate, vessels branch out over the surface of the placenta and further divide to form a network covered by a thin layer of cells. This results in the formation of villous tree structures, on the maternal side, these villous tree structures are grouped into lobules called cotyledons. In humans, the placenta usually has a shape. The placental flora more closely resembles that of the oral microbiome than either the vaginal or rectal microbiome, in non-human animals, part of the microbiome is passed onto offspring even before the offspring are born. Bacteriologists assume that the same holds true for humans. The placenta begins to develop upon implantation of the blastocyst into the maternal endometrium, the outer layer of the blastocyst becomes the trophoblast, which forms the outer layer of the placenta. This outer layer is divided into two layers, the underlying cytotrophoblast layer and the overlying syncytiotrophoblast layer. The syncytiotrophoblast is a continuous cell layer that covers the surface of the placenta.
It forms as a result of differentiation and fusion of the underlying cytotrophoblast cells, the syncytiotrophoblast, thereby contributes to the barrier function of the placenta. Development of the blood supply to the placenta is complete by the end of the first trimester of pregnancy. In preparation for implantation of the blastocyst, the uterine endometrium undergoes decidualisation, spiral arteries in decidua are remodeled so that they become less convoluted and their diameter is increased
Sharks are a group of elasmobranch fish characterized by a cartilaginous skeleton, five to seven 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 and are the group to the rays. Under this broader definition, the earliest known sharks date back to more than 420 million years ago, acanthodians are often referred to as spiny sharks, though they are not part of Chondrichthyes proper, they are a paraphyletic assemblage leading to cartilaginous fish as a whole. Since then, sharks have diversified into over 500 species, Sharks are found in all seas and are common to depths of 2,000 metres. They generally do not live in freshwater although there are a few exceptions, such as the bull shark and the river shark. Sharks have a covering of dermal denticles that protects their skin from damage and they have numerous sets of replaceable teeth. Well-known species such as the white shark, tiger shark, blue shark, mako shark.
Many shark populations are threatened by human activities, until the 16th century, sharks were known to mariners as sea dogs. The etymology of the shark is uncertain. One theory is that it derives from the Yucatec Maya word xok, the Middle English Dictionary records an isolated occurrence of the word shark in a letter written by Thomas Beckington in 1442, which rules out a New World etymology. Evidence for the existence of sharks dates from the Ordovician period, 450–420 million years ago, before land vertebrates existed and before many plants had colonized the continents. Only scales have been recovered from the first sharks and not all agree that these are from true sharks. The oldest generally accepted shark scales are from about 420 million years ago, the first sharks looked very different from modern sharks. The majority of sharks can be traced back to around 100 million years ago. Most fossils are of teeth, often in large numbers, partial skeletons and even complete fossilized remains have been discovered.
Estimates suggest that sharks grow tens of thousands of teeth over a lifetime, the teeth consist of easily fossilized calcium phosphate, an 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 Cladoselache, from about 370 million years ago, which has been found within Paleozoic strata in Ohio and Tennessee
Ichthyosaurs belong to the order known as Ichthyosauria or Ichthyopterygia. In the Late Cretaceous, ichthyosaurs became extinct for unknown reasons, science became aware of the existence of ichthyosaurs during the early nineteenth century when the first complete skeletons were found in England. In 1834, the order Ichthyosauria was named, that century, many excellently preserved ichthyosaur fossils were discovered in Germany, including soft tissue remains. Since the late century, there has been a revived interest in the group leading to an increased number of named ichthyosaurs from all continents. Ichthyosaur species varied from one to sixteen metres in length. Ichthyosaurs resembled both modern fish and dolphins and their limbs had been fully transformed into flippers, which sometimes contained a very large number of digits and phalanges. At least some species possessed a dorsal fin and their heads were pointed, the jaws often equipped with conical teeth to catch smaller prey. Some species had larger bladed teeth to attack large animals, the eyes were very large, probably for deep diving.
