Zoology
Zoology is a biological science that pertains to animals. Animals choose to move whereas plants are moved. Animals feed on bio-organic material and digest it internally. Plants can convert inorganic and organic material into bio-organic material. Cell walls of an animal are flexible. Animal cells possess junctions which are impermeable to fluids (tight junctions), junctions which allow intercellular communication, or the transfer of low molecular-weight substances (gap junctions), and structures which adhere to other cells to form tissue via structural units (desmosomes).

# Genetics
Genetics is the study of the genes, how they affect organisms, and how they are passed from one generation to the next. A scientist that studies genetics is called a geneticist.
Def. a "branch of biology that deals with the transmission and variation of inherited characteristics, in particular chromosomes and DNA" is called genetics.
DNA, deoxyribonucleic acid, is a molecule found in almost every cell of every body of every organism. DNA contains genes, which are the basic instruction codes for making and running a living organism. Strands of DNA make up chromosomes. (Chromosomes are made of DNA, which is made up of genes.)
Genes, through DNA and chromosomes, are passed from one generation to the next by inheritance. Because of this, offspring resemble their parents. Through mixing and occasional mutation, genes can change. When genes change, how the organism forms and how it works may be changed.
How all these changes affect individuals, populations, and entire species is the work of geneticists.
"A recent comparison of the draft sequences of mouse and human genomes has shed light on the selective forces that have predominated in their recent evolutionary histories. In particular, mouse-specific clusters of homologues associated with roles in reproduction, immunity and host defence appear to be under diversifying positive selective pressure, as indicated by high ratios of non-synonymous to synonymous substitution rates. These clusters are also frequently punctuated by homologous pseudogenes. They thus have experienced numerous gene death, as well as gene birth, events. These regions appear, therefore, to have borne the brunt of adaptive evolution that underlies physiological and behavioural innovation in mice. We predict that the availability of numerous animal genomes will give rise to a new field of genome zoology in which differences in animal physiology and ethology are illuminated by the study of genomic sequence variations."

# Biology
Def. the "study of all life or living matter" is called biology.
On the right is a drawing of the archaean Haloquadratum walsbyi.
As the study of biology is the study of all living organisms, zoology is a specialty of biology. All zoologists are also considered biologists.
Other branches of biology include botany (the study of plants), mycology (the study of fungi), phycology (the study of algae), and virology (the study of viruses).

# Animals
The biological definition of animals includes all members of the kingdom Animalia.
Def. "a multicellular organism that is usually mobile, whose cells are not encased in a rigid cell wall (distinguishing it from plants and fungi) and which derives energy solely from the consumption of other organisms" is called an animal.
In colloquial use, as a consequence of anthropocentrism, the term animal is sometimes used nonscientifically to refer only to non-human animals.
Animals are eukaryotic and multicellular, unlike bacteria, which are prokaryotic, and unlike protists, which are eukaryotic but unicellular. Unlike plants and algae, which are autotrophs, producing their own nutrients animals are heterotrophic, feeding on organic material and digesting it internally. With very few exceptions, animals breathe oxygen and respire aerobically. All animals are motile (able to spontaneously move their bodies) during at least part of their life cycle, but some animals, such as sponges, corals, mussels, and barnacles, later become sessile. The blastula is a stage in embryogenesis, embryonic development, that is unique to most animals, allowing cells to be differentiated into specialised tissues and organs.
All animals are composed of cells, surrounded by a characteristic extracellular matrix composed of collagen and elastic glycoproteins. During development, the animal extracellular matrix forms a relatively flexible framework upon which cells can move about and be reorganised, making the formation of complex structures possible. This may be calcified, forming structures such as an exoskeleton of shells, bones, and spicules. In contrast, the cells of other multicellular organisms (primarily algae, plants, and fungi) are held in place by cell walls, and so develop by progressive growth. Animal cells uniquely possess the cell junctions called tight junctions, gap junctions, and desmosomes.
With few exceptionsin particular, the sponges and placozoansanimal bodies are differentiated into tissues. These include muscles, which enable locomotion, and nerve tissues, which transmit signals and coordinate the body. Typically, there is also an internal digestive chamber with either one opening (as in flatworms) or two openings (as in deuterostomes).
Nearly all animals make use of some form of sexual reproduction. They produce haploid gametes by meiosis; the smaller, motile gametes are spermatozoa and the larger, non-motile gametes are ova. These fuse to form zygotes, which develop via mitosis into a hollow sphere, called a blastula. In sponges, blastula larvae swim to a new location, attach to the seabed, and develop into a new sponge. In most other groups, the blastula undergoes more complicated rearrangement. It first invaginates to form a gastrula with a digestive chamber and two separate germ layers, an external ectoderm and an internal endoderm. In most cases, a third germ layer, the mesoderm, also develops between them. These germ layers then differentiate to form tissues and organs.
Repeated instances of inbreeding, mating with a close relative, during sexual reproduction generally leads to inbreeding depression within a population due to the increased prevalence of harmful recessive traits. Animals have evolved numerous mechanisms for inbreeding avoidance, or avoiding close inbreeding. In some species, such as the splendid fairywren (Malurus splendens), females benefit by mating with multiple males, thus producing more offspring of higher genetic quality.
Some animals are capable of asexual reproduction, which often results in a genetic clone of the parent. This may take place through fragmentation; budding, such as in Hydra and other cnidarians; or parthenogenesis, where fertile eggs are produced without mating, such as in aphids.
Animals are categorised into ecological groups depending on how they obtain or consume organic material, including carnivores, herbivores, omnivores, detritivores, and parasites. Interactions between animals form complex food webs. In carnivorous or omnivorous species, predation is a consumer-resource interaction where a predator feeds on another organism (called its prey). Selective pressures imposed on one another lead to an evolutionary arms race between predator and prey, resulting in various anti-predator adaptations. Almost all multicellular predators are animals. Some consumers use multiple methods; for example, in parasitoid wasps, the larvae feed on the hosts' living tissues, killing them in the process, but the adults primarily consume nectar from flowers. Other animals may have very specific feeding behaviours, such as hawksbill sea turtles that primarily eat sponges.
Most animals rely on the energy produced by plants through photosynthesis. Herbivores eat plant material directly, while carnivores, and other animals on higher trophic levels, typically acquire energy (in the form of reduced carbon) by eating other animals. The carbohydrates, lipids, proteins, and other biomolecules are broken down to allow the animal to grow and to sustain biological processes such as locomotion. Animals living close to hydrothermal vents and cold seeps on the dark sea floor do not depend on the energy of sunlight. Rather, archaea and bacteria in these locations produce organic matter through chemosynthesis (by oxidizing inorganic compounds, such as methane) and form the base of the local food web.
Animals originally evolved in the sea. Lineages of arthropods colonised land around the same time as land plants, probably between 510471 million years ago during the Late Cambrian or Early Ordovician. Vertebrates such as the Sarcopterygii, lobe-finned fish, Tiktaalik started to move on to land in the late Devonian, about 375 million years ago. Animals occupy virtually all of earth's habitats and microhabitats, including salt water, hydrothermal vents, fresh water, hot springs, swamps, forests, pastures, deserts, air, and the interiors of animals, plants, fungi and rocks. Animals are however not particularly heat tolerant; very few of them can survive at constant temperatures above 50 C. Only very few species of animals (mostly nematodes) inhabit the most extreme cold deserts of continental Antarctica.

