A larva is a distinct juvenile form many animals undergo before metamorphosis into adults. Animals with indirect development such as insects, amphibians, or cnidarians have a larval phase of their life cycle; the larva's appearance is very different from the adult form including different unique structures and organs that do not occur in the adult form. Their diet may be different. Larvae are adapted to environments separate from adults. For example, some larvae such as tadpoles live exclusively in aquatic environments, but can live outside water as adult frogs. By living in a distinct environment, larvae may be given shelter from predators and reduce competition for resources with the adult population. Animals in the larval stage will consume food to fuel their transition into the adult form. In some species like barnacles, adults are immobile but their larvae are mobile, use their mobile larval form to distribute themselves; some larvae are dependent on adults to feed them. In many eusocial Hymenoptera species, the larvae are fed by female workers.
In Ropalidia marginata the males are capable of feeding larvae but they are much less efficient, spending more time and getting less food to the larvae. The larvae of some species do not develop further into the adult form; this is a type of neoteny. It is a misunderstanding; this could be the case, but the larval stage has evolved secondarily, as in insects. In these cases the larval form may differ more than the adult form from the group's common origin. Within Insects, only Endopterygotes show different types of larvae. Several classifications have been suggested by many entomologists, following classification is based on Antonio Berlese classification in 1913. There are four main types of endopterygote larvae types: Apodous larvae – no legs at all and are poorly sclerotized. Based on sclerotization, three apodous forms are recognized. Eucephalous – with well sclerotized head capsule. Found in Nematocera and Cerambycidae families. Hemicephalus – with a reduced head capsule, retractable in to the thorax.
Found in Tipulidae and Brachycera families. Acephalus – without head capsule. Found in Cyclorrhapha Protopod larvae – larva have many different forms and unlike a normal insect form, they hatch from eggs which contains little yolk. Ex. first instar larvae of parasitic hymenoptera. Polypod larvae – known as eruciform larvae, these larva have abdominal prolegs, in addition to usual thoracic legs, they poorly sclerotized and inactive. They live in close contact with the food. Best example is caterpillars of lepidopterans. Oligopod larvae – have well developed head capsule and mouthparts are similar to the adult, but without compound eyes, they have six legs. No abdominal prolegs. Two types can be seen: Campodeiform – well sclerotized, dorso-ventrally flattened body. Long legged predators with prognathous mouthparts.. Scarabeiform – poorly sclerotized, flat thorax and abdomen. Short legged and inactive burrowing forms.. Crustacean larvae Ichthyoplankton Spawn Non-larval animal juvenile stages and other life cycle stages: In Porifera: olynthus, gemmule In Cnidaria: ephyra, strobila, hydranth, medusa In Mollusca: paralarva, young cephalopods In Platyhelminthes: hydatid cyst In Bryozoa: avicularium In Acanthocephala: cystacanth In Insecta: Nymphs and naiads, immature forms in hemimetabolous insects Subimago, a juvenile that resembles the adult in Ephemeroptera Instar, intermediate between each ecdysis Pupa and chrysalis, intermediate stages between larva and imago Protozoan life cycle stages Apicomplexan life cycle Algal life cycle stages: Codiolum-phase Conchocelis-phase Marine larval ecology Media related to Larvae at Wikimedia Commons The dictionary definition of larva at Wiktionary Arenas-Mena, C.
Indirect development, transdifferentiation and the macroregulatory evolution of metazoans. Philosophical Transactions of the Royal Society B: Biological Sciences. Feb 27, 2010 Vol.365 no.1540 653-669 Brusca, R. C. & Brusca, G. J.. Invertebrates. Sunderland, Mass.: Sinauer Associates. Hall, B. K. & Wake, M. H. eds.. The Origin and Evolution of Larval Forms. San Diego: Academic Press. Leis, J. M. & Carson-Ewart, B. M. eds.. The Larvae of Indo-Pacific Coastal Fishes. An Identification Guide to Marine Fish Larvae. Fauna Malesiana handbooks, vol. 2. Brill, Leiden. Minelli, A.. The larva. In: Perspectives in Animal Phylogeny and Evolution. Oxford University Press. P. 160-170. Link. Shanks, A. L.. An Identification Guide to the Larval Marine Invertebrates of the Pacific Northwest. Oregon State University Press, Corvallis. 256 pp. Smith, D. & Johnson, K. B.. A Guide to Marine Coastal Plankton and Marine Invertebrate Larvae. Kendall/Hunt Plublishing Company. Stanwell-Smith, D. Hood, A. & Peck, L. S.. A field guide to the pelagic invertebrates larvae of the maritime Antarctic.
