Common seadragon or weedy seadragon is a marine fish related to the seahorse. Males have narrower bodies and are darker than females, seadragons have a long dorsal fin along the back and small pectoral fins on either side of the neck, which provide balance. Common seadragons can reach 45 cm in length, the common seadragon is the marine emblem of the Australian State of Victoria. The common seadragon is endemic to Australian waters of the Eastern Indian Ocean and it can be found approximately between Port Stephens and Geraldton, Western Australia, as well as Tasmania. The common seadragon inhabits coastal waters down to at least 50 m deep and it is associated with rocky reefs, seaweed beds, seagrass meadows and structures colonised by seaweed. These fish are slow-moving and rely on their camouflage as protection against predation, they drift in the water and they lack a prehensile tail that enables similar species to clasp and anchor themselves. Individuals are observed either on their own or in pairs, feeding on tiny crustaceans, like seahorses, seadragon males are the sex that cares for the developing eggs.
Females lay around 120 eggs onto the brood patch located on the underside of the males tail, the eggs are fertilised and carried by the male for around a month before the hatchlings emerge. Seadragons and pipefish are among the few known species where the male carries the eggs, the young are independent at birth, beginning to eat shortly after. Common seadragons take about 28 months to reach maturity. Mating in captivity is rare since researchers have yet to understand what biological or environmental factors trigger them to reproduce, in captivity the survival rate for common seadragons is about 60%. In March 2012 the Georgia Aquarium in Atlanta, USA, announced a successful breeding event of common seadragons, as of July 2012, the Monterey Bay Aquarium has successfully bred and hatched out baby common seadragons on exhibit. The common sea dragon is classified as Near Threatened on the IUCN Red List 2006, while the common sea dragon is a desired species in the international aquarium trade, the volume of wild-caught individuals is small and therefore not currently a major threat.
Instead, habitat loss and degradation due to activities and pollution threaten common sea dragons most. The loss of seagrass beds, coupled with natural history traits that make them poor dispersers. This species is not at present a victim of bycatch or a target of trade in Traditional Chinese Medicine and it is illegal to take or export these species in most of the states within which they occur. Captive breeding programs are in place for the sea dragon. The common seadragon is in the subfamily Syngnathinae, which contains all pipefish and it is most closely related to the other member of its genus, the ruby seadragon, and the leafy seadragon
Mammals are any vertebrates within the class Mammalia, a clade of endothermic amniotes distinguished from reptiles by the possession of a neocortex, three middle ear bones and mammary glands. All female mammals nurse their young with milk, secreted from the mammary glands, Mammals include the largest animals on the planet, the great whales. The basic body type is a quadruped, but some mammals are adapted for life at sea, in the air, in trees. The largest group of mammals, the placentals, have a placenta, Mammals range in size from the 30–40 mm bumblebee bat to the 30-meter blue whale. With the exception of the five species of monotreme, all modern mammals give birth to live young, most mammals, including the six most species-rich orders, belong to the placental group. The largest orders are the rodents and Soricomorpha, the next three biggest orders, depending on the biological classification scheme used, are the Primates, the Cetartiodactyla, and the Carnivora. Living mammals are divided into the Yinotheria and Theriiformes There are around 5450 species of mammal, in some classifications, extant mammals are divided into two subclasses, the Prototheria, that is, the order Monotremata, and the Theria, or the infraclasses Metatheria and Eutheria.
The marsupials constitute the group of the Metatheria, and include all living metatherians as well as many extinct ones. Much of the changes reflect the advances of cladistic analysis and molecular genetics, findings from molecular genetics, for example, have prompted adopting new groups, such as the Afrotheria, and abandoning traditional groups, such as the Insectivora. The mammals represent the only living Synapsida, which together with the Sauropsida form the Amniota clade, the early synapsid mammalian ancestors were sphenacodont pelycosaurs, a group that produced the non-mammalian Dimetrodon. At the end of the Carboniferous period, this group diverged from the line that led to todays reptiles. Some mammals are intelligent, with some possessing large brains, self-awareness, Mammals can communicate and vocalize in several different ways, including the production of ultrasound, scent-marking, alarm signals and echolocation. Mammals can organize themselves into fission-fusion societies and hierarchies, most mammals are polygynous, but some can be monogamous or polyandrous.
