Animal law

Animal law is a combination of statutory and case law in which the nature – legal, social or biological – of nonhuman animals is an important factor. Animal law encompasses companion animals, wildlife, animals used in entertainment and animals raised for food and research. The emerging field of animal law is often analogized to the environmental law movement 30 years ago.

Approaches to animal law

Animal law issues encompass a broad spectrum of approaches—from philosophical explorations of the rights of animals to pragmatic discussions about the rights of those who use animals, who has standing to sue when an animal is harmed in a way that violates the law, and what constitutes legal cruelty.^[1] Animal law permeates and affects most traditional areas of the law – including tort, contract, criminal and constitutional law. Examples of this intersection include:

• Animal custody disputes in divorce or separations.^[2]
• Veterinary malpractice cases.
• Housing disputes involving “no pets” policies and discrimination laws.
• Damages cases involving the wrongful death or injury to a companion animal.^[3]
• Enforceable trusts for companions being adopted by states across the country.^[4]
• Criminal law encompassing domestic violence and anti-cruelty laws.

Recent legal changes influenced by animal rights activists

Regarding the campaign to change the status of animals as property, the animal rights activists have seen success in several countries. In 1992, Switzerland amended its constitution to recognize animals as beings and not things.^[13] However, in 1999 the Swiss constitution was completely rewritten. A decade later, Germany guaranteed rights to animals in a 2002 amendment to its constitution, becoming the first European Union member to do so.^[13][14][15] The German Civil Code had been amended correspondingly in 1997. The amendment, however, has not had much impact in German legal practice yet.^{[citation needed]}
The greatest success of the animal rights activists has certainly been the granting of basic rights to five great ape species in New Zealand in 1999. Their use is now forbidden in research, testing or teaching.^[16](It should be noted that the UK government banned experiments on great apes in 1986 ^[17]). Some other countries have also banned or severely restricted the use of non-human great apes in research.
The Seattle-based Great Ape Project (GAP) — founded by Australian philosopher Peter Singer, the author of Animal Liberation, widely regarded as the founding philosophical work of the animal liberation movement^[18] — is campaigning for the United Nations to adopt its Declaration on Great Apes, which would see chimpanzees, bonobos, gorillas and orang-utans included in a "community of equals" with human beings. The declaration wants to extend to the non-human apes the protection of three basic interests: the right to life, the protection of individual liberty, and the prohibition of torture ^[19] (see also Great ape personhood).

Animal welfare

Animal welfare is the physical and psychological well-being of animals.^[1] The term animal welfare can also mean human concern for animal welfare or a position in a debate on animal ethics and animal rights.^[2] Welfare is measured by indicators such as behavior, physiology, animal choices, longevity, production and reproduction.^[3]
Systematic concern for animal welfare can be based on awareness that non-human animals are sentient and that consideration should be given to their well-being, especially when they are used by humans.^[4] These concerns can include how animals are killed for food, how they are used for scientific research, how they are kept as pets, and how human activities affect the survival of endangered species.
An ancient object of concern in some civilizations, animal welfare began to take a larger place in western public policy in 19th-century Britain. Today it is a significant focus of interest or activity in veterinary science, in ethics, and in animal welfare organizations.
There are two forms of criticism of the concept of animal welfare, coming from diametrically opposite positions. One view, dating back centuries, asserts that animals are not consciously aware and hence are unable to experience poor welfare. The other view is based on the animal rights position that animals should not be regarded as property and any use of animals by humans is unacceptable. Some authorities thus treat animal welfare and animal rights as two opposing positions.^[2] Accordingly, some animal right proponents argue that the perception of better animal welfare facilitates continued and increased exploitation of animals.^[5][6] Others see the increasing concern for animal welfare as incremental steps towards animal rights.