British Antarctic Survey, Cambridge. Thyssen, P. J.. Keys for Identification of Immature Insects. In: Amendt, J. et al.. Current Concepts in Forensic Entomology, chapter 2, pp. 25–42. Springer: Dordrecht
Chironex fleckeri known as the sea wasp, is a species of venomous box jellyfish found in coastal waters from northern Australia and New Guinea north to the Philippines and Vietnam. It has been described as "the most lethal jellyfish in the world", with at least 63 known deaths in Australia from 1884 to 1996. Notorious for its sting, C. fleckeri has tentacles up to 3 m long covered with millions of cnidocytes which, on contact, release microscopic darts delivering an powerful venom. Being stung results in excruciating pain, if the sting area is significant, an untreated victim may die in two to five minutes; the amount of venom in one animal is said to be enough to kill 60 adult humans, although most stings are mild and do not require hospitalization. Chironex fleckeri was named after radiologist Doctor Hugo Flecker. "On January 20, 1955, when a 5-year-old boy died after being stung in shallow water at Cardwell, North Queensland, Flecker found three types of jellyfish. One was an unidentified box-shaped jellyfish with groups of tentacles arising from each corner.
Flecker sent it to Dr Ronald Southcott in Adelaide, on December 29, 1955, Southcott published his article introducing it as a new genus and species of lethal box jellyfish. He named it Chironex fleckeri, the name being derived from the Greek cheiro meaning "hand", the Latin nex meaning "murderer", "fleckeri" in honour of its discoverer." Chironex fleckeri is the largest of the cubozoans, many of which may carry toxic venom. Its bell grows to about the size of a basketball. From each of the four corners of the bell trails a cluster of 15 tentacles; the pale blue bell has faint markings. Since it is transparent, the creature is nearly impossible to see in its habitat, posing particular danger to swimmers; when the jellyfish are swimming, the tentacles contract so they are about 15 cm long and about 5 mm in diameter. The tentacles are covered with a high concentration of stinging cells called cnidocytes, which are activated by pressure and a chemical trigger. Box jellyfish are day hunters. In common with other box jellyfish, C. fleckeri has four eye-clusters with 24 eyes.
Some of these eyes seem capable of forming images, but whether they exhibit any object recognition or object tracking is debated. During a series of tests by marine biologists including Australian jellyfish expert Jamie Seymour, a single jellyfish was put in a tank. Two white poles were lowered into the tank; the creature appeared unable to swam straight into them, thus knocking them over. Similar black poles were placed into the tank; this time, the jellyfish seemed aware of them, swam around them in a figure-eight. To see if the specimen could see colour, a single red pole was stood in the tank; when the jellyfish became aware of the object in its tank, it was repelled by it and remained at the far edge of the tank. Following these experiments, the Australian researchers put forward the idea of red safety nets for beaches; the test was repeated, with similar results, on Irukandji jellyfish, another toxic species of box jelly. Chironex fleckeri lives on a diet of prawns and small fish, are prey to turtles, whose thick skin is impenetrable to the cnidocytes of the jellyfish.
The medusa is pelagic and has been documented from coastal waters of Australia and New Guinea north to the Philippines and Vietnam. In Australia, it is known from the northern coasts from Exmouth to Agnes Water, but its full distribution outside Australia has not been properly identified. To further confuse, the related and dangerously venomous Chironex yamaguchii was first described from Japan in 2009; this species has been documented from the Philippines, meaning the non-Australian records of C. fleckeri need to be rechecked. Breeding occurs in lower levels of rivers and mangrove channels. Chironex fleckeri is best known for its powerful and fatal "sting"; the sting can produce an excruciating pain accompanied by an intense burning sensation, like being branded with a red hot iron. In Australia, fatalities are most caused by the larger specimens of C. fleckeri. In Australia, C. fleckeri has caused at least 64 deaths since the first report in 1883, but most encounters appear to result only in mild envenomation.
