Animals comprise an expansive group of living organisms. From single-celled life forms to multicellular creatures with intricate body systems, animal life has changed over time.
Animal science encompasses several branches. These include zoology, physiology, ecology and evolutionary biology.
Definition
An animal is any multicellular, eukaryotic organism in the kingdom Animalia (named after the Greek words for “breath” and “spirit,” anima and animus). These organisms are heterotrophs, feeding on organic matter such as plants, other animals or decaying plant materials. Animals possess a digestive tract, immune system, nervous system and specialized sensory organs for recognizing stimuli and responding accordingly.
These organisms have evolved independently from bacteria and protists, which are single-celled organisms without internal membranes. While some unicellular eukaryotes such as fungi can classified as animals, they lack the same complex body systems and organs found in mammals.
Since the late 1800s, biological classification of animals based on genetic information has been use to establish their phylogeny (evolutionary relationships). Additionally, this classification helps distinguish traits such as size, weight, shape, and colour.
In the 18th century, Carl Linnaeus coined the term “animal,” naming it after two Greek words for “breath” or “spirit.” His Systema Naturae (1758) divided animalkind into 3 phyla based on their ability to breathe: Metazoa, Fungi and Protozoa.
Modern biology classifies animals into the kingdom Animalia, which contains over 1.5 million species. Birds, fish, reptiles and mammals are some of the most widespread creatures within this realm.
During embryonic development, most animals formed from a large, hollow sphere of cells known as a blastula and grow to adulthood. They possess sexual reproduction ability and several physical characteristics such as muscle mass that allow them to move around and perform various tasks.
They possess various body systems, such as the respiratory system, digestive system, vascular system and immune system. Each of these is composed of tissues that perform specific metabolic tasks – epithelial tissues, connective tissues, muscular tissues and nervous tissues being some examples.
Plants and fungi, which are not classify as animals, possess a nucleus that contains DNA. This DNA can copy into new cells to create more animals; hence they are known as eukaryotes–cell structures composed of multiple cells bound together by membranes.
Life Cycles
An animal life cycle is the sequence of stages a living creature goes through from birth to death. There are various types of animals, and their lifespans can differ drastically; some insects live only a few weeks or days, while other creatures can live for centuries.
When it comes to animals, there are four primary stages: birth, growth, reproduction and death. For most mammals and birds, the first stage is birth when a mother lays an egg in her uterus.
Other creatures’ reproduction cycle is more intricate, involving multiple sperm cells and eggs from different individuals that fertilize each other to create new offspring. In these animals, reproduction also includes the development and growth of the offspring.
Metamorphosis, also known as growth stage transformations in animals, occurs during this period. Changes to body structure such as size and shape of the animal or skin color may take place during metamorphosis.
Other changes involve the type of cells in an organism’s body. For instance, butterfly wings develop from an eggless egg to fully developed wings during metamorphosis.
Though these examples differ greatly, they serve as a clear illustration of the difficulty it can be to distinguish various stages in an animal’s life cycle. Animal morphologies or physical forms evolve over time and the ease with which one can identify distinct phases has an important impact on how diverse these processes appear.
It is beneficial to think more deeply about animal life cycles. They form the basis for most evolutionary research, so it’s essential to gain a better understanding of them.
Metazoan life cycles are highly intricate. Not only do they often feature regular alternations between sexual and asexual generations (metagenesis), but some species even exhibit polyphenism or parthenogenesis – a reproductive event that separates distinct generations – such as polyphenism in some mosses, ferns and flowering plants.
Animal Body System
An animal’s body is an intricate structure that serves a variety of functions. These include mechanical and chemical breakdown of food to provide energy, filtration of blood and waste, immune defense against external threats, as well as production of reproductive cells.
An organ is a special cell that plays an essential role in the body of an animal. It collaborates with other organs to perform vital functions that keep an animal alive.
For instance, the stomach must collaborate with the mouth and oesophagus (gullet) to absorb food, while then working with the intestines to break it down for absorption by blood and lymph systems. Likewise, the respiratory system works in concert with lungs and circulatory system to guarantee all cells within the body receive enough oxygen.
Some animals possess simple organs that perform multiple functions, such as flatworms with two cerebral ganglia–nerve clusters in the head that function as the brain–connected to nerve cords throughout their bodies. On the other hand, some invertebrates lack any internal organs at all and rely solely on their bodies’ internal skeletons for support.
Higher vertebrates typically have a skeletal system composed of bones and cartilages, while invertebrates have external coverings such as chitin that hold their bodies together. Some land-dwelling vertebrates like mammals and amphibians possess kidneys which convert waste materials from digestion into urea before mixing it with water to excreted as urine.
The circulatory system is the system responsible for pumping blood around an animal’s body. Fish have the most basic circulatory systems among vertebrates: blood flows unidirectionally from two-chambered hearts through their gills and then to other parts of their anatomy. Amphibians, reptiles, and birds possess more sophisticated systems which mix oxygenated with deoxygenated blood before circulated throughout their bodies.
The nervous system is an essential organ of every animal’s body, enabling them to send and receive messages between their brain and other parts of the body. Additionally, the nervous system allows an animal to control its muscles and movements by sending nerve impulses to specific muscles which contract or relax according to preprogramed commands.
Animal Tissues
An animal’s body is composed of different tissues. These organs serve specific functions and help ensure the overall wellbeing and well-being of an animal.
Tissues are composed of cells with similar structure and function as one cohesive unit. An intercellular matrix fills the spaces between these cells in tissues, though its amount may vary between tissues.
Epithelial tissue is a type of connective tissue that covers surfaces in contact with the outside world, lines internal cavities and forms glands. It lacks blood vessels and relies solely on connective tissue for nutrition.
Connective tissue is a type of connective tissue that holds cells and organs together, especially fat, bone, blood, cartilage, and adipose tissues in an animal’s body. It serves to protect the body from mechanical shocks by helping organs withstand stretching or tearing forces.
Muscle tissue is a special type of connective tissue that contracts to move an area of the body. It consists of thin, elongated cells known as muscle fibers that contract when activated and relax once the trigger has applied.
Nervous tissue is a type of organ that processes information from throughout the body. It consists of highly specialized cells (neurons and glia) which receive signals from other cells, process them, and send it on to other cells or muscles as necessary.
Nerve cells are long, string-like structures that transmit impulses between the brain, spinal cord and other parts of the nervous system.
Endocrine cells reside in nervous tissue, which release chemicals into the bloodstream when they detect changes in environment. These hormones are essential to keeping our bodies running optimally.
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