They provided, and continue to provide, power for transport and agriculture, as well as commodities such as meat, dairy products, wool. Mammals are hunted or raced for sport, and are used as model organisms in science, Mammals have been depicted in art since Palaeolithic times, and appear in literature, film and religion. Defaunation of mammals is primarily driven by anthropogenic factors, such as poaching and habitat destruction, Mammal classification has been through several iterations since Carl Linnaeus initially defined the class. No classification system is accepted, McKenna & Bell and Wilson & Reader provide useful recent compendiums. Though field work gradually made Simpsons classification outdated, it remains the closest thing to a classification of mammals
Scorpions are predatory arachnids of the order Scorpiones. Scorpions range in size from 9 mm /0.3 in. to 23 cm /9 in, the evolutionary history of scorpions goes back to the Silurian era 430 million years ago. They have adapted to a range of environmental conditions and can now be found on all continents except Antarctica. Scorpions number about 1750 described species, with 13 extant families recognised to date, only about 25 of these species are known to have venom capable of killing a human being. The taxonomy has undergone changes and is likely to change further, Scorpion stings are painful but are usually harmless to humans. For stings from species found in North America, no treatment is normally needed for healthy adults, although care should be sought for children. More harmful stings from species found in South America, Scorpions are found on all major land masses except Antarctica. The greatest diversity of scorpions in the Northern Hemisphere is to be found in the areas lying between latitudes 23° N and 38° N.
Above these latitudes, the diversity decreases, with the northernmost natural occurrence of scorpions being the northern scorpion Paruroctonus boreus at Medicine Hat, five colonies of scorpions have established themselves in Sheerness on the Isle of Sheppey in the United Kingdom. This small population has been resident since the 1860s, having arrived with imported fruit from Africa. This scorpion species is small and completely harmless to humans, at just over 51° N, this marks the northernmost limit where scorpions live in the wild. There are thirteen families and about 1,750 described species and subspecies of scorpions, in addition, there are 111 described taxa of extinct scorpions. This classification is based on that of Soleglad & Fet, which replaced the older, additional taxonomic changes are from papers by Soleglad et al. The following classification covers extant taxa to the rank of family, L. Koch,1837 Family Buthidae C. L. The oldest known scorpions lived around 430 million years ago in the Silurian period and these first scorpions were believed to have had gills instead of the present forms book lungs though this has subsequently been refuted.
The oldest Gondwanan scorpiones comprise the earliest known terrestrial animals from Gondwana, currently,111 fossil species of scorpion are known. Unusually for arachnids, there are species of Palaeozoic scorpion than Mesozoic or Cenozoic ones. The eurypterids, marine creatures that lived during the Palaeozoic era, share several traits with scorpions
Viviparity literally means giving live birth, from the Latin vivus and parere. The term is in use both in zoology and in botany as a term for certain classes of modes of reproduction, the adjective viviparous is derived from viviparity and describes both the various modes of reproduction and the organisms that reproduce in any such mode. In botany vivipary is an alternative form of the term. In botany too, the usage of the term is largely a matter of convenience and preference, five modes of reproduction have been differentiated in animals based on relations between zygote and parents. The five include two nonviviparous modes, with external fertilisation, and oviparity, with internal fertilisation, in the latter, the female lays zygotes as eggs with a large yolk, this occurs in all birds, most reptiles, and some fishes. Hemotrophic viviparity, nutrients are provided by the female, often some form of placenta. In the frog Gastrotheca ovifera, embryos are fed by the mother through specialized gills, the skink Pseudemoia entrecasteauxii and most mammals exhibit a hemotrophic viviparity.