Among 225 analyzed C. fleckeri stings in Australia's Top End from 1991 to 2004, only 8% required hospital admission, 5% received antivenom and there was a single fatality. 26 % experienced severe pain. Most deaths in recent decades have been children, as their smaller body mass puts them at a higher risk of fatal envenomation; when people do die, it is caused by a cardiac arrest occurring within minutes of the sting. Researchers at the University of Hawaii's Department of Tropical Medicine have found that the venom causes cells to become porous enough to allow potassium leakage, causing hyperkalemia which can lead to cardiovascular collapse and death as as within two to five minutes with an LD50 of 0.04 mg/kg, making it the most venomous jellyfish in the world. It was postulated that a zinc compound may be developed as an antidot
A micrograph or photomicrograph is a photograph or digital image taken through a microscope or similar device to show a magnified image of an object. This is opposed to a macrograph or photomacrograph, an image, taken on a microscope but is only magnified less than 10 times. Micrography is the art of using microscopes to make photographs. A micrograph contains extensive details of microstructure. A wealth of information can be obtained from a simple micrograph like behavior of the material under different conditions, the phases found in the system, failure analysis, grain size estimation, elemental analysis and so on. Micrographs are used in all fields of microscopy. A light micrograph or photomicrograph is a micrograph prepared using an optical microscope, a process referred to as photomicroscopy. At a basic level, photomicroscopy may be performed by connecting a camera to a microscope, thereby enabling the user to take photographs at reasonably high magnification. Scientific use began in England in 1850 by Prof Richard Hill Norris FRSE for his studies of blood cells.
Roman Vishniac was a pioneer in the field of photomicroscopy, specializing in the photography of living creatures in full motion. He made major developments in light-interruption photography and color photomicroscopy. Photomicrographs may be obtained using a USB microscope attached directly to a home computer or laptop. An electron micrograph is a micrograph prepared using an electron microscope. Micrographs have micron bars, or magnification ratios, or both. Magnification is a ratio between the size of an object on its real size. Magnification can be a misleading parameter as it depends on the final size of a printed picture and therefore varies with picture size. A scale bar, or micron bar, is a line of known length displayed on a picture; the bar can be used for measurements on a picture. When the picture is resized the bar is resized making it possible to recalculate the magnification. Ideally, all pictures destined for publication/presentation should be supplied with a scale bar. All but one of the micrographs presented on this page do not have a micron bar.
The microscope has been used for scientific discovery. It has been linked to the arts since its invention in the 17th century. Early adopters of the microscope, such as Robert Hooke and Antonie van Leeuwenhoek, were excellent illustrators. After the invention of photography in the 1820s the microscope was combined with the camera to take pictures instead of relying on an artistic rendering. Since the early 1970s individuals have been using the microscope as an artistic instrument. Websites and traveling art exhibits such as the Nikon Small World and Olympus Bioscapes have featured a range of images for the sole purpose of artistic enjoyment; some collaborative groups, such as the Paper Project have incorporated microscopic imagery into tactile art pieces as well as 3D immersive rooms and dance performances. Close-up Digital microscope Macro photography Microphotograph Microscopy USB microscope Make a Micrograph – This presentation by the research department of Children's Hospital Boston shows how researchers create a three-color micrograph.
Shots with a Microscope – a basic, comprehensive guide to photomicrography Scientific photomicrographs – free scientific quality photomicrographs by Doc. RNDr. Josef Reischig, CSc. Micrographs of 18 natural fibres by the International Year of Natural Fibres 2009 Seeing Beyond the Human Eye Video produced by Off Book - Solomon C. Fuller bio Charles Krebs Microscopic Images Dennis Kunkel Microscopy Andrew Paul Leonard, APL Microscopic Cell Centered Database - Montage Nikon Small World Olympus Bioscapes Other examples
Cnidaria is a phylum under Kingdom Animalia containing over 11,000 species of animals found in aquatic environments: they are predominantly marine. Their distinguishing feature is cnidocytes, specialized cells that they use for capturing prey, their bodies consist of mesoglea, a non-living jelly-like substance, sandwiched between two layers of epithelium that are one cell thick. They have two basic body forms: swimming medusae and sessile polyps, both of which are radially symmetrical with mouths surrounded by tentacles that bear cnidocytes. Both forms have a single body cavity that are used for digestion and respiration. Many cnidarian species produce colonies that are single organisms composed of medusa-like or polyp-like zooids, or both. Cnidarians' activities are coordinated by simple receptors. Several free-swimming species of Cubozoa and Scyphozoa possess balance-sensing statocysts, some have simple eyes. Not all cnidarians reproduce sexually, with many species having complex life cycles of asexual polyp stages and sexual medusae.