Definitions

In animal ethics, the term animal welfare often means animal welfarism.
In Saunders Comprehensive Veterinary Dictionary, animal welfare is defined as "the avoidance of abuse and exploitation of animals by humans by maintaining appropriate standards of accommodation, feeding and general care, the prevention and treatment of disease and the assurance of freedom from harassment, and unnecessary discomfort and pain."^[7]
Donald Broom defines the welfare of an animal as "its state as regards its attempts to cope with its environment. This state includes how much it is having to do to cope, the extent to which it is succeeding in or failing to cope, and its associated feelings." He states that "Welfare will vary over a continuum from very good to very poor and studies of welfare will be most effective if a wide range of measures is used."^[8]
Yew-Kwang Ng defines animal welfare in terms of welfare economics: "Welfare biology is the study of living things and their environment with respect to their welfare (defined as net happiness, or enjoyment minus suffering). Despite difficulties of ascertaining and measuring welfare and relevancy to normative issues, welfare biology is a positive science."^[9]

[

Animal suffering

Singer writes that commentators on all sides of the debate now accept that animals suffer and feel pain, although it was not always so. Bernard Rollin, a philosopher and professor of animal sciences, writes that Descartes' influence continued to be felt until the 1980s. Veterinarians trained in the U.S. before 1989 were taught to ignore pain, he writes, and at least one major veterinary hospital in the 1960s did not stock narcotic analgesics for animal pain control. In his interactions with scientists, he was often asked to "prove" that animals are conscious, and to provide "scientifically acceptable" evidence that they could feel pain.^[87]
According to Singer, scientific publications have made it clear over the last two decades that the majority of researchers do believe animals suffer and feel pain, though it continues to be argued that their suffering may be reduced by an inability to experience the same dread of anticipation as humans, or to remember the suffering as vividly.^[88] In the most recent edition of Animal Liberation, Singer cites research indicating that animal impulses, emotions, and feelings are located in the diencephalon, a region well developed in mammals and birds.^[89] He also relies on the work of Richard Sarjeant. Sarjeant pointed out that non-human animals possess anatomical complexity of the cerebral cortex and neuroanatomy that is nearly identical to that of the human nervous system, arguing that, "[e]very particle of factual evidence supports the contention that the higher mammalian vertebrates experience pain sensations at least as acute as our own. To say that they feel less because they are lower animals is an absurdity; it can easily be shown that many of their senses are far more acute than ours."^[90]
The problem of animal suffering, and animal consciousness in general, arises primarily because animals have no language, leading scientists to argue that it is impossible to know when an animal is suffering. This situation may change as increasing numbers of chimps are taught sign language, although skeptics question whether their use of it portrays real understanding. Singer writes that, following the argument that language is needed to communicate pain, it would often be impossible to know when humans are in pain. All we can do is observe pain behavior, he writes, and make a calculated guess based on it. As Ludwig Wittgenstein argued, if someone is screaming, clutching a part of their body, moaning quietly, or apparently unable to function, especially when followed by an event we believe would cause pain in ourselves, that is in large measure what it means to be in pain.^[91] Singer argues that there is no reason to suppose animal pain behavior would have a different meaning.

1824: Society for the Prevention of Cruelty to Animals

Richard Martin soon realized that magistrates did not take the Martin Act seriously, and that it was not being reliably enforced. Several members of parliament decided to form a society to bring prosecutions under the Act. The Reverend Arthur Broome, a Balliol man who had recently become the vicar of Bromley-by-Bow, arranged a meeting in Old Slaughter's Coffee House in St. Martin's Lane, a London café frequented by artists and actors. The group met on June 16, 1824, and included a number of MPs: Richard Martin, Sir James Mackintosh, Sir Thomas Buxton, William Wilberforce, and Sir James Graham, who had been an MP, and who became one again in 1826. They decided to form a "Society instituted for the purpose of preventing cruelty to animals," or the Society for the Prevention of Cruelty to Animals, as it became known. It determined to send men to inspect Smithfield Market, where livestock had been sold since the 10th century, as well as slaughterhouses, and the practices of coachmen toward their horses.^[33] The Society became the Royal Society in 1840, when it was granted a royal charter by Queen Victoria, herself strongly opposed to vivisection.^[38]
Noel Molland writes that, in 1824, Catherine Smithies, an anti-slavery activist, set up an SPCA youth wing called the Bands of Mercy. It was a children's club modeled on the Temperance Society's Bands of Hope, which were intended to encourage children to campaign against drinking and gambling. The Bands of Mercy were similarly meant to encourage a love of animals.^[39] Molland writes that some of its members responded with more enthusiasm than Smithies intended, and became known for engaging in direct action against hunters by sabotaging their rifles, although Kim Stallwood writes that he has never been able to find solid evidence to support this.^[40] Whether the story is true or apocryphal, the idea of the youth group was revived by Ronnie Lee in 1972, when he and Cliff Goodman set up the Band of Mercy as a militant, anti-hunting guerrilla group, which slashed hunters' vehicles' tires and smashed their windows. In 1976, some of the same activists, sensing that the Band of Mercy name sounded too accommodating, founded the Animal Liberation Front.^[39]

