The circulatory system in chordates
Blood vessels and nerves are branched structures that travel together to supply almost every tissue in the body. Blood vessels are composed of endothelial cells and sometimes pericytes or smooth-muscle cells; nerves consist of nerve axons and supporting Schwann cells. In terms of function, blood flow is unidirectional, arterial to venous, just as information passes along incoming (sensory) and outgoing (motor and autonomic) nerve pathways. Also, blood vessels and nerves are physically interrelated, e.g., the vasa nervorum and the perivascular sympathetic plexus controlling vascular tone. It is not surprising, therefore, that investigators have wondered whether these two systems are established together or independently
The circulatory system in chordates has a characteristic pattern. In tunicates and vertebrates, the blood is propelled by a distinct heart; in cephalochordates, by contraction of the blood vessels. Unoxygenated blood is driven forward via a vessel called the ventral aorta. It then passes through a series of bronchial arteries in the gills, where gas exchange takes place, and the oxygenated blood flows to the body, much of it returning to its origin via a dorsal aorta. The blood of vertebrates passes through the tissues via tiny vessels called capillaries. In tunicates and cephalochordates, capillaries are absent, and the blood passes through spaces in the muscles instead.
Fish gills are organs that allow fish to breathe underwater. Most fish exchange gases like oxygen and carbon dioxide using gills that are protected under gill covers on both sides of the pharynx (throat). Gills are tissues that are like short threads, protein structures called filaments. These filaments have many functions including the transfer of ions and water, as well as the exchange of oxygen, carbon dioxide, acids and ammonia. Each filament contains a capillary network that provides a large surface area for exchanging oxygen and carbon dioxide.
Fish exchange gases by pulling oxygen-rich water through their mouths and pumping it over their gills. In some fish, capillary blood flows in the opposite direction to the water, causing counter-current exchange. The gills push the oxygen-poor water out through openings in the sides of the pharynx. Some fish, like sharks and lampreys, possess multiple gill openings. However, bony fish have a single gill opening on each side. This opening is hidden beneath a protective skinny cover called the operculum.
This is an egg mass laid by a frog. Each of those black dots is made up of a bunch of cells that will eventually grow into a tadpole. Depending on the species, these egg masses can be made up of several hundred eggs. Within a few days, the eggs develop into tadpoles. Tadpoles live entirely underwater. After a few weeks, a hormone in the tadpole’s thyroid gland initiates metamorphosis. Over about a 24 hour period, the tadpole develops into a frog. This means almost every organ has to change so the tadpole can go from living underwater to living on land as an adult frog. In this picture, you can see the tadpole has started to grow hind legs. After the hind legs have started to form, a pair of front legs will begin to develop, and the tail will begin to disappear. You might also notice that the tadpole had already begun to form a frog-like face. The tadpole’s skull is made out of cartilage (the same stuff your nose and ears are made out of), but during metamorphosis, the cartilage is replaced with bone. When the tadpole reaches the froglet stage, it is almost a full adult. At this point, the tadpole’s gills have disappeared, and its lungs have enlarged. This means it is ready to leave the water and live on land. Once its tail goes, it will become an adult frog.
The form of the lungs and the methods of irrigating them may also influence activity by affecting the efficiency of gas exchange. In snakes, the lungs are simple saclike structures having small pockets, or alveoli, in the walls. In the lungs of all crocodiles and many lizards and turtles, the surface area is increased by the development of partitions that, in turn, have alveoli. Because the exchange of respiratory gases takes place across surfaces, an increase of the ratio of surface area to volume leads to a rise in respiratory efficiency. In this regard, the lungs of snakes are not as capable as the lungs of crocodiles. The elaboration of the internal surface of lungs in reptiles is simple, however, compared with that reached by mammalian lungs, with their enormous number of very fine alveoli.
Most reptiles breathe by changing the volume of the body cavity. By contractions of the muscles moving the ribs, the size of the body cavity is increased, creating a negative pressure, which is restored to atmospheric level by air rushing into the lungs. By contraction of body muscles, the volume of the body cavity is reduced, forcing air out of the lungs.
This system applies to all modern reptiles except turtles, which, because of the fusion of the ribs with a rigid shell, are unable to breathe by this means; they do use the same mechanical principle of changing pressure in the body cavity, however. Contraction of two flank muscles enlarges the body cavity, causing inspiration. Contraction of two other muscles, coincident with the relaxation of the first two, forces the viscera upward against the lungs, causing exhalation.