# Theoretical zoology
Def. "that part of biology which relates to the animal kingdom, including the structure, embryology, evolution, classification, habits, and distribution of all animals, both living and extinct" is called zoology.
"Compare both photos, in the photo above and the "civil war soldiers". All this is model of a pterodactyl. Not sure of the age, but I'm sure it was like a practical joke back in the day. The "bigfoot" of it's day so to speak. Compare the photos and you'll see that it's the exact same model. The above photo would be a better looking fake than the civil war photo. Any real dead animal is limp until rigor sets in, and even then, it has to  something right? Why is the guy in the center of the above photo not holding up the main weight of the body? He's practically leaning on it. It's because it's a stiff model that supports itself. And I'm willing to bet that if you were to look at some older photos around the turn of the 19th century of some paleontology exhibits, you'd find this exact same model at some point. The Bone Wars started in the 1880s and made dinosaurs all the rage."

# Zoography
Zoography is descriptive zoology. Animals are described in comparison with other organisms, and their surroundings. This comparison takes into account variation among individuals and populations, and organizes it into systems and classifications that are used for making predictions, such as the evolutionary relationships among organisms.
The branches of zoology focusing on a particular taxonomic group (mammals, etc.) are often considered part of zoography.
Def. a "description of animals, their forms, and habits; descriptive zoology" is called zoography.

# Vertebrate Zoology
Vertebrate zoology is the study of vertebrates, the animals that have a spinal column. This covers only about 3% of all known species.

# Mammalogy
Def. the "study of mammals" is called mammalogy.
Scientists who study mammalogy are called mammalogists.
Mammals are endothermic vertebrates that have fur or hair and feed milk to their young. Milk is a fluid produced by mammary glands.
There are about 5000 species of living mammal known to science. Mammals live in every habitat on Earth. They range in size from the Etruscan shrew (Suncus etruscus), the smallest living mammal, to the blue whale (Balaenoptera musculus) which is the largest animal to have ever lived. Humans (Homo sapiens) are also mammals. They are placed in the taxonomic class Mammalia.
Def. a "taxonomic subclass within the class Reptilia  (or a class or clade within superclass Tetrapoda) - the extinct mammal-like reptiles " is called the Synapsida.
Def. a "taxonomic clade within the clade Reptiliomorpha  the most recent common ancestor of extant mammals and reptiles, and all its descendants; the reptiles, birds and mammals, collectively, all of which have an amnion during development" is called the Amniota.
External Resources:
-  a journal that consists of species accounts of mammals from around the world.

# Archosaurology
Def. the study of the Archosauria is called archosaurology.
The Archosauria include the Pseudosuchia and the Ornithodira.
The Pseudosuchia include the Crocodylomorpha: Crocodylia, Aetosauria, Phytosauria, Rauisuchia and the unplaced families: Atoposauridae, Bernissartiidae, and Peirosauridae.
The Ornithodira include the Dinosauromorpha and Pterosauromorpha.
The Dinosauromorpha include the Dinosauria.

# Ornithology
Def. "the scientific study of birds" is called ornithology.
Scientists who study ornithology are called ornithologists.
Birds are endothermic vertebrates that have feathers and lay eggs. Birds are considered the only living direct descendants of dinosaurs. They belong to the taxonomic class Aves.
There are about 10,000 species of living bird known to science. Birds live in every habitat on Earth. They range in size from the Bee Hummingbird (Mellisuga helenae), the smallest living bird, to the Common Ostrich (Struthio camelus), which standing up to 8 feet (2.4 m) tall, is the tallest living bird.
Def. the group containing the most recent common ancestor of archosaurs and lepidosaurs and all its descendants is called the Sauria.
Sauria may be a crowned-base grouping of diapsids. However, recent genomic studies and comprehensive studies in the fossil record suggest that turtles are closely related to archosaurs, not to parareptiles as previously thought.
External Resources:
-  a world bird database.
Buried "beneath the dry, dusty plains of Central Argentina  the fossilized bones of Argentavis magnificens, now the world's largest known flying bird. With a wingspan of twenty-five feet, and stretching eleven feet from the tip of its bill to the tip of its tail  The giant bird, a new genus and species of teratorn (from the Greek teratos-, meaning wonder, and ornis, meaning bird), was discovered a few years ago by two well-known Argentinian paleontologists, Drs. Eduardo Tonni and Rosendo Pascual.  Argentavis magnificens is the oldest known teratorn. It has been dated at between five and eight million years old, or from the late part of the Miocene epoch, on the basis of the fossil mammals found in the same deposits as the fossil bird. Fossils of these same species of mammals have been found at other sites in Argentina for which radiaometric dates are available."

# Dinosaurology
Def. the "branch of paleontology that focuses on studying dinosaurs" is called dinosaurology.
Usually allied with geology rather than biology, paleontology is the study of past life. Dinosaurology is the specific branch of paleontology that is the study of dinosaurs. Because birds are the living descendants of dinosaurs, ornithology might be considered a branch of dinosaurology.
Scientists who study dinosaurs are usually called paleontologists, but might also be called dinosaurologists.
The non-avian dinosaurs lived during the Mesozoic Era, the time period between roughly 252 and 65 million years ago. The first dinosaurs evolved about 225 million years ago, and all but the birds became extinct by the end of the Mesozoic Era.
Dinosaurologists, like other paleontologists need a good grounding in geology and anatomy, as well as whatever other fields their particular study may involve.

# Crocodylomorphology
Def. the study of the Crocodylomorpha is called crocodylomorphology.

# Archelosaurology
Def. the "study of the Archelosauria" is called archelosaurology.
The Archelosauria include the Crurotarsi and Pan-Testudines.
The Crurotarsi include the Archosauria.
The Pan-Testudines include the Testudinata (turtles and tortoises) and Sauropterygia.