Moral status of animals in the ancient world

The 21st-century debates about how humans should treat animals can be traced to the ancient world. The idea that the use of animals by humans—for food, clothing, entertainment, and as research subjects—is morally acceptable, springs mainly from two sources. First, there is the idea of a divine hierarchy based on the theological concept of "dominion," from Genesis (1:20–28), where Adam is given "dominion over the fish of the sea, and over the fowl of the air, and over the cattle, and over all the earth, and over every creeping thing that creepeth upon the earth." Although the concept of dominion need not entail property rights, it has been interpreted over the centuries to imply ownership. There is also the idea that animals are inferior because they lack rationality and language, and as such are worthy of less consideration than humans, or even none.^[6] Springing from this is the idea that individual animals have no separate moral identity: a pig is simply an example of the class of pigs, and it is to the class, not to the individual, that human stewardship should be applied. This leads to the argument that the use of individual animals is acceptable so long as the species is not threatened with extinction.^[7]

Animal rights

Animal rights, also known as animal liberation, is the idea that the most basic interests of non-human animals should be afforded the same consideration as the similar interests of human beings. Advocates approach the issue from different philosophical positions, ranging from the protectionist side of the movement, presented by philosopher Peter Singer—with a utilitarian focus on suffering and consequences, rather than on the concept of rights—to the abolitionist side, represented by law professor Gary Francione, who argues that animals need only one right: the right not to be property. Despite the different approaches, advocates broadly agree that animals should be viewed as non-human persons and members of the moral community, and should not be used as food, clothing, research subjects, or entertainment.^[3]
The idea of awarding rights to animals has the support of legal scholars such as Alan Dershowitz and Laurence Tribe of Harvard Law School. Animal rights is routinely covered in universities in philosophy or applied ethics courses, and as of spring 2010 animal law was taught in 125 law schools in the United States and Canada. Toronto lawyer Clayton Ruby argued in 2008 that the movement had reached the stage the gay rights movement was at 25 years earlier.^[4]
Critics of the idea argue that animals are unable to enter into a social contract or make moral choices, and for that reason cannot be regarded as possessors of rights, a position summed up by the philosopher Roger Scruton, who wrote in 2000 that only humans have duties and therefore only humans have rights. There has also been criticism, including from within the animal rights movement itself, of certain forms of animal rights activism, in particular the destruction of fur farms and animal laboratories by the Animal Liberation Front. A parallel argument is that there is nothing inherently wrong with using animals as resources so long there is no unnecessary suffering, a view known as the animal welfare position.^[5]

Animal

Etymology

The word "animal" comes from the Latin word animalis, meaning "having breath".^[1] In everyday colloquial usage, the word usually refers to non-human animals.^[2] Frequently, only closer relatives of humans such as mammals and other vertebrates are meant in colloquial use.^[3] The biological definition of the word refers to all members of the kingdom Animalia, encompassing creatures as diverse as sponges, jellyfish, insects and humans.^[4]

Characteristics

Animals have several characteristics that set them apart from other living things. Animals are eukaryotic and mostly multicellular,^[5] which separates them from bacteria and most protists. They are heterotrophic,^[6] generally digesting food in an internal chamber, which separates them from plants and algae.^[7] They are also distinguished from plants, algae, and fungi by lacking rigid cell walls.^[8] All animals are motile,^[9] if only at certain life stages. In most animals, embryos pass through a blastula stage,^[10] which is a characteristic exclusive to animals.