Placental viviparity is arguably the most highly developed form of vivipary, Placental mammals, including humans, are the best-known example, but adaptations in some other animals have incorporated this principle or close analogies. Other examples include species of scorpions and cockroaches, certain genera of sharks and snakes. Ovoviviparity, a less developed form of animal vivipary, occurs in most vipers for instance. However, there are arguments in favour of abandoning the term as obsolete, in at least one species of skink in the large genus Trachylepis, placental transport accounts for nearly all of the provisioning of nutrients to the embryos before birth. In the uterus, the eggs are small, about 1mm in diameter, with very little yolk. The shell membrane is vestigial and transient, its disintegration permits the absorption of nutrients from uterine secretions, the embryo produces invasive chorionic tissues that grow between the cells of the uterine lining till they can absorb nutrients from maternal blood vessels.
As it penetrates the lining, the embryonic tissue grows aggressively till it forms sheets of tissue beneath the uterine epithelium and they eventually strip it away and replace it, making direct contact with maternal capillaries. In several respects, the phenomenon is of importance in theoretical zoology. The authors remark that such an endotheliochorial placenta is fundamentally different from that of any known viviparous reptile, there is no relationship between sex-determining mechanisms and whether a species bears live young or lays eggs. Temperature-dependent sex determination, which function in an aquatic environment, is seen only in terrestrial viviparous reptiles. Viviparous plants produce seeds or embryos that begin to develop before they detach from the parent, some Crassulaceae, such as Bryophyllum and drop plantlets from notches in their leaves, ready to grow
A fish is any member of a group of animals that consist of all gill-bearing aquatic craniate animals that lack limbs with digits. They form a group to the tunicates, together forming the olfactores. Included in this definition are the living hagfish and cartilaginous, tetrapods emerged within lobe-finned fishes, so cladistically they are fish as well. However, traditionally fish are rendered obsolete or paraphyletic by excluding the tetrapods, because in this manner the term fish is defined negatively as a paraphyletic group, it is not considered a formal taxonomic grouping in systematic biology. The traditional term pisces is considered a typological, but not a phylogenetic classification, the earliest organisms that can be classified as fish were soft-bodied chordates that first appeared during the Cambrian period. Although they lacked a true spine, they possessed notochords which allowed them to be more agile than their invertebrate counterparts, fish would continue to evolve through the Paleozoic era, diversifying into a wide variety of forms.
Many fish of the Paleozoic developed external armor that protected them from predators, the first fish with jaws appeared in the Silurian period, after which many became formidable marine predators rather than just the prey of arthropods. Fish are abundant in most bodies of water and they can be found in nearly all aquatic environments, from high mountain streams to the abyssal and even hadal depths of the deepest oceans. With 33,100 described species, fish exhibit greater species diversity than any group of vertebrates. Fish are an important resource for humans worldwide, especially as food and subsistence fishers hunt fish in wild fisheries or farm them in ponds or in cages in the ocean. They are caught by fishers, kept as pets, raised by fishkeepers. Fish have had a role in culture through the ages, serving as deities, religious symbols, fish do not represent a monophyletic group, and therefore the evolution of fish is not studied as a single event. Early fish from the record are represented by a group of small, jawless.
Jawless fish lineages are mostly extinct, an extant clade, the lampreys may approximate ancient pre-jawed fish. The first jaws are found in Placodermi fossils, the diversity of jawed vertebrates may indicate the evolutionary advantage of a jawed mouth. It is unclear if the advantage of a hinged jaw is greater biting force, improved respiration, fish may have evolved from a creature similar to a coral-like sea squirt, whose larvae resemble primitive fish in important ways. The first ancestors of fish may have kept the form into adulthood. Fish are a group, that is, any clade containing all fish contains the tetrapods
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
Menstruation, known as a period or monthly, is the regular discharge of blood and mucosal tissue from the inner lining of the uterus through the vagina. The first period usually begins between twelve and fifteen years of age, a point in time known as menarche, periods may occasionally start as young as eight years old and still be considered normal. The average age of the first period is in the developing world. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, bleeding usually lasts around 2 to 7 days. Menstruation stops occurring after menopause, which occurs between 45 and 55 years of age. Periods stop during pregnancy and typically do not resume during the months of breastfeeding. Up to 80% of women report having some symptoms prior to menstruation, common signs and symptoms include acne, tender breasts, feeling tired and mood changes. These may interfere with life, therefore qualifying as premenstrual syndrome. In 3 to 8%, symptoms are severe, a lack of periods, known as amenorrhea, is when periods do not occur by age 15 or have not occurred in 90 days.