# Cheloniology
Def. the scientific study of turtles is called cheloniology.
Def. the scientific study of tortoises is called testudinology.

# Lepidosaurology
Def. a "taxonomic superorder   the scaled reptiles" is called the Lepidosauria.
Def. a "study of Lepidosauria" is called lepidosaurology.

# Saurology
Def. any "reptile of the order Squamata , usually having four legs, external ear openings, and movable eyelids " is called a lizard.
Def. a "study of lizards" is called saurology.

# Ophiology
Def. the "study of snakes"is called ophiology.
Ophiology is a specialization of herpetology. Scientists who study ophiology are called herpetologists.
The name of this specialty comes from the Greek word for snake, ophis.

# Herpetology
Def. the "branch of biology dealing with reptiles (Reptilia) and amphibians" is called herpetology.
Scientists who study herpetology are called herpetologists.
Reptiles and amphibians are ectothermic vertebrates that have scales or naked skin. Reptiles lay shelled eggs or give birth to live young. Amphibians lay eggs without shells.
There are more than 10,000 species of reptile.
Reptiles also live in every terrestrial habitat on Earth except the artic and antarctic, but some species also occur in marine environments. Reptiles comprise the taxonomic class Reptilia. The living reptiles include lizards, snakes, tuataras, crocodilians, and turtles.
External Resources:

# Batrachology
Def. the "study of amphibians" is called batrachology.
Batrachology is a specialty within herpetology. Scientists who study Batrachology are called herpetologists.
The name of this specialty comes from the Ancient Greek word for frog, batrakhos.
There are about 8000 species of living amphibian known to science.
Amphibians live in every terrestrial habitat on Earth except in the arctic and antarctic. Amphibians are also absent from marine environments. The amphibians comprise the taxonomic class Amphibia. The living amphibians include frogs and toads, salamanders and newts, and caecilians.
Def. a "taxonomic  within the superclass Tetrapoda  the reptile-like amphibians, which gave rise to the amniotes in the Carboniferous" is called the Reptiliomorpha.

# Ichthyology
Def. "the study of fish" is called ichthyology.
Scientists who study ichthyology are called ichthyologists.
Fishes are endothermic vertebrates that live in the fresh or marine water.
Living species are extremely diverse, and belong to three taxonomic groups, Superclass Agnatha (jawless fishes), Class Chondrichthyes (sharks and rays), and the Osteichthyes (the bony fishes).
There are about 30,000 species of living fish known to science. Fish live in every aquatic habitat on Earth.
About the Photo: Considered a "living fossil" because it is an ancient class of lobe-limbed fish that otherwise went extinct more than 70 million years ago, the first living coelacanth was caught off the east coast of South Africa near the Chalumna River in 1938. Today, at least two major populations of coelacanths are known: one off east Africa, and another near Indonesia. Coelacanths cannot be eaten; their flesh contains an oil that is highly distasteful to human beings. This specimen in the image above weighs 160 pounds (72.6 kg) and measures 5.5 feet (1.67 m) in length.
See also:
Fish
External Resources:
-  at the Florida Museum, part of the University of Florida.

# Invertebrate Zoology
Invertebrate zoology is the study of animals that lack a spinal column. This covers approximately 97% of all known living species.

# Echinodermology
Def. study of the echinoderms is called echinodermology.
Echinodermology is a specialty of invertebrate zoology that studies echinoderms. The echinoderms include sea stars, urchins, and crinoids. All species are marine, so this is also a specialty of marine biology. Scientists who study echinoderms most often are called either marine biologists or invertebrate zoologists.

# Malacology
Def. a "soft-bodied invertebrate , " is called a mollusc.
Def. the "study of molluscs" is called malacology.

# Teuthology
Def. "the scientific study of squid (often extended to all cephalopods)" is called teuthology.

# Arthropodology
Def. the "study of arthropods" is called arthropodology.
The Arthropoda include the Chelicerata, Crustacea, Hexapoda, Myriapoda, Marrellomorpha and the Trilobitomorpha.
The classes of the Arthropoda include the Arachnida, Branchiopoda, Cephalocarida, Chilopoda, Diplopoda, Entognatha, Insecta, Malacostraca, Maxillopoda, Merostomata, Ostracoda, Pauropoda, Pycnogonida, Remipedia and Symphyla.

# Chelicerata
The Chelicerata contains the classes: Arachnida, Merostomata, Pycnogonida, Aglaspidida, Chasmataspidida, and Eurypterida.
The subphylum Chelicerata constitutes one of the major subdivisions of the phylum Arthropoda that contains the sea spiders, arachnids (including scorpions, spiders, and potentially horseshoe crabs), and several extinct lineages, such as the eurypterids.

# Arachnology
Arachnology is the study of spiders, scorpions, and related arthropods. Scientists who study arachnology are called arachnologists.
Def. the "study of the Arachnida" is called arachnology.

# Arachnida
The Arachnida include the following orders (Ordines): Amblypygi, Araneae, Haptopoda, Holothyrida, Ixodida, Mesostigmata, Opilioacarida, Opiliones, Palpigradi, Phalangiotarbida, Pseudoscorpiones, Ricinulei, Sarcoptiformes, Schizomida, Scorpiones, Solifugae, Thelyphonida, Trigonotarbida, Trombidiformes, Uraraneida.

# Acarology
Def. the "study of ticks and mites" is called acarology.
"The 6 ordines Holothyrida, Ixodida, Mesostigmata, Opilioacarida, Sarcoptiformes and Trombidiformes are sometimes grouped together as Acari, but there is no consensus on classification above ordinal level, making a Linnean classification difficult."

# Araneology
Def. the "study of the spider, a branch of arachnology" is called araneology.

# Scorpiology
Def. the "scientific study of scorpions" is called scorpiology.
The order (Ordo) is Scorpiones.

# Xiphosurology
Xiphosura is an order of arthropods related to arachnids, or, according to one recent study, actual arachnids, sometimes called horseshoe crabs (a name applied more specifically to the only extant family, Limulidae).
Def. the study of the Merostomata, or Xiphosura, is called Xiphosurology.

# Pantopodology
Def. the study of the Pycnogonida is called pantopodology.

# Carcinology
Def. the "study of crustaceans" is called carcinology.

# Crustacea
The Crustacea contain the classes: Branchiopoda, Cephalocarida, Hexanauplia, Ichthyostraca, Malacostraca, Maxillopoda, Ostracoda and Remipedia.