Some, omit either the polyp or the medusa stage. Cnidarians were grouped with ctenophores in the phylum Coelenterata, but increasing awareness of their differences caused them to be placed in separate phyla. Cnidarians are classified into four main groups: the wholly sessile Anthozoa. Staurozoa have been recognised as a class in their own right rather than a sub-group of Scyphozoa, the parasitic Myxozoa and Polypodiozoa were only recognized as cnidarians in 2007. Most cnidarians prey on organisms ranging in size from plankton to animals several times larger than themselves, but many obtain much of their nutrition from dinoflagellates, a few are parasites. Many are preyed on by other animals including starfish, sea slugs, fish and other cnidarians. Many scleractinian corals—which form the structural foundation for coral reefs—possess polyps that are filled with symbiotic photo-synthetic zooxanthellae. While reef-forming corals are entirely restricted to warm and shallow marine waters, other cnidarians can be found at great depths, in polar regions, in freshwater.
Recent phylogenetic analyses support monophyly of cnidarians, as well as the position of cnidarians as the sister group of bilaterians. Fossil cnidarians have been found in rocks formed about 580 million years ago, other fossils show that corals may have been present shortly before 490 million years ago and diversified a few million years later. However, molecular clock analysis of mitochondrial genes suggests a much older age for the crown group of cnidarians, estimated around 741 million years ago 200 million years before the Cambrian period as well as any fossils. Cnidarians form a phylum of animal that are more complex than sponges, about as complex as ctenophores, less complex than bilaterians, which include all other animals. Both cnidarians and ctenophores are more complex than sponges as they have: cells bound by inter-cell connections and carpet-like basement membranes. Cnidarians are distinguished from all other animals by having cnidocytes that fire harpoon like structures and are used to capture prey.
In some species, cnidocytes can be used as anchors. Like sponges and ctenophores, cnidarians have two main layers of cells that sandwich a middle layer of jelly-like material, called the mesoglea in cnidarians. Hence and ctenophores have traditionally been labelled diploblastic, along with sponges. However, both cnidarians and ctenophores have a type of muscle that, in more complex animals, arises from the middle cell layer; as a result, some recent text books classify ctenophores as triploblastic, it has been suggested that cnidarians evolved from triploblastic ancestors. Most adult cnidarians appear as either free-swimming medusae or sessile polyps, many hydrozoans species are known to alternate between the two forms. Both are radially symmetrical, like a tube respectively. Since these animals have no heads, their ends are described as "oral" and "aboral". Most have fringes of tentacles equipped with cnidocytes around their edges, medusae have an inner ring of tentacles around the mouth; some hydroids may consist of colonies of zooids that serve different purposes, such as defense and catching prey.