Structure

With a few exceptions, most notably the sponges (Phylum Porifera) and Placozoa, animals have bodies differentiated into separate tissues. These include muscles, which are able to contract and control locomotion, and nerve tissues, which send and process signals. Typically, there is also an internal digestive chamber, with one or two openings.^[11] Animals with this sort of organization are called metazoans, or eumetazoans when the former is used for animals in general.^[12]
All animals have eukaryotic cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins.^[13] This may be calcified to form structures like shells, bones, and spicules.^[14] During development, it forms a relatively flexible framework^[15] upon which cells can move about and be reorganized, making complex structures possible. In contrast, other multicellular organisms, like plants and fungi, have cells held in place by cell walls, and so develop by progressive growth.^[11] Also, unique to animal
cells are the following intercellular junctions: tight junctions, gap junctions, and desmosomes.^[16]

Reproduction and development

Nearly all animals undergo some form of sexual reproduction.^[17] They have a few specialized reproductive cells, which undergo meiosis to produce smaller, motile spermatozoa or larger, non-motile ova.^[18] These fuse to form zygotes, which develop into new individuals.^[19]
Many animals are also capable of asexual reproduction.^[20] This may take place through parthenogenesis, where fertile eggs are produced without mating, budding, or fragmentation.^[21]
A zygote initially develops into a hollow sphere, called a blastula,^[22] which undergoes rearrangement and differentiation. In sponges, blastula larvae swim to a new location and develop into a new sponge.^[23] In most other groups, the blastula undergoes more complicated rearrangement.^[24] It first invaginates to form a gastrula with a digestive chamber, and two separate germ layers — an external ectoderm and an internal endoderm.^[25] In most cases, a mesoderm also develops between them.^[26] These germ layers then differentiate to form tissues and organs.^[27]

Food and energy sourcing

Main article: Animal nutrition

All animals are heterotrophs, meaning that they feed directly or indirectly on other living things.^[28] They are often further subdivided into groups such as carnivores, herbivores, omnivores, and parasites.^[29]
Predation is a biological interaction where a predator (a heterotroph that is hunting) feeds on its prey (the organism that is attacked).^[30] Predators may or may not kill their prey prior to feeding on them, but the act of predation always results in the death of the prey.^[31] The other main category of consumption is detritivory, the consumption of dead organic matter.^[32] It can at times be difficult to separate the two feeding behaviours, for example, where parasitic species prey on a host organism and then lay their eggs on it for their offs pring to feed on its decaying corpse. Selective pressures imposed on one another has led to an evolutionary arms race between prey and predator, resulting in various antipredator adaptations.^[33]
Most animals feed indirectly from the energy of sunlight. Plants use this energy to convert sunlight into simple sugars using a process known as photosynthesis. Starting with the molecules carbon dioxide (CO₂) and water (H₂O), photosynthesis converts the energy of sunlight into chemical energy stored in the bonds of glucose (C₆H₁₂O₆) and releases oxygen (O₂). These sugars are then used as the building blocks which allow the plant to grow.^[11] When animals eat these plants (or eat other animals which have eaten plants), the sugars produced by the plant are used by the animal.^[34] They are either used directly to help the animal grow, or broken down, releasing stored solar energy, and giving the animal the energy required for motion.^[35] This process is known as glycolysis.^[36]
Animals living close to hydrothermal vents and cold seeps on the ocean floor are not dependent on the energy of sunlight.^[37] Instead chemosynthetic archaea and bacteria form the base of the food chain.^[38]

O

Animals are generally considered to have evolved from a flagellated eukaryote.^[40] Their closest known living relatives are the choanoflagellates, collared flagellates that have a morphology similar to the choanocytes of certain sponges.^[41] Molecular studies place animals in a supergroup called the opisthokonts, which also include the choanoflagellates, fungi and a few small parasitic protists.^[42] The name comes from the posterior location of the flagellum in motile cells, such as most animal spermatozoa, whereas other eukaryotes tend to have anterior flagella.^[43]
The first fossils that might represent animals appear in the Trezona Formation at Trezona Bore, West Central Flinders, South Australia.^[44] These fossils are interpreted as being early sponges. They were found in 665-million-year-old rock.^[44]
The next oldest possible animal fossils are found towards the end of the Precambrian, around 610 million years ago, and are known as the Ediacaran or Vendian biota.^[45] These are difficult to relate to later fossils, however. Some may represent precursors of modern phyla, but they may be separate groups, and it is possible they are not really animals at all.^[46]
Aside from them, most known animal phyla make a more or less simultaneous appearance during the Cambrian period, about 542 million years ago.^[47] It is still disputed whether this event, called the Cambrian explosion, represents a rapid divergence between different groups or a change in conditions that made fossilization possible.
Some paleontologists suggest that animals appeared much earlier than the Cambrian explosion, possibly as early as 1 billion years ago.^[48] Trace fossils such as tracks and burrows found in the Tonian era indicate the presence of triploblastic worms, like metazoans, roughly as large (about 5 mm wide) and complex as earthworms.^[49] During the beginning of the Tonian period around 1 billion years ago, there was a decrease in Stromatolite diversity, which may indicate the appearance of grazing animals, since Stromatolites diversity increased when grazing animals went extinct at the End Permian and End Ordovician extinction events, and decreased shortly after the grazer populations recovered. However the discovery that tracks very similar to these early trace fossils are produced today by the giant single-celled protist Gromia sphaerica casts doubt on their interpretation as evidence of early animal evolution.^[50][51]