# Branchiopodology
Def. the study of Branchiopoda is called branchiopodology.

# Brachypodology
The Cephalocarida are a class in the subphylum Crustacea comprising only 12 benthic species discovered in 1955 by Howard L. Sanders, and are commonly referred to as horseshoe shrimps.
Def. the study of the Brachypoda is called brachypodology.

# Malacostracology
Def. the study of the Malacostraca is called Malacostracology.

# Maxillopodology
Def. the study of the Maxillopoda is called maxillopodology.
Maxillopoda is a diverse class of crustaceans including barnacles, copepods and a number of related animals that does not appear to be a monophyletic group, and no single character unites all the members.

# Ostracodology
Def. the study of the Ostracoda is called ostracodology.

# Remipediology
Def. the study of the Remipedia is called remipediology.

# Hexapoda
The Hexapoda consist of the following classes: Entognatha and Insecta.
"Phylogenetic tree of  Hexapoda . Hexapoda (insects and their six-legged relatives) comprise more than half of all described species and dominate terrestrial and freshwater ecosystems. The tree shown is from a maximum likelihood analysis of 8 genes, calibrated by 89 fossils. Membership of major clades is denoted by coloration of the ring (grey: Entognatha, black: basal insects, cyan: Palaeoptera, magenta: Polyneoptera, green: Paraneoptera, red: Holometabola). Changes in branch coloration denote diversification shifts identified using TurboMEDUSA. Branch colors identify regions of the tree with the same underlying diversification model. Symbols at shifts denote a net upshift (diamond) or down shift (circle). Coloration of symbols reflects the robustness of the shift event across 500-scaled samples taken from the post-burin MCMC chain (black: shift recovered in >80% of samples, grey with black outline: recovery >50%, grey with pale outline: recovery >30%, pale grey: recovery<30%). Black rings are shown at 100 Ma increments from the present."

# Entognathology
Def. the study of the Entognatha is called entognathology.

# Entomology
Entomology is the study of insects (Insecta). Scientists who study entomology are called entomologists.
Def. the "scientific study of insects" is called entomology.
The orders (Ordines) for the Insecta include: Archaeognatha, "Blattodea", Coleoptera, Dermaptera, Diptera, Embiidina, Ephemeroptera, Hemiptera, Hymenoptera, Isoptera, Lepidoptera, Mantodea, "Mecoptera", Megaloptera, Neuroptera, Notoptera, Odonata, Orthoptera, Phasmatodea, Phthiraptera, Plecoptera, "Psocoptera", Raphidioptera, Siphonaptera, Strepsiptera, Thysanoptera, Trichoptera, Zoraptera, and Zygentoma.

# Coleopterology
Def. the "scientific study of beetles" is called coleopterology.

# Lepidopterology
Def. the "scientific study of butterflies and moths" is called lepidopterology.
On the right is an example of Callioratis grandis from Mulanje Mountain, Africa.
Lepidopterology is the study of the Lepidoptera, the group of insects that includes butterflies and moths. It is a specialty of entomology, which is the overall study of insects. Most scientists who study the lepidoptera consider themselves to be entomologists.

# Myrmecology
Def. the "study of ants" is called myrmecology.

# Myriapodology
Def. the "scientific study of myriapods" is called myriapodology.

# Myriapoda
The Myriapoda include the classes: Chilopoda, Diplopoda, Pauropoda, Symphyla, and Arthropleuridea.

# Chilopodology
Def. the study of the Chilopoda is called chilopodology.
Most centipedes are generally venomous and could inflict a painful bite, injecting their venom through pincer-like appendage known as forcipules, where despite the name, centipedes can have a varying number of legs, ranging from 30 to 354 and centipedes always have an odd number of pairs of legs. Therefore, no centipede has exactly 100 legs. Similar to spiders and scorpions, centipedes are  predominantly carnivorous.

# Diplopodology
Def. the study of the Diplopoda is called diplopodology.

# Pauropodology
Def. the study of the Pauropoda is called pauropodology.

# Symphylology
Def. the study of the Symphyla is called symphylology.

# Arthropleurideology
Def. the study of the Arthropleuridea is called arthropleurideology.

# Marrellomorpha
Marrellomorphs are a group of arthropods known from the Cambrian to the Early Devonian.

# Trilobitomorpha
Trilobites meaning "three lobes" are a group of extinct marine arachnomorph arthropods that form the class Trilobita. The trilobites were among the most successful of all early animals, existing in oceans for almost 300 million years.
Trilobites are often placed within the arthropod subphylum Schizoramia within the superclass Arachnomorpha (equivalent to the Arachnata), although several alternative taxonomies are found in the literature.
Trilobites had many lifestyles; some moved over the sea bed as predators, scavengers, or filter feeders, and some swam, feeding on plankton; lifestyles expected of modern marine arthropods are seen in trilobites, with the possible exception of parasitism (where scientific debate continues). Some trilobites (particularly the family Olenidae) are even thought to have evolved a symbiotic relationship with sulfur-eating bacteria from which they derived food.