The mesoglea of polyps is thin and soft, but that of medusae is thick and springy, so that it returns to its original shape after muscles around the edge have contracted to squeeze water out, enabling medusae to swim by a sort of jet propulsion. In medusae the only supporting structure is the mesoglea. Hydra and most sea anemones close their mouths when they are not feeding, the water in the digestive cavity acts as a hydrostatic skeleton, rather like a water-filled balloon. Other polyps such as Tubularia use columns of water-filled cells for support. Sea pens stiffen the mesoglea with calcium carbonate spicules and tough fibrous proteins, rather like sponges. In some colonial polyps, a chitinous periderm gives support and some protection to the connecting sections and to the lower parts of individual polyps. Stony corals secrete massive calcium carbonate exoske
Spiders are air-breathing arthropods that have eight legs and chelicerae with fangs able to inject venom. They are the largest order of arachnids and rank seventh in total species diversity among all orders of organisms. Spiders are found worldwide on every continent except for Antarctica, have become established in nearly every habitat with the exceptions of air and sea colonization; as of November 2015, at least 45,700 spider species, 113 families have been recorded by taxonomists. However, there has been dissension within the scientific community as to how all these families should be classified, as evidenced by the over 20 different classifications that have been proposed since 1900. Anatomically, spiders differ from other arthropods in that the usual body segments are fused into two tagmata, the cephalothorax and abdomen, joined by a small, cylindrical pedicel. Unlike insects, spiders do not have antennae. In all except the most primitive group, the Mesothelae, spiders have the most centralized nervous systems of all arthropods, as all their ganglia are fused into one mass in the cephalothorax.
Unlike most arthropods, spiders have no extensor muscles in their limbs and instead extend them by hydraulic pressure. Their abdomens bear appendages that have been modified into spinnerets that extrude silk from up to six types of glands. Spider webs vary in size and the amount of sticky thread used, it now appears that the spiral orb web may be one of the earliest forms, spiders that produce tangled cobwebs are more abundant and diverse than orb-web spiders. Spider-like arachnids with silk-producing spigots appeared in the Devonian period about 386 million years ago, but these animals lacked spinnerets. True spiders have been found in Carboniferous rocks from 318 to 299 million years ago, are similar to the most primitive surviving suborder, the Mesothelae; the main groups of modern spiders and Araneomorphae, first appeared in the Triassic period, before 200 million years ago. The species Bagheera kiplingi was described as herbivorous in 2008, but all other known species are predators preying on insects and on other spiders, although a few large species take birds and lizards.
It is estimated that the world's 25 million tons of spiders kill 400–800 million tons of prey per year. Spiders use a wide range of strategies to capture prey: trapping it in sticky webs, lassoing it with sticky bolas, mimicking the prey to avoid detection, or running it down. Most detect prey by sensing vibrations, but the active hunters have acute vision, hunters of the genus Portia show signs of intelligence in their choice of tactics and ability to develop new ones. Spiders' guts are too narrow to take solids, so they liquefy their food by flooding it with digestive enzymes, they grind food with the bases of their pedipalps, as arachnids do not have the mandibles that crustaceans and insects have. To avoid being eaten by the females, which are much larger, male spiders identify themselves to potential mates by a variety of complex courtship rituals. Males of most species survive a few matings, limited by their short life spans. Females weave silk egg-cases. Females of many species care for their young, for example by carrying them around or by sharing food with them.
A minority of species are social, building communal webs that may house anywhere from a few to 50,000 individuals. Social behavior ranges from precarious toleration, as in the widow spiders, to co-operative hunting and food-sharing. Although most spiders live for at most two years and other mygalomorph spiders can live up to 25 years in captivity. While the venom of a few species is dangerous to humans, scientists are now researching the use of spider venom in medicine and as non-polluting pesticides. Spider silk provides a combination of lightness and elasticity, superior to that of synthetic materials, spider silk genes have been inserted into mammals and plants to see if these can be used as silk factories; as a result of their wide range of behaviors, spiders have become common symbols in art and mythology symbolizing various combinations of patience and creative powers. An abnormal fear of spiders is called arachnophobia. Spiders are chelicerates and therefore arthropods; as arthropods they have: segmented bodies with jointed limbs, all covered in a cuticle made of chitin and proteins.
Being chelicerates, their bodies consist of two tagmata, sets of segments that serve similar functions: the foremost one, called the cephalothorax or prosoma, is a complete fusion of the segments that in an insect would form two separate tagmata, the head and thorax. In spiders, the cephalothorax and abdomen are connected by the pedicel; the pattern of segment fusion that forms chelicerates' heads is unique among arthropods, what would be the first head segment disappears at an early stage of development, so that chelicerates lack the antennae typical of most arthropods. In fact, chelicerates' only appendages ahead of the mouth are a pair of chelicerae, they lack anything that would function directly as "jaws"; the first appendages behind the mouth are called pedipalps, serve different functions within different groups of chelicerates. Spiders and scorpions are members of the arachnids. Scorpions' chelicerae are used in feeding. Spiders' chelicerae have two sections and terminate in fangs that are venomous, fold away behind the upper sections while not in use.