Groups of animals

Porifera, Radiata and basal Bilateria

he sponges (Porifera) were long thought to have diverged from other animals early.^[52] They lack the complex organization found in most other phyla.^[53] Their cells are differentiated, but in most cases not organized into distinct tissues.^[54] Sponges typically feed by drawing in water through pores.^[55] Archaeocyatha, which have fused skeletons, may represent sponges or a separate phylum.^[56] However, a phylogenomic study in 2008 of 150 genes in 29 animals across 21 phyla revealed that it is the Ctenophora or comb jellies which are the basal lineage of animals, at least among those 21 phyla. The authors speculate that sponges—or at least those lines of sponges they investigated—are not so primitive, but may instead be secondarily simplified.^[57] Among the other phyla, the Ctenophora and the Cnidaria, which includes sea anemones, corals, and jellyfish, are radially symmetric and have digestive chambers with a single opening, which serves as both the mouth and the anus.^[58] Both have distinct tissues, but they are not organized into organs.^[59] There are only two main germ layers, the ectoderm and endoderm, with only scattered cells between them. As such, these animals are sometimes called diploblastic.^[60] The tiny placozoans are similar, but they do not have a permanent digestive chamber.
The remaining animals form a monophyletic group called the Bilateria. For the most part, they are bilaterally symmetric, and often have a specialized head with feeding and sensory organs. The body is triploblastic, i.e. all three germ layers are well-developed, and tissues form distinct organs. The digestive chamber has two openings, a mouth and an anus, and there is also an internal body cavity called a coelom or pseudocoelom. There are exceptions to each of these characteristics, however — for instance adult echinoderms are radially symmetric, and certain parasitic worms have extremely simplified body structures.
Genetic studies have considerably changed our understanding of the relationships within the Bilateria. Most appear to belong to two major lineages: the deuterostomes and the protostomes, the latter of which includes the Ecdysozoa, Platyzoa, and Lophotrochozoa. In addition, there are a few small groups of bilaterians with relatively similar structure that appear to have diverged before these major groups. These include the Acoelomorpha, Rhombozoa, and Orthonectida. The Myxozoa, single-celled parasites that were originally considered Protozoa, are now believed to have developed from the Medusozoa as well.

Deuterostomes

Superb Fairy-wren, Malurus cyaneus

Deuterostomes differ from the other Bilateria, called protostomes, in several ways. In both cases there is a complete digestive tract. However, in protostomes, the initial opening (the archenteron) develops into the mouth, and an anus forms separately. In deuterostomes this is reversed. In most protostomes, cells simply fill in the interior of the gastrula to form the mesoderm, called schizocoelous development, but in deuterostomes, it forms through invagination of the endoderm, called enterocoelic pouching. Deuterostomes also have a dorsal, rather than a ventral, nerve chord and their embryos undergo different cleavage.
All this suggests the deuterostomes and protostomes are separate, monophyletic lineages. The main phyla of deuterostomes are the Echinodermata and Chordata. The former are radially symmetric and exclusively marine, such as starfish, sea urchins, and sea cucumbers. The latter are dominated by the vertebrates, animals with backbones. These include fish, amphibians, reptiles, birds, and mammals.
In addition to these, the deuterostomes also include the Hemichordata, or acorn worms. Although they are not especially prominent today, the important fossil graptolites may belong to this group.
The Chaetognatha or arrow worms may also be deuterostomes, but more recent studies suggest protostome affinities.