# Trilobitology
Def. the "study of trilobites" is called trilobitology.

# Annelida
The earliest indisputable fossils of the Annelida appear in the Tertiary period, which began 66 million years ago.
There are over 22,000 living annelid species, ranging in size from microscopic to the Australian giant Gippsland earthworm and Amynthas mekongianus (Cognetti, 1922), which can both grow up to 3 meters long. Although research since 1997 has radically changed scientists' views about the evolutionary family tree of the annelids, most textbooks use the  traditional classification into the following sub-groups:
- Polychaetes (about 12,000 species). As their name suggests, they have multiple chetae ("hairs") per segment. Polychaetes have parapodia that function as limbs, and nuchal organs that are thought to be chemosensors. Most are marine animals, although a few species live in fresh water and even fewer on land.
- Clitellates (about 10,000 species ). These have few or no chetae per segment, and no nuchal organs or parapodia. However, they have a unique reproductive organ, the ring-shaped  clitellum ("pack saddle") around their bodies, which produces a pupa (cocoon) that stores and nourishes fertilized eggs until they hatch  or, in moniligastrids, yolky eggs that provide nutrition for the embryos. The clitellates are sub-divided into:
Oligochaetes ("with few hairs"), which includes earthworms. Oligochaetes have a sticky pad in the roof of the mouth. Most are burrowers that feed on wholly or partly decomposed organic materials.
Hirudinea, whose name means "leech-shaped" and whose best known members are leeches. Marine species are mostly blood-sucking parasites, mainly on fish, while most freshwater species are predators. They have suckers at both ends of their bodies, and use these to move rather like inchworms.
- Oligochaetes ("with few hairs"), which includes earthworms. Oligochaetes have a sticky pad in the roof of the mouth. Most are burrowers that feed on wholly or partly decomposed organic materials.
- Hirudinea, whose name means "leech-shaped" and whose best known members are leeches. Marine species are mostly blood-sucking parasites, mainly on fish, while most freshwater species are predators. They have suckers at both ends of their bodies, and use these to move rather like inchworms.
The Archiannelida, minute annelids that live in the spaces between grains of marine sediment, were treated as a separate class because of their simple body structure, but are now regarded as polychaetes. Some other groups of animals have been classified in various ways, but are now widely regarded as annelids:
- Pogonophora / Siboglinidae were first discovered in 1914, and their lack of a recognizable gut made it difficult to classify them. They have been classified as a separate phylum, Pogonophora, or as two phyla, Pogonophora and Vestimentifera. More recently they have been re-classified as a family, Siboglinidae, within the polychaetes.
- The Echiura have a checkered taxonomic history: in the 19th century they were assigned to the phylum "Gephyrea", which is now empty as its members have been assigned to other phyla; the Echiura were next regarded as annelids until the 1940s, when they were classified as a phylum in their own right; but a molecular phylogenetics analysis in 1997 concluded that echiurans are annelids.
- Myzostomida live on crinoids and other echinoderms, mainly as parasites. In the past they have been regarded as close relatives of the trematode flatworms or of the tardigrades, but in 1998 it was suggested that they are a sub-group of polychaetes. However, another analysis in 2002 suggested that myzostomids are more closely related to flatworms or to rotifers and acanthocephales.
- Sipuncula  was originally classified as annelids, despite the complete lack of segmentation, bristles and other annelid characters. The phylum Sipuncula was later allied with the Mollusca, mostly on the basis of developmental and larval characters. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida. Subsequent analysis of the mitochondrion's DNA has confirmed their close relationship to the Myzostomida and Annelida (including echiurans and Siboglinidae (pogonophorans). It has also been shown that a rudimentary neural segmentation similar to that of annelids occurs in the early larval stage, even if these traits are absent in the adults.
On the right, "This is a parchment worm (Thelepus cincinatus) that is out of its tube. It is also called a Bristleworm and puts out its tentacles to collect its microscopic food."

# Brachiopodology
Def. the study of the Brachiopoda is called brachiopodology.
Lineages of brachiopods that have both fossil and extant taxa appeared in the early Cambrian, Ordovician, and Carboniferous periods, respectively.
The largest brachiopods known  Gigantoproductus and Titanaria, reaching 30 to 38 cm in width  occurred in the upper part of the Lower Carboniferous.
Brachiopod shells can be classified according to the angle between the cardinal plane and the plane where the shells join (commissure); anacline shells have an angle of less than 90, whereas aspacline shells have a higher angle.
Impunctate shells as solid without any tissue inside them, and pseudopunctate shells are only known from fossil forms.

# Bryozoology
Def. a "branch of zoology specializing in Bryozoa" is called bryozoology.

# Chaetognatha
The Chaetognatha fossil range is Lower Cambrian to Recent.
There are more than 120 modern species assigned to over 20 genera. Despite the limited diversity of species, the number of individuals is large.
All chaetognaths are carnivorous, preying on other planktonic animals.
Two chaetognath species, Caecosagitta macrocephala and Eukrohnia fowleri, have bioluminescent organs on their fins.
Chaetognaths swim in short bursts using a dorso-ventral undulating body motion, where their tail fin assists with propulsion and the body fins with stabilization and steering. Some species are known to use the neurotoxin tetrodotoxin to subdue prey.

# Cnidariology
Def. the study of the Cnidaria is called cnidariology.
Def. the study of the Myxozoa is called myxozoology.
Myxozoans were originally considered protozoan, and were included among other non-motile forms in the group Sporozoa. As their distinct nature became clear through 18S ribosomal DNA (rDNA) sequencing, they were relocated in the metazoa. Detailed classification within the metazoa was however long hindered by conflicting rDNA evidence: although 18S rDNA suggested an affinity with Cnidaria, other rDNA sampled, and the HOX genes of two species, were more similar to those of the Bilateria.
Further testing resolved the genetic conundrum by sourcing the first three previously identified discrepant HOX genes  (Myx1-3) to the bryozoan Cristatella mucedo and the fourth (Myx4) to Northern pike, the respective hosts of the two corresponding Myxozoa samples. This explained the confusion: the original experiments had used samples contaminated by tissue from host organisms, leading to false positives for a position among the Bilateria. More careful cloning of 50 coding genes from Buddenbrockia firmly established the clade as severely modified members of the phylum Cnidaria, with medusozoans as their closest relatives. Similarities between myxozoan polar capsules and cnidarian nematocysts had been drawn for a long time, but were generally assumed to be the result of convergent evolution.
Taxonomists now recognize the outdated subgroup Actinosporea as a life-cycle phase of Myxosporea.

# Ctenophorology
Def. the study of the Ctenophora is called ctenophorology.
The Beroida, also known as Nuda, have no feeding appendages, but their large pharynx, just inside the large mouth and filling most of the saclike body, bears "macrocilia" at the oral end which are fused bundles of several thousand large cilia are able to "bite" off pieces of prey that are too large to swallow whole  almost always other ctenophores.
In front of the field of macrocilia, on the mouth "lips" in some species of Beroe, is a pair of narrow strips of adhesive epithelial cells on the stomach wall that "zip" the mouth shut when the animal is not feeding, by forming intercellular connections with the opposite adhesive strip, which streamlines the front of the animal when it is pursuing prey.

# Cycliophorology
Def. the study of the Cycliophora is called cycliophorology.

# Gastrotrichology
Def. the study of the Gastrotricha is called gastrotrichology.
As of 2011, around 790 species have been described. The phylum contains a single class, divided into two orders: the Macrodasyida and the Chaetonotida. The Macrodasyida are wholly marine, but two rare and poorly known species, Marinellina flagellata and Redudasys fornerise, are known from fresh water. The Chaetonotida comprise both marine and freshwater species.
Phylogenetics places the Gastrotricha as close relatives of the flatworms Platyhelminthes, the Ecdysozoa or the Lophotrochozoa.