The upper sections have thick "beards
An arthropod is an invertebrate animal having an exoskeleton, a segmented body, paired jointed appendages. Arthropods form the phylum Euarthropoda, which includes insects, arachnids and crustaceans; the term Arthropoda as proposed refers to a proposed grouping of Euarthropods and the phylum Onychophora. Arthropods are characterized by their jointed limbs and cuticle made of chitin mineralised with calcium carbonate; the arthropod body plan consists of each with a pair of appendages. The rigid cuticle inhibits growth, so arthropods replace it periodically by moulting. Arthopods are bilaterally symmetrical and their body possesses an external skeleton; some species have wings. Their versatility has enabled them to become the most species-rich members of all ecological guilds in most environments, they have over a million described species, making up more than 80 per cent of all described living animal species, some of which, unlike most other animals, are successful in dry environments. Arthropods range in size from the microscopic crustacean Stygotantulus up to the Japanese spider crab.
Arthropods' primary internal cavity is a haemocoel, which accommodates their internal organs, through which their haemolymph – analogue of blood – circulates. Like their exteriors, the internal organs of arthropods are built of repeated segments, their nervous system is "ladder-like", with paired ventral nerve cords running through all segments and forming paired ganglia in each segment. Their heads are formed by fusion of varying numbers of segments, their brains are formed by fusion of the ganglia of these segments and encircle the esophagus; the respiratory and excretory systems of arthropods vary, depending as much on their environment as on the subphylum to which they belong. Their vision relies on various combinations of compound eyes and pigment-pit ocelli: in most species the ocelli can only detect the direction from which light is coming, the compound eyes are the main source of information, but the main eyes of spiders are ocelli that can form images and, in a few cases, can swivel to track prey.
Arthropods have a wide range of chemical and mechanical sensors based on modifications of the many setae that project through their cuticles. Arthropods' methods of reproduction and development are diverse; the evolutionary ancestry of arthropods dates back to the Cambrian period. The group is regarded as monophyletic, many analyses support the placement of arthropods with cycloneuralians in a superphylum Ecdysozoa. Overall, the basal relationships of Metazoa are not yet well resolved; the relationships between various arthropod groups are still debated. Aquatic species use either external fertilization. All arthropods lay eggs, but scorpions give birth to live young after the eggs have hatched inside the mother. Arthropod hatchlings vary from miniature adults to grubs and caterpillars that lack jointed limbs and undergo a total metamorphosis to produce the adult form; the level of maternal care for hatchlings varies from nonexistent to the prolonged care provided by scorpions. Arthropods contribute to the human food supply both directly as food, more indirectly as pollinators of crops.
Some species are known to spread severe disease to humans and crops. The word arthropod comes from the Greek ἄρθρον árthron, "joint", πούς pous, i.e. "foot" or "leg", which together mean "jointed leg". Arthropods are invertebrates with jointed limbs; the exoskeleton or cuticles consists of a polymer of glucosamine. The cuticle of many crustaceans, beetle mites, millipedes is biomineralized with calcium carbonate. Calcification of the endosternite, an internal structure used for muscle attachments occur in some opiliones. Estimates of the number of arthropod species vary between 1,170,000 and 5 to 10 million and account for over 80 per cent of all known living animal species; the number of species remains difficult to determine. This is due to the census modeling assumptions projected onto other regions in order to scale up from counts at specific locations applied to the whole world. A study in 1992 estimated that there were 500,000 species of animals and plants in Costa Rica alone, of which 365,000 were arthropods.
