Vertebrate

Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a spine or backbone), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordate subphylum Vertebrata, i.e. vertebrates. Well-known phyla of invertebrates include arthropods, molluscs, annelids, echinoderms, flatworms, cnidarians, and sponges.

The majority of animal species are invertebrates; one estimate puts the figure at 97%. Many invertebrate taxa have a greater number and diversity of species than the entire subphylum of Vertebrata. Invertebrates vary widely in size, from 10 μm (0.0004 in) myxozoans to the 9–10 m (30–33 ft) colossal squid.

Amphibians are ectothermic, anamniotic, four-limbed vertebrate animals that constitute the class Amphibia. In its broadest sense, it is a paraphyletic group encompassing all tetrapods, but excluding the amniotes (tetrapods with an amniotic membrane, such as modern reptiles, birds and mammals). All extant (living) amphibians belong to the monophyletic subclass Lissamphibia, with three living orders: Anura (frogs and toads), Urodela (salamanders), and Gymnophiona (caecilians). Evolved to be mostly semiaquatic, amphibians have adapted to inhabit a wide variety of habitats, with most species living in freshwater, wetland or terrestrial ecosystems (such as riparian woodland, fossorial and even arboreal habitats). Their life cycle typically starts out as aquatic larvae with gills known as tadpoles, but some species have developed behavioural adaptations to bypass this.

Young amphibians generally undergo metamorphosis from an aquatic larval form with gills to an air-breathing adult form with lungs. Amphibians use their skin as a secondary respiratory interface, and some small terrestrial salamanders and frogs even lack lungs and rely entirely on their skin. They are superficially similar to reptiles like lizards, but unlike reptiles and other amniotes, require access to water bodies to breed. With their complex reproductive needs and permeable skins, amphibians are often ecological indicators to habitat conditions; in recent decades there has been a dramatic decline in amphibian populations for many species around the globe.

A frog is any member of a diverse and largely semiaquatic group of short-bodied, tailless amphibian vertebrates composing the order Anura (coming from the Ancient Greek ἀνούρα, literally 'without tail'). Frog species with rough skin texture due to wart-like parotoid glands tend to be called toads, but the distinction between frogs and toads is informal and purely cosmetic, not from taxonomy or evolutionary history.

Frogs are widely distributed, ranging from the tropics to subarctic regions, but the greatest concentration of species diversity is in tropical rainforest and associated wetlands. They account for around 88% of extant amphibian species, and are one of the five most diverse vertebrate orders. The oldest fossil "proto-frog" Triadobatrachus is known from the Early Triassic of Madagascar (250 million years ago), but molecular clock dating suggests their divergence from other amphibians may extend further back to the Permian, 265 million years ago.

Mosquitoes, the Culicidae, are a family of small flies consisting of 3,600 species. The word mosquito (formed by mosca and diminutive -ito) is Spanish and Portuguese for little fly. Mosquitoes have a slender segmented body, one pair of wings, three pairs of long hair-like legs, and specialized, highly elongated, piercing-sucking mouthparts. All mosquitoes drink nectar from flowers; females of many species have adapted to also drink blood. The group diversified during the Cretaceous period. Evolutionary biologists view mosquitoes as micropredators, small animals that parasitise larger ones by drinking their blood without immediately killing them. Medical parasitologists view mosquitoes as vectors of disease, carrying protozoan parasites or bacterial or viral pathogens from one host to another.

The mosquito life cycle consists of four stages: egg, larva, pupa, and adult. Eggs are laid on the water surface; they hatch into motile larvae that feed on aquatic algae and organic material. These larvae are important food sources for many freshwater animals, such as dragonfly nymphs, many fish, and some birds. Adult females of many species have mouthparts adapted to pierce the skin of a host and feed on blood of a wide range of vertebrate hosts, and some invertebrates, primarily other arthropods. Some species only produce eggs after a blood meal.

Porphyrins (/ˈpɔːrfərɪns/ POR-fər-ins) are heterocyclic, macrocyclic, organic compounds, composed of four modified pyrrole subunits interconnected at their α carbon atoms via methine bridges (=CH−). In vertebrates, an essential member of the porphyrin group is heme, which is a component of hemoproteins, whose functions include carrying oxygen in the bloodstream. In plants, an essential porphyrin derivative is chlorophyll, which is involved in light harvesting and electron transfer in photosynthesis.

The parent of porphyrins is porphine, a rare chemical compound of exclusively theoretical interest. Substituted porphines are called porphyrins. With a total of 26 π-electrons the porphyrin ring structure is a coordinated aromatic system. One result of the large conjugated system is that porphyrins absorb strongly in the visible region of the electromagnetic spectrum, i.e. they are deeply colored. The name "porphyrin" derives from Greek πορφύρα (porphyra) 'purple'.