# Gnathostomulidology
Def. the study of the Gnathostomulida is called gnathostomulidology.
The Gnathostomulida were first recognised and described in 1956.
In many Bursovaginoidea, one of the major group of gnathostomulids, the neck region is slightly narrower than the rest of the body, giving them a distinct head.
Like flatworms they have a ciliated epidermis, but in contrast to flatworms, they have one cilium per cell.
The only sense organs are modified cilia, which are especially common in the head region.
The basal plate is used to scrape smaller organisms off of the grains of sand that make up their anoxic seabed mud habitat.
The mouth opens into a blind-ending tube in which digestion takes place; there is no true anus. However, there is tissue connecting the intestine to the epidermis which may serve as an anal pore.

# Hemichordatology
Def. the study of the Hemichordata is called hemichordatology.
The extinct class Graptolithina is closely related to the pterobranchs.
Acorn worms are solitary worm-shaped organisms that generally live in burrows (the earliest secreted tubes).
While the family Torquaratoridae are free living detritivores, many are well known for their production and accumulation of various halogenated phenols and pyrroles. Pterobranchs are filter-feeders, mostly colonial, living in a collagenous tubular structure called a coenecium.
A post-anal tail is present in juvenile member of the acorn worm family Harrimaniidae.

# Kamptozoology
Def. the study of the Kamptozoa is called kamptozoology.
Entoprocta, coined in 1870, means "anus inside". The alternative name "Kamptozoa", meaning "bent" or "curved" animals, where the prefix "campto-" is explained was assigned in 1929. Some authors use Entoprocta while others prefer Kamptozoa.
Most species are colonial, and their members are known as "zooids", since they are not fully independent animals. Zooids are typically 1 mm long but range from 0.1 to 7 mm long.

# Kinorhynchology
Def. the study of the Kinorhyncha is called kinorhynchology.
Modern species of the Kinorhyncha are 1 mm or less, but Cambrian forms could reach 4 cm.
Unlike some similar invertebrates, they do not have external cilia, but instead have a number of spines along the body, plus up to seven circles of spines around the head.
The head is completely retractable, and is covered by a set of neck plates called placids when retracted. The short hind-gut is lined by cuticle, and empties into an anus at the posterior end of the trunk. The excretory system consists of two protonephridia emptying through pores in the final segment. Some species have simple ocelli on the head, and all species have tiny bristles on the body to provide a sense of touch.

# Loriciferology
Def. the study of the Loricifera is called loriciferology.
Loricifera is a phylum of very small to microscopic marine cycloneuralian sediment-dwelling animals with 37 described species, in nine genera. Aside from these described species, there are approximately 100 more that have been collected and not yet described. Their sizes range from 100 m to ca. 1 mm. Their habitat is in the spaces between marine gravel to which they attach themselves. The phylum was discovered in 1983 by Reinhardt Kristensen, in Roscoff, France. They are among the most recently discovered groups of Metazoans. They attach themselves quite firmly to the substrate, and hence remained undiscovered for so long. The first specimen was collected in the 1970s, and later described in 1983. They are found at all depths, in different sediment types, and in all latitudes.
The armor-like lorica consists of a protective external shell or case of encircling plicae. Many of the larvae are acoelomate, with some adults being pseudocoelomate, and some remaining acoelomate.
Fossils have been dated to the late Cambrian.
Morphological studies have traditionally placed the phylum in the Vinctiplicata with the Priapulida; this plus the Kinorhyncha constitutes the taxon Scalidophora, or Cephalorhyncha, where the three phyla share four characters in common  chitinous cuticle, rings of scalids on the introvert, flosculi, and two rings of introvert retracts.  However, mounting molecular evidence indicates a closer relationship with the Panarthropoda.

# Micrognathozoology
Def. the study of the Micrognathozoa is called micrognathozoology.
Limnognathia maerski is a microscopic platyzoan freshwater animal, discovered living in warm springs on Disko Island, Greenland in 1994, that has variously been assigned as a class or subphylum in the phylum Gnathifera or as a phylum in a Gnathifera superphylum, named Micrognathozoa, related to the rotifers and gnathostomulids, grouped together as the Gnathifera.

# Nematodology
Def. the study of the Nematoda is called nematodology.
The nematodes or roundworms  constitute the phylum Nematoda, also called Nemathelminthes.
A 2013 survey of animal biodiversity published in the mega journal Zootaxa puts the number of species at over 25,000.
Nematods are found in every part of the earth's lithosphere, even at great depths, 0.93.6 km below the surface of the Earth in gold mines in South Africa. They represent  90% of all animals on the ocean floor. In total, 4.4  1020 nematodes inhabit the Earth's topsoil, or approximately 60 billion for each human, with the highest densities observed in tundra and boreal forests. Their numerical dominance, often exceeding a million individuals per square meter and accounting for about 80% of all individual animals on earth, their diversity of lifecycles, and their presence at various trophic levels point to an important role in many ecosystems. They have been shown to play crucial roles in polar ecosystem. The roughly 2,271 genera are placed in 256 families. The many parasitic forms include pathogens in most plants and animals. A third of the genera occur as parasites of vertebrates; about 35 nematode species occur in humans.
"In short, if all the matter in the universe  except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its  mountains, hills, vales, rivers, lakes, and  oceans represented by a film of nematodes. The location of towns would  be decipherable, since for every massing of human beings, there would be a corresponding massing of certain nematodes. Trees would still stand  in ghostly rows representing  our streets and highways. The location of the various plants and  animals would still be decipherable, and, had  we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites."

# Nematomorphology
Def. the study of the Nematomorpha is called nematomorphology.
The adult worms are free-living, but the larvae are parasitic on arthropods, such as beetles, cockroaches, mantids, orthopterans, and crustaceans. About 351 freshwater species are known and a conservative estimate suggests that there may be about 2000 freshwater species worldwide. The name "Gordian" stems from the legendary Gordian knot. This relates to the fact that nematomorpha often tie themselves in knots.
The nervous system consists of a nerve ring near the anterior end of the animal, and a ventral nerve cord running along the body.

# Nemerteology
Def. the study of the Nemertea is called nemerteology.
Nemertea is a phylum of invertebrate animals also known as ribbon worms or proboscis worms. Alternative names for the phylum have included Nemertini, Nemertinea and Rhynchocoela.
In 1995, a total of 1,149 species had been described and grouped into 250 genera.
Nemertea are named after the Greek sea-nymph Nemertes, one of the daughters of Nereus and Doris. Alternative names for the phylum have included Nemertini, Nemertinea, and Rhynchocoela. The Nemertodermatida are a separate phylum, whose closest relatives appear to be the Acoela.