The rate of respiration, like so many physiological activities of reptiles, is highly variable, depending in part upon the temperature of the environment and in part upon the emotional state of the animal.
2
The eggs of amniotes contain three additional extra-embryonic membranes: the chorion, amnion, and allantois. Extra-embryonic membranes are those present in amniotic eggs that are not a part of the body of the developing embryo. While the inner amniotic membrane surrounds the embryo itself, the chorion surrounds the embryo and yolk sac. The chorion facilitates the exchange of oxygen and carbon dioxide between the embryo and the egg’s external environment. The amnion protects the embryo from mechanical shock and supports hydration. The allantois stores nitrogenous wastes produced by the embryo and also facilitates respiration.
After the ovulation, the York moves through specialized regions of the oviduct while collecting specific components of the egg and the albumen are also deposited. The albumen and the yolk then travel through a specialized segment of the oviduct called the white isthmus. From here there is the secretion of eggshell membrane precursors. This results in a meshwork of fires which are organized into morphologically distinct inner and outer sheets which encloses the egg albumen. The membrane fire contains collagen. The inner membranes remain unclassified, while the fibres of the outer shell membrane become mineralized hence become incorporated into the base of the eggshell.
One of the features of all mammals is the presence of interlocking teeth which results in improved chewing, the upper and lower teeth in the jaw work together to cut or crush and grind food but, the interlocking cusps of upper and lower teeth cannot be maintained if the teeth are continuously lost and replaced. This led to the evolution of just two sets of teeth one set replacing the other in the juvenile. This altered design means the early egg-laying mammals must have evolved milk glands and lactation. Once the milk is available, the young can be born with few or no teeth that can later appear when the jaw is more massive and closer to the adult size. Without milk, a newborn mammal would need a full set of teeth to eat and survive.
Throughout the Permian period, the synapsids included the dominant carnivores and several vital herbivores. In the subsequent Triassic period, however, a previously obscure group of sauropsids, the archosaurs, became the dominant vertebrates. The mammalian forms appeared during this period; their superior sense of smell backed up by a large brain, facilitated entry into nocturnal niches with less exposure to archosaur predation. The nocturnal lifestyle may have contributed significantly to the development of mammalian traits such as endothermy and hair. Later in the Mesozoic, after theropod dinosaurs replaced saurischians as the dominant carnivores, mammals spread into other ecological niches. For example, some became aquatic, some were gliders, and some even fed on juvenile dinosaurs.
Most of the evidence consists of fossils. For many years, fossils of Mesozoic mammals and their immediate ancestors were scarce and fragmentary; but, since the mid-1990s, there have been many remarkable new finds, especially in China. The relatively new techniques of molecular phylogenetics have also shed light on some aspects of mammalian evolution by estimating the timing of essential divergence points for new species. When used carefully, these techniques often, but not always, agree with the fossil record. Although mammary glands are a signature feature of modern mammals, little is known about the evolution of lactation as these soft tissues are not often preserved in the fossil record. Most research concerning the development of mammals centres on the shapes of the teeth, the hardest parts of the tetrapod body.
3
The various modern and basics of flight mechanisms in birds include,lift, gliding, flapping and drag.
Lift. Lift force is produced by the action of airflow on the wing, which is an airfoil. The lift force occurs because the air has a lower pressure just above the wing and higher pressure below.
Gliding. When gliding, birds obtain both, a vertical and a propulsive force from their wings. This is possible because the lift force is generated at right angles to the airflow. The lift force, therefore, has a forward component that counteracts drag.
Drag. Apart from its weight, there are three major drag forces that impede a bird’s aerial flight: frictional drag (caused by the friction of air and body surfaces), form drag (due to frontal area of the bird, also known as pressure drag), and lift-induced drag (caused by the wingtip vortices). These forces are reduced by streamlining the bird’s body and wings.
Flapping. When a bird flaps, its wings continue to develop lift, but the lift is rotated forward so providing thrust, which counteracts drag and increases its speed.