The neck was short and species had a stiff trunk. These had a vertical tail fin, used for a powerful propulsive stroke. The vertebral column, made of simplified disc-like vertebrae, continued into the lobe of the tail fin. Ichthyosaurs were air-breathing, bore live young, and were probably warm-blooded, the first known illustrations of ichthyosaur bones and limb elements were published by the Welshman Edward Lhuyd in his Lithophylacii Brittannici Ichnographia of 1699. Lhuyd thought that they represented fish remains, in 1708, the Swiss naturalist Johann Jakob Scheuchzer described two ichthyosaur vertebrae assuming they belonged to a man drowned in the Universal Deluge. In 1766, a jaw with teeth was found at Weston near Bath. In 1783, this piece was exhibited by the Society for Promoting Natural History as those of a crocodilian, in 1779, ichthyosaur bones were illustrated in John Walcotts Descriptions and Figures of Petrifications. Towards the end of the century, British fossil collections quickly increased in size.
Those of the naturalists Ashton Lever and John Hunter were acquired in their totality by museums, later, it was established that they contained dozens of ichthyosaur bones and teeth. The bones had typically been labelled as belonging to fish, dolphins or crocodiles, the demand by collectors led to more intense commercial digging activities
The yolk is the portion of an egg with the primary function to supply food for the development of the animal embryo. Reproductive systems in which the mothers body supplies the embryo directly are said to be matrotrophic, in many species, such as all birds, and most reptiles and insects, the yolk takes the form of a special storage organ constructed in the reproductive tract of the mother. In many other animals, especially small species such as some fishes and invertebrates, the yolk material is not in a special organ. Yolks, being mainly stored food, tend to be very concentrated, the proteins function partly as food in their own right, and partly in controlling the storage and supply of the other nutrients. For example, in some species the amount of yolk in an egg cell affects the developmental processes that follow fertilization. Yolk is not living cell material like protoplasm, but largely passive material, the food material, plus associated control structures, is supplied by the maternal body during the process called oogenesis.
Apart from animals, other organisms, like algae, specially in the oogamous, in gymnosperms, the remains of the female gametophyte serve as food supply, and in flowering plants, the endosperm. In the avian egg, the yolk usually is some shade of yellow in colour and it is spherical and is suspended in the egg white by one or two spiral bands of tissue called the chalazae. The yolk mass, together with the egg cell or ovum properlly are enclosed by the vitelline membrane, whose structure is different from a cell membrane. The yolk is mostly extracellular to the oolemma, being not accumulated inside the cytoplasm of the egg cell, contrary to the claim that the egg cell. After the fertilization, the cleavage of the leads to the formation of the germinal disc. As food, the egg yolk is a major source of vitamins. It contains all of the fat and cholesterol, and nearly half of the protein. If left intact when an egg is fried, the yellow yolk surrounded by a flat blob of egg white creates a distinctive sunny-side up form, mixing the two components together before cooking results in a pale yellow mass, as in omelettes and scrambled eggs.
The developing embryo inside the egg uses the yolk as sustenance and it is at times separated from the egg white and used in cooking. It is used in painting as a component of traditional egg-tempera and it is used in the production of egg-yolk agar plate medium, useful in testing for the presence of Clostridium perfringens. Egg yolk contains an antibody called antiglobulin, the antibody transfers from the laying hen to the egg yolk by passive immunity to protect both embryo and hatchling from microorganism invasion. Egg yolk can be used to make such as Advocaat or eggnog
The Plesiosauria or plesiosaurs are an order or clade of Mesozoic marine reptiles, belonging to the Sauropterygia. Plesiosaurs first appeared in the latest Triassic Period, possibly in the Rhaetian stage and they had a worldwide oceanic distribution. Plesiosaurs were among the first fossil reptiles discovered, in the beginning of the nineteenth century, scientists realised how distinctive their build was and they were named as a separate order in 1835. The first plesiosaurian genus, the eponymous Plesiosaurus, was named in 1821, since then, more than a hundred valid species have been described. In the early twenty-first century, the number of discoveries has increased, leading to an understanding of their anatomy, relationships. Plesiosaurs had a flat body and a short tail. Their limbs had evolved into four long flippers, which were powered by muscles attached to wide bony plates formed by the shoulder girdle. The flippers made a movement through the water. Plesiosaurs breathed air, and bore live young, there are indications that they were warm-blooded, plesiosaurs showed two main morphological types.