# Onychophorology
Def. the study of the Onychophora is called onychophorology.
Onychophora, commonly known as velvet worms (due to their velvety texture and somewhat wormlike appearance) or more ambiguously as peripatus (after the first described genus, Peripatus), is a phylum of elongate, soft-bodied, many-legged panarthropods. In appearance they have variously been compared to worms with legs, caterpillars, and slugs.
"Because they resemble worms with legs ... Superficially they resemble caterpillars, but have also been compared with slugs."
It is the only phylum within Animalia that is wholly endemic to terrestrial environments. Velvet worms are considered close relatives of the Arthropoda and Tardigrada, with which they form the taxon Panarthropoda.

# Phoronidology
Def. the study of the Phoronida is called phoronidology.
Phoronis is the name of one of the two genera of Phoronids.
As of 2010 there are no indisputable body fossils of phoronids.
Phoronids may be members of the protostome super-phylum Lophotrochozoa.
Phoronids and brachiopods may be sister-groups, while others place phoronids as a sub-group within brachiopoda.
The diagram on the right describes the anatomy of an adult phoronid.
Most adult phoronids are 2 to 20 cm long and about 1.5 mm wide,
 although the largest are 50 cm long. Their skins have no cuticle but secrete rigid tubes of chitin, similar to the material used in arthropods' exoskeletons, and sometimes reinforced with sediment particles and other debris. Most species' tubes are erect, but those of Phoronis vancouverensis are horizontal and tangled.  Phoronids can move within their tubes but never leave them. The bottom end of the body is an ampulla (a flask-like swelling in a tube-like structure), which anchors the animal in the tube and enables it to retract its body when threatened, reducing the body to 20 percent of its maximum length. Longitudinal muscles retract the body very quickly, while circular muscles slowly extend the body by compressing the internal fluid.
For feeding and respiration each phoronid has at the top end a lophophore, a "crown" of tentacles with which the animal filter-feeds. In small species the "crown" is a simple circle, in medium-size species it is bent into the shape of a horseshoe with tentacles on the outer and inner sides, and in the largest species the ends of the horseshoe wind into complex spirals. These more elaborate shapes increase the area available for feeding and respiration. The tentacles are hollow, held upright by fluid pressure, and can be moved individually by muscles.
The mouth is inside the base of the crown of tentacles but to one side. The gut runs from the mouth to one side of the stomach, in the bottom of the ampulla. The intestine runs from the stomach, up the other side the body, and exits at the anus, outside and a little below the crown of tentacles. The gut and intestine are both supported by two mesenteries (partitions that run the length of the body) connected to the body wall, and another mesentery connects the gut to the intestine.
The body is divided into coeloms, compartments lined with mesothelium. The main body cavity, under the crown of tentacles, is called the metacoelom, and the tentacles and their base share the mesocoelom. Above the mouth is the epistome, a hollow lid which can close the mouth. The cavity in the epistome is sometimes called the protocoelom, although other authors disagree that it is a coelom and Ruppert, Fox and Barnes think it is built by a different process.

# Placozoology
Def. the study of the Placozoa is called placozoology.
The Placozoa are a basal form of free-living (non-parasitic) multicellular organism. They are the simplest in structure of all animals. Three genera have been found: the classical Trichoplax adhaerens, Hoilungia hongkongensis, and Polyplacotoma mediterranea, where the last appears most basal. The last two have been found only since 2017. Although the Placozoa were first discovered in 1883 and since the 1970s more systematically analyzed, a common name does not yet exist for the taxon; the scientific name means "flat animals".

# Platyhelminthology
Def. the study of the Platyhelminthes is called platyhelminthology.
Like other bilaterians, they have three main cell layers (endoderm, mesoderm, and ectoderm), while the radially symmetrical cnidarians and ctenophores (comb jellies) have only two cell layers. Beyond that, they are "defined more by what they do not have than by any particular series of specializations." Unlike other bilaterians, Platyhelminthes have no internal body cavity, so are described as acoelomates. They also lack specialized circulatory and respiratory organs, both of these facts are defining features when classifying a flatworm's anatomy. Their bodies are soft and unsegmented.

# Poriferology
Def. the study of the Porifera is called poriferology.
Def. a "branch of zoology concerning sponges or Porifera" is called spongiology.

# Rhombozoology
Def. the study of the Rhombozoa is called rhombozoology.
Classification is controversial. Traditionally, dicyemids have been grouped with the Orthonectida in the Mesozoa, and, as of 2017, molecular evidence appears to confirm this.
However, other molecular phylogenies have placed the dicyemids more closely related to the Nematoda (roundworms). Additional molecular evidence suggests that this phylum is derived from the Lophotrochozoa.
The phylum is not divided in classes or orders, but contains three families, Conocyemidae, Dicyemidae, and Kantharellidae.

# Rotiferology
Def. the study of the Rotifera is called rotiferology.
Def. any "of many minute aquatic multicellular organisms, of the phylum Rotifera, that have a ring of cilia resembling a wheel" is called a rotifer.

# Scalidophorology
Def. the study of the Scalidophora is called scalidophorology.
Scalidophora is a group of marine pseudocoelomate protostomes that was proposed on morphological grounds to unite three phyla: the Kinorhyncha, the Priapulida and the Loricifera. The three phyla have four characters in common  chitinous cuticle that is moulted, rings of scalids on the introvert, flosculi, and two rings of introvert retracts. However, the monophyly of the Scalidophora is not supported by molecular studies, where the position of the Loricifera was uncertain or as sister to the Panarthropoda.
The group has also been considered a single group, Cephalorhyncha, with three classes.
The group is named after the spines (scalids) covering the introvert (head that can be retracted into the trunk).

# Sipunculology
Def. the study of the Sipuncula is called sipunculology.
The worm Sipunculus nudus was first described in 1767. In 1814, the word "Sipuncula" was used to describe the family (now Sipunculidae), and in time, the term came to be used for the whole phylum. This is a relatively understudied group, and it is estimated there may be around 162 species worldwide.
The phylogenetic placement of this phylum in the past has proved troublesome. Originally classified as annelids, despite the complete lack of segmentation, bristles and other annelid characters, the phylum Sipuncula was later allied with the Mollusca, mostly on the basis of developmental and larval characters. Currently these two phyla have been included in a larger group, the Lophotrochozoa, that also includes the annelids, the nemertea (ribbon worms) and several other phyla. Phylogenetic analyses based on 79 ribosomal proteins indicated a position of Sipuncula within Annelida. Subsequent analysis of the mitochondrion's DNA has confirmed their close relationship to the Myzostomida and Annelida (including echiurans and Siboglinidae (pogonophorans)). It has also been shown that a rudimentary neural segmentation similar to that of annelids occurs in the early larval stage, even if these traits are absent in the adults.