Other problems with the cycle include painful periods and abnormal bleeding such as bleeding between periods or heavy bleeding. Menstruation in other animals occurs in primates, such as apes and monkeys, as well as bats, the menstrual cycle occurs due to the rise and fall of hormones. This cycle results in the thickening of the lining of the uterus, the egg is released from an ovary around day fourteen in the cycle, the thickened lining of the uterus provides nutrients to an embryo after implantation. If pregnancy does not occur, the lining is released in what is known as menstruation, the first menstrual period occurs after the onset of pubertal growth, and is called menarche. The average age of menarche is 12 to 15, however, it may start as early as eight. The average age of the first period is in the developing world. The average age of menarche has changed little in the United States since the 1950s, Menstruation is the most visible phase of the menstrual cycle and its beginning is used as the marker between cycles.
The first day of bleeding is the date used for the last menstrual period. The typical length of time between the first day of one period and the first day of the next is 21 to 45 days in young women, the medical definition of menopause is one year without a period and typically occurs between 45 and 55 in Western countries
Pregnancy in fish
Pregnancy has been traditionally defined as the period during which developing embryos are incubated in the body after egg-sperm union. There are two types of viviparity in fish, in histotrophic viviparity, the zygotes develop in the females oviducts, but she provides no direct nutrition, the embryos survive by eating her eggs or their unborn siblings. In hemotrophic viviparity, the zygotes are retained within the female and are provided with nutrients by her, in seahorses and pipefish, it is the male that becomes pregnant. Pregnancy has been defined as the period during which developing embryos are incubated in the body after egg-sperm union. Although the term often refers to eutherian animals, it has been used in the titles of many international, peer-reviewed, scientific articles on fish. Five modes of reproduction can be differentiated in fish based on relations between the zygote and parents, Fertilization of eggs is external, zygotes develop externally, Fertilization of eggs is internal, zygotes develop externally as eggs with large vitellus.
Ovoviviparity, Fertilization is internal, zygotes are retained in the female, the embryos depend upon their yolk for survival. There are two types of viviparity among fish, histotrophic viviparity, The zygotes develop in the females oviducts, but she provides no direct nutrition. The embryos survive by eating her eggs or their unborn siblings, hemotrophic viviparity, The zygotes are retained within the female and are provided with nutrients by her, often through some form of placenta. Some species of rockfish and sculpins produce rather weak larvae with no egg membrane and are also, by definition, ovoviviparity occurs in most live-bearing bony fishes. Viviparous fish include the families Goodeidae, Jenynsiidae, Embiotocidae, the Bythitidae are viviparous although one species, Dinematichthys ilucoeteoides, is reported to be ovoviviparous. Aquarists commonly refer to ovoviviparous and viviparous fish as livebearers, examples include guppies, moonfish, four-eyed fish and swordtails. All of these varieties exhibit signs of their pregnancy before the live fry are born, other terms relating to pregnancy in fish relate to the differences in the mode and extent of support the female gives the developing offspring.