Some species, with the build, had long necks and small heads. Other species, some of them reaching a length of up to seventeen metres, had the pliosauromorph build with a neck and a large head. The two types are related to the strict division of the Plesiosauria into two suborders, the long-necked Plesiosauroidea and the short-neck Pliosauroidea. Modern research, indicates that several long-necked groups might have had some short-necked members or vice versa, the purely descriptive terms plesiosauromorph and pliosauromorph have been introduced, which do not imply a direct relationship. Plesiosauroidea and Pliosauroidea today have a limited meaning. The term plesiosaur is properly used to refer to the Plesiosauria as a whole, but informally it is meant to indicate only the long-necked forms. Skeletal elements of plesiosaurs are among the first fossils of extinct reptiles recognised as such, the Welshman Edward Lhuyd in his Lithophylacii Brittannici Ichnographia from 1699 included depictions of plesiosaur vertebrae that again were considered fish vertebrae or Ichthyospondyli.
In 1719, William Stukeley described a skeleton of a plesiosaur. The stone plate came from a quarry at Fulbeck and had used, with the fossil at its underside
Southern grass skink
The southern grass skink is a species of skink endemic to Australia, where it is found in the south-east of the continent, as well as in Tasmania and the islands of Bass Strait. Although it occurs in a variety of habitats, it is most commonly found in grassy woodlands. Southern grass skinks have a lifespan of about 5 or 6 years and they grow up to 7.5 cm in length. Male skinks change colouration during the breeding season, the specific name, entrecasteauxii, is in honor off French naval officer and explorer Antoine Bruni dEntrecasteaux. The southern grass skink has become a species for reproductive biology in reptiles because it gives birth to live young. Unlike the majority of live bearing reptiles, Pseudemoia develop complex placentae, pregnancy in squamates is supported by the evolution of a novel state of gene regulation. When food is limiting, females will cannibalize their offspring, lipid transport in this species most likely occurs through the yolk sac placenta and is facilitated in part by the production of the protein lipoprotein lipase.
The first observation of a pregnancy in a reptile was found in this species. The extra-uterine embryo did not invade maternal tissue, suggesting differences between the nature and evolution of placentation in southern grass skinks and eutherian mammals
In vertebrates, other than mammals, the passageway from the ovaries to the outside of the body is known as the oviduct. In female mammals this passageway is known as the tube or Fallopian tube. The eggs travel along the oviduct and these eggs will either be fertilized by sperm to become a zygote, or will degenerate in the body. Normally, these are paired structures, but in birds and some fishes, one or the other side fails to develop. Except in teleosts, the oviduct does not directly contact the ovary, the most anterior portion ends in a funnel-shaped structure called the infundibulum, which collects eggs as they are released by the ovary into the body cavity. The only female vertebrates to lack oviducts are the jawless fishes, in these species, the single fused ovary releases eggs directly into the body cavity. The fish eventually extrudes the eggs through small genital pores towards the rear of the body, in amphibians and lungfishes, the oviduct is a simple ciliated tube, lined with mucus-secreting glands that produce the jelly that surrouns the ovum.
In all other vertebrates, there is some degree of specialisation of the tube. In cartilaginous fishes, the portion of the tube develops as a shell gland. The first portion of this secretes the egg white, while the lower portion secretes a hard, horny. Below the shell gland is the ovisac, a region in which eggs are stored prior to laying. In ovoviviparous species, the egg remains within the ovisac until it hatches, some cartilaginous fishes, are truly viviparous, giving birth to live young, and producing no egg shell. In these forms, the ovisac nurtures the developing embryo, often with the aid of vascular outgrowths similar to, but much simpler than, the ovary itself is hollow, with eggs being shed into the central cavity, and thence passing directly into the oviduct. In amniotes – reptiles and mammals – the egg is enclosed with a layer, or amnion. In reptiles and monotremes, the part of the oviduct is a muscular tube. This part of the oviduct is lined with glands that secrete the components of the egg white, the lower portion of the oviduct, or uterus, has a thicker layer of smooth muscle and contains the glands that secrete the egg shell.