# Tardigradology
Def. the study of the Tardigrada is called tardigradology.
Tardigrades, known colloquially as water bears or moss piglets, are a phylum of water-dwelling eight-legged segmented micro-animals. They were first described by the German zoologist Johann August Ephraim Goeze in 1773, who called them little water bears. In 1777, the Italian biologist Lazzaro Spallanzani named them Tardigrada, which means "slow steppers".
They have been found everywhere, from mountaintops to the deep sea and mud volcanoes, and from tropical rain forests to the Antarctic. Tardigrades are among the most resilient animals known, with individual species able to survive extreme conditionssuch as exposure to extreme temperatures, extreme pressures (both high and low), air deprivation, radiation, dehydration, and starvationthat would quickly kill most other known forms of life. Tardigrades have survived exposure to outer space. About 1,150 known species form the phylum Tardigrada, a part of the superphylum Ecdysozoa. The group includes fossils dating from 530 million years ago, in the Cambrian period.
Tardigrades are usually about 0.5 mm long when fully grown. They are short and plump, with four pairs of legs, each ending in claws (usually four to eight) or sucking disks. Tardigrades are prevalent in mosses and lichens and feed on plant cells, algae, and small invertebrates. When collected, they may be viewed under a very low-power microscope, making them accessible to students and amateur scientists.

# Xenacoelomorphology
Def. the study of the Xenacoelomorpha is called xenacoelomorphology.
Xenacoelomorpha is a basal bilaterian phylum of small and very simple animals, grouping the xenoturbellids with the acoelomorphs. This grouping was suggested by morphological synapomorphies, and confirmed by phylogenomic analyses of molecular data.
The clade Xenacoelomorpha, grouping Acoelomorpha and the genus Xenoturbella, was revealed by molecular studies. Initially it was considered to be a member of the deuterostomes, but a more recent transcriptome analysis concluded that it is the sister group to the Nephrozoa, which includes the protostomes and the deuterostomes, being therefore the basalmost bilaterian clade.

# Systematics & Taxonomy
Superregnum: Eukaryota
Systematics is the study of the evolutionary relationships among organisms. It includes taxonomy, which is the study of the names of organisms and their organization into categories. Scientists who study systematics are called systematists, while taxonomists primarily study taxonomy. Most systematists are also taxonomists.
Living organisms are grouped into a hierarchy of categories. The main categories are Kingdom (or Regnum, pl. Regna), Phylum (pl. Phyla), Class, Order, Family, Genus (pl. Genera), and Species (pl. Species). Every living organism is classified into one, and only one, of each of those categories. There are additional categories used for some groups, but not all.
The classification of organisms can change as more is learned about the living world around us.
Below are two ideas of how life may be divided into Kingdoms (Regna).
- Four Regna listed by Whittaker & Margulis (1978): Animalia - Plantae - Fungi - Protista
- Five Regna listed Cavalier-Smith (1981): Animalia - Plantae - Fungi - Chromista - Protozoa
Below is one recent list of the Phyla of within the Kingdom Animalia. Note number 8, Chordata. That is the Phylum that includes vertebrates, the animals with a spinal column that includes humans.
Regnum: Animalia
Phyla (36):
- Acanthocephala
- Annelida
- Arthropoda
- Brachiopoda
- Bryozoa
- Cephalorhyncha
- Chaetognatha
- Chordata
- Cnidaria
- Ctenophora
- Cycliophora
- Echinodermata
- Echiura
- Gastrotricha
- Gnathostomulida
- Hemichordata
- Kamptozoa
- Kinorhyncha
- Loricifera
- Micrognathozoa
- Mollusca
- Nematoda
- Nematomorpha
- Nemertea
- Onychophora
- Orthonectida
- Phoronida
- Placozoa
- Platyhelminthes
- Porifera
- Rhombozoa
- Rotifera
- Sipuncula
- Tardigrada
- Xenacoelomorpha

# Parasitology
"Humans are hosts to nearly 300 species of parasitic worms and over 70 species of protozoa, some derived from our primate ancestors and some acquired from the animals we have domesticated or come in contact with during our relatively short history on Earth".
Parasitology is the study of parasites. In this context, a parasite is usually an invertebrate, and as such parasitology is a  specialization of invertebrate zoology. Scientists that study parasites are called parasitologists.
Parasites are organisms that live on or in, and take their nutrition from, another living organism called a host. Rarely do parasites kill their hosts, since finding a new host can be quite difficult. Individual parasites may do little harm to their host, but large numbers of them can cause great problems. This kind of relationship where one organism lives off another giving either nothing in return or causing harm is called parasitism.
Parasites may be found in many invertebrate taxonomic groups. Commonly encountered parasites include tapeworms, roundworms, and lice.

# Helminthology
Def. the "branch of zoology related to the study of helminths (parasitic worms)" is called helminthology.

# Acanthocephalology
Def. the study of the Acanthocephala is called acanthocephalology.
The "Acanthocephala are descended from, and should be considered as, highly modified rotifers. Genetic research has determined this is unequivocal; the Acanthocephalans are modified rotifers".
Acanthocephalans have complex life cycles, involving at least two hosts, which may include invertebrates, fish, amphibians, birds, and mammals. About 1420 species have been described.

# Orthonectidology
Def. the study of the Orthonectida is called orthonectidology.
Orthonectida is a small phylum of poorly known parasites of marine invertebrates.
The adults are microscopic wormlike animals, consisting of a single layer of ciliated outer cells surrounding a mass of sex cells, that swim freely within the bodies of their hosts, which include flatworms, polychaete worms, bivalve molluscs, and echinoderms, and are gonochoristic, with separate male and female individuals.
When they are ready to reproduce, the adults leave the host, and sperm from the males penetrate the bodies of the females to achieve internal fertilisation, where the resulting zygote develops into a ciliated larva that escapes from the mother to seek out new hosts so as to lose its cilia and develop into a syncytial plasmodium larva, which, in turn, breaks up into numerous individual cells that become the next generation of adults.

# Hypotheses
An hypothesis is a scientific statement made to explain some part of the world. It is not a guess. It is testable by scientific means, and is falsifiable (able to be disproved) by those means. Coming up with hypotheses and testing them are the main occupation of science.
- The genetic classification of animals may not match the current classification before genetic evidence.

# Acknowledgements
The content on this page was first contributed by: Henry A. Hoff.
Initial content for this page in some instances came from Wikiversity.