Internal thermoregulation contributes to the birds’ ability to maintain homeostasis within a specific range of temperatures. As internal body temperature rises, physiological processes are affected, such as enzyme activity. Although enzyme activity initially increases with temperature, enzymes begin to denature and lose their function at higher temperatures. Endosperms regulate their own body temperature through internal metabolic processes and usually maintain a narrow range of internal temperatures. Heat is generally generated from the animal’s healthy metabolism, but under conditions of excessive cold or low activity, an endotherm generates additional heat by shivering.
Most of the birds’ organs and even large muscles are always located near the centre of gravity that is to say, slightly behind the wings to provide better balance during flights. Birds eat also helps them stay light for flight. Most consume energy-packed foods rich in calories – like seeds, fruits, and meat. These are foods that add as little as possible to a bird’s payload and deliver a lot of bang for the buck. And what birds eat is processed rapidly, so they aren’t weighed down by waste. Feathers provide insulation, waterproofing, and a means to fly, and they’re extraordinarily lightweight. Other features also help minimize the pull of gravity, mostly by reducing body weight.
4
One of the smallest marine mammals is the sea otter. The sea otter is the largest member of the family Mustelidae.the average length of an adult male otter is approximately 1.4m and weighs between 23 to 45 kg. The female adult sea otter has an average length of 1.2m and weighs about 20kg. The sea otters have a highly buoyant, elongated body, blunt snout and small, wide head.
The sea otters have a number of adaptations that assist them to survive their marine environment, which is always challenging. The sea otters have long whiskers which assist them in the detection of vibrations in murky waters and sensitive forepaws, with retractable claws, help them to groom, locate and capture prey underwater, and use tools. The sea otters have the hind feet which are webbed and flipped like and are used in combination with its lower body which assists it to propel in the water. The sea otters have a long flattened tail which they use as a rudder for further propulsion. They have blunt teeth, unlike other marine mammals which they use for crushing food. Apart from the pads of its paw and the nose, the sea otters have their bodies covered with a dense two-layer fur with the top layer of long waterproof guard hairs helps to keep the underfur layer dry by keeping cold water away from the skin.
The strong forelegs paws are used to locate and capture prey. Pockets of loose skin under each foreleg are used to store prey it has gathered on the seafloor for the ascent to the surface. Rocks are often used as tools to dislodge prey on the seafloor and to break open the hard outer shells of some prey items upon returning to the surface. Floating belly-up in the water, they place rocks on their chests and repeatedly pound hard-shelled prey against them to gain access to the meat inside. While eating, an otter will repeatedly roll in the water to wash away food scraps from its chest. Unlike most other marine mammals, sea otters commonly drink seawater. Although most of the animal’s water needs are met through the consumption of prey, its large kidneys allow it to extract freshwater from seawater.
The sea otter uses their feet in the reduction and maximization of heat loss when the water temperature turns too hot or too cold. When the temperatures of the water become too cold, the sea otters reduces heat loss by floating on their backs with their feet out of the water and vice versa to lose heat. To preserve body heat, sea otters tend to spread out or fold up their feet. The sea otters also have developed an adaptation of increasing or decreasing their buoyancy in response to fluctuation in water temperatures.
When it comes to challenges, the sea otters face many physiological and energetic challenges, which includes heat loss, foraging demands, high metabolic rates and surface swimming behaviours which they have to overcome for survival. These challenges are widely affecting the younger sea otters. The young sea otters always experience higher metabolic rates; they have very limited foraging and experience more top thermal energetic challenges which are rare in adults.
Work cited
Wilson, Christopher M., et al. “Phylogeny and effects of anoxia on hyperpolarization-activated cyclic nucleotide-gated channel gene expression in the heart of a primitive chordate, the Pacific hagfish (Eptatretus stout).” Journal of Experimental Biology 216.23 (2013): 4462-4472.
Mallatt, Jon, and Nicholas Holland. “Pikaia graceless Walcott: stem chordate, or already specialized in the Cambrian?.” Journal of Experimental Zoology Part B: Molecular and Developmental Evolution 320.4 (2013): 247-271.
Salazar, Aubrey. Advanced Chordate Zoology. Scientific e-Resources, 2018.
Holzer, Guillaume, and Vincent Laudet. “Thyroid hormones and postembryonic development in amniotes.” Current topics in developmental biology. Vol. 103. Academic Press, 2013. 397-425.
Manley, Geoffrey A. “The mammalian Cretaceous cochlear revolution.” Hearing Research 352 (2017): 23-29.