Animals are multicellular, eukaryotic organisms comprising the biological kingdom Animalia (/ˌænɪˈmeɪliə/). With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a hollow sphere of cells, the blastula, during embryonic development. Animals form a clade, meaning that they arose from a single common ancestor. Over 1.5 million living animal species have been described, of which around 1.05 million are insects, over 85,000 are molluscs, and around 65,000 are vertebrates. It has been estimated there are as many as 7.77 million animal species on Earth. Animal body lengths range from 8.5 μm (0.00033 in) to 33.6 m (110 ft). They have complex ecologies and interactions with each other and their environments, forming intricate food webs. The scientific study of animals is known as zoology, and the study of animal behaviour is known as ethology.

The animal kingdom is divided into five major clades, namely Porifera, Ctenophora, Placozoa, Cnidaria and Bilateria. Most living animal species belong to the clade Bilateria, a highly proliferative clade whose members have a bilaterally symmetric and significantly cephalised body plan, and the vast majority of bilaterians belong to two large clades: the protostomes, which includes organisms such as arthropods, molluscs, flatworms, annelids and nematodes; and the deuterostomes, which include echinoderms, hemichordates and chordates, the latter of which contains the vertebrates. The much smaller basal phylum Xenacoelomorpha have an uncertain position within Bilateria.

A body plan, Bauplan (pl. German: Baupläne), or ground plan is a set of morphological features common to many members of a phylum of animals. The vertebrates share one body plan, while invertebrates have many.

This term, usually applied to animals, envisages a "blueprint" encompassing aspects such as symmetry, layers, segmentation, nerve, limb, and gut disposition. Evolutionary developmental biology seeks to explain the origins of diverse body plans.

Soft-bodied organisms are organisms that lack rigid physical skeletons or frame, roughly corresponds to the group Vermes as proposed by Carl von Linné. The term typically refers to non-panarthropod invertebrates from the kingdom Animalia, although many non-vascular plants (mosses and algae), fungi (such as jelly fungus), lichens and slime molds can also be seen as soft-bodied organisms by definition.

All animals have a muscular system of some sort but, since myocytes are tensile actuator units that can only contract and pull but never push, some animals evolved rigid body parts upon which the muscles can attach and act as levers/cantilevers to redirect force and produce locomotive propulsion. These rigid parts also serve as structural elements to resist gravity and ambient pressure, as well as sometimes provide protective surfaces shielding internal structures from trauma and exposure to external thermal, chemical and pathogenic insults. Such physical structures are the commonly referred "skeletons", which may be internal (as in vertebrates, echinoderms and sponges) or external (as in arthropods and non-coleoid molluscs). However, many soft-bodied animals do still have a functional skeleton maintained by body fluid hydrostatics known as a hydroskeleton, such as that of earthworms, jellyfish, tapeworms, squids and an enormous variety of invertebrates from almost every phyla of the animal kingdom; and many have hardened teeth that allow them to chew, bite and burrow despite the rest of body being soft.

Fish began evolving about 530 million years ago during the Cambrian explosion. It was during this time that the early chordates developed the skull and the vertebral column, leading to the first craniates and vertebrates. The first fish lineages belong to the Agnatha, or jawless fish. Early examples include Haikouichthys. During the late Cambrian, eel-like jawless fish called the conodonts, and small mostly armoured fish known as ostracoderms, first appeared. Most jawless fish are now extinct; but the extant lampreys may approximate ancient pre-jawed fish. Lampreys belong to the Cyclostomata, which includes the extant hagfish, and this group may have split early on from other agnathans.

The earliest jawed vertebrates probably developed during the late Ordovician period. They are first represented in the fossil record from the Silurian by two groups of fish: the armoured fish known as placoderms, which evolved from the ostracoderms; and the Acanthodii (or spiny sharks). The jawed fish that are still extant in modern days also appeared during the late Silurian: the Chondrichthyes (or cartilaginous fish) and the Osteichthyes (or bony fish). The bony fish evolved into two separate groups: the Actinopterygii (or ray-finned fish) and Sarcopterygii (which includes the lobe-finned fish).

The evolution of tetrapods began about 400 million years ago in the Devonian Period with the earliest tetrapods evolved from lobe-finned fishes. Tetrapods (under the apomorphy-based definition used on this page) are categorized as animals in the biological superclass Tetrapoda, which includes all living and extinct amphibians, reptiles, birds, and mammals. While most species today are terrestrial, little evidence supports the idea that any of the earliest tetrapods could move about on land, as their limbs could not have held their midsections off the ground and the known trackways do not indicate they dragged their bellies around. Presumably, the tracks were made by animals walking along the bottoms of shallow bodies of water. The specific aquatic ancestors of the tetrapods, and the process by which land colonization occurred, remain unclear. They are areas of active research and debate among palaeontologists at present.

Most amphibians today remain semiaquatic, living the first stage of their lives as fish-like tadpoles. Several groups of tetrapods, such as the snakes and cetaceans, have lost some or all of their limbs. In addition, many tetrapods have returned to partially aquatic or fully aquatic lives throughout the history of the group (modern examples of fully aquatic tetrapods include cetaceans and sirenians). The first returns to an aquatic lifestyle may have occurred as early as the Carboniferous Period whereas other returns occurred as recently as the Cenozoic, as in cetaceans, pinnipeds, and several modern amphibians.