Ecdysozoa

Yellow-winged darter, Sympetrum flaveolum

The Ecdysozoa are protostomes, named after the common trait of growth by moulting or ecdysis. The largest animal phylum belongs here, the Arthropoda, including insects, spiders, crabs, and their kin. All these organisms have a body divided into repeating segments, typically with paired appendages. Two smaller phyla, the Onychophora and Tardigrada, are close relatives of the arthropods and share these traits.
The ecdysozoans also include the Nematoda or roundworms, perhaps the second largest animal phylum. Roundworms are typically microscopic, and occur in nearly every environment where there is water. A number are important parasites. Smaller phyla related to them are the Nematomorpha or horsehair worms, and the Kinorhyncha, Priapulida, and Loricifera. These groups have a reduced coelom, called a pseudocoelom.
The remaining two groups of protostomes are sometimes grouped together as the Spiralia, since in both embryos develop with spiral cleavage.

Platyzoa

Pseudobiceros bedfordi, (Bedford's flatworm)

The Platyzoa include the phylum Platyhelminthes, the flatworms. These were originally considered some of the most primitive Bilateria, but it now appears they developed from more complex ancestors.^[61] A number of parasites are included in this group, such as the flukes and tapeworms. Flatworms are acoelomates, lacking a body cavity, as are their closest relatives, the microscopic Gastrotricha.^[62]
The other platyzoan phyla are mostly microscopic and pseudocoelomate. The most prominent are the Rotifera or rotifers, which are common in aqueous environments. They also include the Acanthocephala or spiny-headed worms, the Gnathostomulida, Micrognathozoa, and possibly the Cycliophora.^[63] These groups share the presence of complex jaws, from which they are called the Gnathifera.

Lophotrochozoa

Roman snail, Helix pomatia

The Lophotrochozoa include two of the most successful animal phyla, the Mollusca and Annelida.^[64][65] The former, which is the second-largest animal phylum by number of described species, includes animals such as snails, clams, and squids, and the latter comprises the segmented worms, such as earthworms and leeches. These two groups have long been considered close relatives because of the common presence of trochophore larvae, but the annelids were considered closer to the arthropods because they are both segmented.^[66] Now, this is generally considered convergent evolution, owing to many morphological and genetic differences between the two phyla.^[67]
The Lophotrochozoa also include the Nemertea or ribbon worms, the Sipuncula, and several phyla that have a ring of ciliated tentacles around the mouth, called a lophophore.^[68] These were traditionally grouped together as the lophophorates.^[69] but it now appears that the lophophorate group may be paraphyletic,^[70] with some closer to the nemerteans and some to the molluscs and annelids.^[71][72] They include the Brachiopoda or lamp shells, which are prominent in the fossil record, the Entoprocta, the Phoronida, and possibly the Bryozoa or moss animals.^[73]

Model organisms

 

Because of the great diversity found in animals, it is more economical for scientists to study a small number of chosen species so that connections can be drawn from their work and conclusions extrapolated about how animals function in general. Because they are easy to keep and breed, the fruit fly Drosophila melanogaster and the nematode Caenorhabditis elegans have long been the most intensively studied metazoan model organisms, and were among the first life-forms to be genetically sequenced. This was facilitated by the severely reduced state of their genomes, but as many genes, introns, and linkages lost, these ecdysozoans can teach us little about the origins of animals in general. The extent of this type of evolution within the superphylum will be revealed by the crustacean, annelid, and molluscan genome projects currently in progress. Analysis of the starlet sea anemone genome has emphasised the importance of sponges, placozoans, and choanoflagellates, also being sequenced, in explaining the arrival of 1500 ancestral genes unique to the Eumetazoa.^[74]
An analysis of the homoscleromorph sponge Oscarella carmela also suggests that the last common ancestor of sponges and the eumetazoan animals was more complex than previously assumed.^[75]
Other model organisms belonging to the animal kingdom include the mouse (Mus musculus) and zebrafish (Danio rerio).

History of classification

Aristotle divided the living world between animals and plants, and this was followed by Carolus Linnaeus (Carl von Linné), in the first hierarchical classification. Since then biologists have begun emphasizing evolutionary relationships, and so these groups have been restricted somewhat. For instance, microscopic protozoa were originally considered animals because they move, but are now treated separately.
In Linnaeus's original scheme, the animals were one of three kingdoms, divided into the classes of Vermes, Insecta, Pisces, Amphibia, Reptila, Aves, and Mammalia. Since then the last five have all been subsumed into a single phylum, the Chordata, whereas the various other forms have been separated out. The above lists represent our current understanding of the group, though there is some variation from source to source.