Lecithotrophy occurs when the provisions the oocyte with all the resources it needs prior to fertilization. Many members of the fish family Poeciliidae are considered to be lecithotrophic, aplacentral viviparity occurs when the female retains the embryos during the entire time of development but without any transfer of nutrients to the young. The yolk sac is the source of nutrients for the developing embryo. There are at least two exceptions to this, some sharks gain nourishment by eating unfertilized eggs produced by the mother or by eating their unborn siblings, matrotrophy occurs when the embryo exhausts its yolk supply early in gestation and the mother provides additional nutrition. Viviparous fish have developed ways of providing their offspring with nutrition
Gestational age is a measure of the age of a pregnancy where the origin from womans last normal menstrual period, or the corresponding age as estimated by other methods. Such methods include adding 14 days to a known duration since fertilization, the initiation of pregnancy for the calculation of gestational age can be different from definitions of initiation of pregnancy in context of the abortion debate or beginning of human personhood. In case of in vitro fertilization, calculating days since oocyte retrieval or co-incubation, gestational age can be estimated by calculating days from ovulation if it was estimated from related signs or ovulation tests, and adding 14 days by convention. Following are diagrams for estimating gestational age from obstetric ultrasound, by various target parameters and it usually occurs within a day of ovulation, which, in turn, occurs on average 14.6 days after the beginning of the preceding menstruation.6. In a reference group representing all women, the 95% prediction interval of the LMP-to-ovulation is 8.2 to 20.5 days.
Also, vaginal bleeding occurs during 15-25% of first trimester pregnancies, a fetus born prior to the 37th week of gestation is considered to be preterm. A preterm baby is likely to be premature and consequently faces increased risk of morbidity and mortality, an estimated due date is given by Naegeles rule. The events of development usually occur at specific gestational ages. The gestational timing of a toxin exposure or infection can be used to predict the consequences to the fetus. Only if such newborns survived seven days were classified as live births. If, they died within that interval, they were classified as stillbirths, if they survived that interval but died within the first 365 days they were classified as infant deaths. More recently, thresholds for fetal death continue to vary widely internationally, the gestational age for fetal viability ranges from 16 weeks in Norway, to 20 weeks in the US and Australia,24 weeks in the UK, and 26 weeks in Italy and Spain. Gestational age is sometimes used postnatally to estimate various risk factors, for example, it is a better predictor than postnatal age for risk of intraventricular hemorrhage in premature babies treated with extracorporeal membrane oxygenation.
Family-based studies showed that age is partially determined by genetic factors
Animals are multicellular, eukaryotic organisms of the kingdom Animalia. The animal kingdom emerged as a clade within Apoikozoa as the group to the choanoflagellates. Animals are motile, meaning they can move spontaneously and independently at some point in their lives and their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis in their lives. All animals are heterotrophs, they must ingest other organisms or their products for sustenance, most known animal phyla appeared in the fossil record as marine species during the Cambrian explosion, about 542 million years ago. Animals can be divided broadly into vertebrates and invertebrates, vertebrates have a backbone or spine, and amount to less than five percent of all described animal species. They include fish, reptiles and mammals, the remaining animals are the invertebrates, which lack a backbone. These include molluscs, annelids, flatworms, ctenophores, the study of animals is called zoology.
The word animal comes from the Latin animalis, meaning having breath, the biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish and humans. Aristotle divided the world between animals and plants, and this was followed by Carl Linnaeus, in the first hierarchical classification. In Linnaeuss original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Pisces, Amphibia and Mammalia. Since the last four have all been subsumed into a single phylum, in 1874, Ernst Haeckel divided the animal kingdom into two subkingdoms and Protozoa. The protozoa were moved to the kingdom Protista, leaving only the metazoa, thus Metazoa is now considered a synonym of Animalia. Animals have several characteristics that set apart from other living things. Animals are eukaryotic and multicellular, which separates them from bacteria and they are heterotrophic, generally digesting food in an internal chamber, which separates them from plants and algae.
They are distinguished from plants and fungi by lacking cell walls. All animals are motile, if only at life stages. In most animals, embryos pass through a stage, which is a characteristic exclusive to animals. With a few exceptions, most notably the sponges and Placozoa and these include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals
Reptiles are tetrapod animals in the class Reptilia, comprising todays turtles, snakes, lizards and their extinct relatives. The study of these traditional reptile orders, historically combined with that of amphibians, is called herpetology. Because some reptiles are more related to birds than they are to other reptiles. For this reason, many scientists prefer to consider the birds part of Reptilia as well. Some early examples include the lizard-like Hylonomus and Casineria, in addition to the living reptiles, there are many diverse groups that are now extinct, in some cases due to mass extinction events. In particular, the K–Pg extinction wiped out the pterosaurs, plesiosaurs and sauropods, as well as species of theropods, crocodyliforms. Modern non-avian reptiles inhabit every continent with the exception of Antarctica, several living subgroups are recognized, approximately 400 species, Sphenodontia,1 species, over 9,600 species, Crocodilia,25 species, and Aves,10,000 species. Reptiles are tetrapod vertebrates, creatures that either have four limbs or, unlike amphibians, reptiles do not have an aquatic larval stage.