In marsupials and placental mammals, the uterus becomes lined by an endometrium, in many placental mammals, the uteri of each side become partially or wholly fused into a single organ, although in marsupials they remain completely separate. In mammals, the portion of the oviduct above the uterus is referred to as the Fallopian tube, for birds, the oviduct is composed of, Infundibulum Magnum Isthmus Shell gland Vaginal homologue Fallopian tube Romer, Alfred Sherwood, Thomas S
Osmotic pressure is a measure of the tendency of water to move into one solution from another by osmosis. The higher the pressure of a solution, the more water tends to move into it. Pressure must be exerted on the side of a selectively permeable membrane to prevent diffusion of water by osmosis from the side containing pure water. Two major types of osmoregulation are osmoconformers and osmoregulators, osmoconformers match their body osmolarity to their environment actively or passively. Most marine invertebrates are osmoconformers, although their ionic composition may be different from that of seawater, osmoregulators tightly regulate their body osmolarity, maintaining constant internal conditions. They are more common in the animal kingdom, osmoregulators actively control salt concentrations despite the salt concentrations in the environment. The gills actively uptake salt from the environment by the use of mitochondria-rich cells, water will diffuse into the fish, so it excretes a very hypotonic urine to expel all the excess water. A marine fish has an internal osmotic concentration lower than that of the seawater, so it tends to lose water.
It actively excretes salt out from the gills, most fish are stenohaline, which means they are restricted to either salt or fresh water and cannot survive in water with a different salt concentration than they are adapted to. However, some show a tremendous ability to effectively osmoregulate across a broad range of salinities, fish with this ability are known as euryhaline species. Flounder have been observed to inhabit two utterly disparate environments — marine and fresh water — and it is inherent to adapt to both by bringing in behavioral and physiological modifications. Some marine fish, like sharks, have adopted a different, efficient mechanism to conserve water and they retain urea in their blood in relatively higher concentration. Urea damages living tissues so, to cope with this problem and this provides a better solution to ureas toxicity. Sharks, having slightly higher concentration, do not drink water like fresh water fish. Strong winds, low humidity and high temperatures all increase evapotranspiration from leaves, abscisic acid is an important hormone in helping plants to conserve water — it causes stomata to close and stimulates root growth so that more water can be absorbed.
Plants share with animals the problems of obtaining water but, unlike in animals, certain plants have evolved methods of water conservation. Xerophytes are plants that can survive in dry habitats, such as deserts, succulent plants such as the cacti store water in the vacuoles of large parenchyma tissues. Other plants have modifications to reduce water loss, such as needle-shaped leaves, sunken stomata
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water and excretes carbon dioxide. The gills of species, such as hermit crabs, have adapted to allow respiration on land provided they are kept moist. The microscopic structure of a gill presents a surface area to the external environment. Many microscopic aquatic animals, and some larger but inactive ones, can absorb oxygen through the entire surface of their bodies. However, more complex or more active aquatic organisms usually require a gill or gills, Gills usually consist of thin filaments of tissue, branches, or slender, tufted processes that have a highly folded surface to increase surface area. A high surface area is crucial to the gas exchange of aquatic organisms, a cubic meter of air contains about 250 grams of oxygen at STP. The concentration of oxygen in water is lower than in air, in fresh water, the dissolved oxygen content is approximately 8 cm3/L compared to that of air which is 210 cm3/L.
Water is 777 times more dense than air and is 100 times more viscous, oxygen has a diffusion rate in air 10,000 times greater than in water. The use of sac-like lungs to remove oxygen from water would not be efficient enough to sustain life, rather than using lungs, aseous exchange takes place across the surface of highly vascularised gills over which a one-way current of water is kept flowing by a specialised pumping mechanism. The density of the water prevents the gills from collapsing and lying on top of each other, with the exception of some aquatic insects, the filaments and lamellae contain blood or coelomic fluid, from which gases are exchanged through the thin walls. The blood carries oxygen to other parts of the body, carbon dioxide passes from the blood through the thin gill tissue into the water. Gills or gill-like organs, located in different parts of the body, are found in groups of aquatic animals, including mollusks, insects, fish. The gills of vertebrates typically develop in the walls of the pharynx, most species employ a countercurrent exchange system to enhance the diffusion of substances in and out of the gill, with blood and water flowing in opposite directions to each other.
The gills are composed of filaments, the gill lamellae. When a fish breathes, it draws in a mouthful of water at regular intervals, it draws the sides of its throat together, forcing the water through the gill openings, so it passes over the gills to the outside. Fish gill slits may be the ancestors of the tonsils, thymus glands. Adjacent slits are separated by a gill arch from which projects a cartilaginous gill ray. This gill ray is the support for the sheet-like interbranchial septum, the base of the arch may support gill rakers, projections into the pharyngeal cavity that help to prevent large pieces of debris from damaging the delicate gills