They are important members of marine, freshwater and air ecosystems, are one of only two major animal groups that have adapted to life in dry environments. One arthropod sub-group, insects, is the most species-rich member of all ecological guilds in land and freshwater environments; the lightest insects weigh less than 25 micrograms. Some living crustaceans are much larger; the embryos of all arthropods are segmented, built from a series of repeated modules. The last common ancestor of living arthropods consisted of a series of undifferentiated segments, each with a pair of appendages that functioned as limbs. However, all known living and fossil arthropods have grouped segments into tagmata in which segments and their limbs are specialized in various ways; the three-
Crustaceans form a large, diverse arthropod taxon which includes such familiar animals as crabs, crayfish, krill and barnacles. The crustacean group is treated as a subphylum, because of recent molecular studies it is now well accepted that the crustacean group is paraphyletic, comprises all animals in the Pancrustacea clade other than hexapods; some crustaceans are more related to insects and other hexapods than they are to certain other crustaceans. The 67,000 described species range in size from Stygotantulus stocki at 0.1 mm, to the Japanese spider crab with a leg span of up to 3.8 m and a mass of 20 kg. Like other arthropods, crustaceans have an exoskeleton, they are distinguished from other groups of arthropods, such as insects and chelicerates, by the possession of biramous limbs, by their larval forms, such as the nauplius stage of branchiopods and copepods. Most crustaceans are free-living aquatic animals, but some are terrestrial, some are parasitic and some are sessile; the group has an extensive fossil record, reaching back to the Cambrian, includes living fossils such as Triops cancriformis, which has existed unchanged since the Triassic period.
More than 10 million tons of crustaceans are produced by fishery or farming for human consumption, the majority of it being shrimp and prawns. Krill and copepods are not as fished, but may be the animals with the greatest biomass on the planet, form a vital part of the food chain; the scientific study of crustaceans is known as carcinology, a scientist who works in carcinology is a carcinologist. The body of a crustacean is composed of segments, which are grouped into three regions: the cephalon or head, the pereon or thorax, the pleon or abdomen; the head and thorax may be fused together to form a cephalothorax, which may be covered by a single large carapace. The crustacean body is protected by the hard exoskeleton, which must be moulted for the animal to grow; the shell around each somite can be divided into a dorsal tergum, ventral sternum and a lateral pleuron. Various parts of the exoskeleton may be fused together; each somite, or body segment can bear a pair of appendages: on the segments of the head, these include two pairs of antennae, the mandibles and maxillae.
The abdomen bears pleopods, ends in a telson, which bears the anus, is flanked by uropods to form a tail fan. The number and variety of appendages in different crustaceans may be responsible for the group's success. Crustacean appendages are biramous, meaning they are divided into two parts, it is unclear whether the biramous condition is a derived state which evolved in crustaceans, or whether the second branch of the limb has been lost in all other groups. Trilobites, for instance possessed biramous appendages; the main body cavity is an open circulatory system, where blood is pumped into the haemocoel by a heart located near the dorsum. Malacostraca have haemocyanin as the oxygen-carrying pigment, while copepods, ostracods and branchiopods have haemoglobins; the alimentary canal consists of a straight tube that has a gizzard-like "gastric mill" for grinding food and a pair of digestive glands that absorb food. Structures that function as kidneys are located near the antennae. A brain exists in the form of ganglia close to the antennae, a collection of major ganglia is found below the gut.
In many decapods, the first pair of pleopods are specialised in the male for sperm transfer. Many terrestrial crustaceans return to the sea to release the eggs. Others, such as woodlice, lay their eggs on land, albeit in damp conditions. In most decapods, the females retain the eggs; the majority of crustaceans are aquatic, living in either marine or freshwater environments, but a few groups have adapted to life on land, such as terrestrial crabs, terrestrial hermit crabs, woodlice. Marine crustaceans are as ubiquitous in the oceans; the majority of crustaceans are motile, moving about independently, although a few taxonomic units are parasitic and live attached to their hosts, adult barnacles live a sessile life – they are attached headfirst to the substrate and cannot move independently. Some branchiurans are able to withstand rapid changes of salinity and will switch hosts from marine to non-marine species. Krill are the bottom layer and the most important part of the food chain in Antarctic animal communities.
Some crustaceans are significant invasive species, such as the Chinese mitten crab, Eriocheir sinensis, the Asian shore crab, Hemigrapsus sanguineus. The majority of crustaceans have separate sexes, reproduce sexually. A small number are hermaphrodites, including barnacles and Cephalocarida; some may change sex during the course of their life. Parthenogenesis is widespread among crustaceans, where viable eggs are produced by a female without needing fertilisation by a male; this occurs in many branchiopods, some os