As amniotes, reptile eggs are surrounded by membranes for protection and transport, many of the viviparous species feed their fetuses through various forms of placenta analogous to those of mammals, with some providing initial care for their hatchlings. In the 18th century, the reptiles were, from the outset of classification, the terms reptile and amphibian were largely interchangeable, reptile being preferred by the French. Josephus Nicolaus Laurenti was the first to use the term Reptilia for an expanded selection of reptiles. Today, the two groups are commonly treated under the same heading as herptiles. He subsequently proposed the names of Sauropsida and Ichthyopsida for the two groups. In 1866, Haeckel demonstrated that vertebrates could be divided based on their strategies, and that reptiles, birds. The terms Sauropsida and Theropsida were used again in 1916 by E. S, Goodrich to distinguish between lizards and their relatives on the one hand and mammals and their extinct relatives on the other.
Goodrich supported this division by the nature of the hearts and blood vessels in each group, according to Goodrich, both lineages evolved from an earlier stem group, Protosauria in which he included some animals today considered reptile-like amphibians, as well as early reptiles. Watson observed that the first two groups diverged very early in history, so he divided Goodrichs Protosauria between them. He reinterpreted Sauropsida and Theropsida to exclude birds and mammals, thus his Sauropsida included Procolophonia, Millerosauria, Squamata, Crocodilia, non-avian dinosaurs, pterosaurs and sauropterygians
Invertebrates are animals that neither possess nor develop a vertebral column, derived from the notochord. This includes all animals apart from the subphylum Vertebrata, familiar examples of invertebrates include insects, crabs and their kin, clams and their kin, sea-urchins and their kin and worms. The majority of species are invertebrates, one estimate puts the figure at 97%. Many invertebrate taxa have a number and variety of species than the entire subphylum of Vertebrata. Some of the invertebrates, such as the Tunicata and Cephalochordata are more closely related to the vertebrates than to other invertebrates. This makes the term invertebrate paraphyletic and hence almost meaningless for taxonomic purposes, the word invertebrate comes from the form of the Latin word vertebra, which means a joint in general, and sometimes specifically a joint from the spinal column of a vertebrate. In turn the jointed aspect of vertebra derived from the concept of turning, coupled with the prefix in-, meaning not or without.
The term invertebrates is not always precise among non-biologists since it does not accurately describe a taxon in the way that Arthropoda. Each of these describes an valid taxon, subphylum or family. Invertebrata is a term of convenience, not a taxon, it has very little circumscriptional significance except within the Chordata, the Vertebrata as a subphylum comprises such a small proportion of the Metazoa that to speak of the kingdom Animalia in terms of Vertebrata and Invertebrata has limited practicality. That would at least circumscribe the Chordata, even the notochord would be a less fundamental criterion than aspects of embryological development and symmetry or perhaps bauplan. The following text reflects earlier scientific understanding of the term and of animals which have constituted it. According to this understanding, invertebrates do not possess a skeleton of bone and they include hugely varied body plans. Many have fluid-filled, hydrostatic skeletons, like jellyfish or worms, others have hard exoskeletons, outer shells like those of insects and crustaceans.
The most familiar invertebrates include the Protozoa, Coelenterata, Nematoda, Echinodermata, Arthropoda include insects and arachnids. By far the largest number of described species are insects. The following table lists the number of described extant species for major invertebrate groups as estimated in the IUCN Red List of Threatened Species,2014.3. The IUCN estimates that 66,178 extant vertebrate species have been described, the trait that is common to all invertebrates is the absence of a vertebral column, this creates a distinction between invertebrates and vertebrates