• Organization of the animal kingdom

We explain to you what is the classification of the animal kingdom, its history and what are the phyla and sub-phyla that are used today.

Many different species coexist in the animal kingdom.

Organization of the animal kingdom

The biology lists animals as members of the Animal Kingdom or Animalia, one of the great chapters in which the life known, distinct from the realm of plants, the mushrooms or those of microorganisms. However, within this kingdom there are very numerous species of different animals, which deserve an extensive classification by specialists.

This classification is always changing, as the nature of life is understood more and better. It had its beginnings in the antiquity itself, since the desire to human being By understanding and categorizing what surrounds him, he does not exclude, far from it, the animals with whom he shares the Earth.

The first systems of animal classification date from the 4th century BC. C., and they remained more or less in vogue until practically the eighteenth century, when the Scientific Revolutions and the Modernity allowed the formal emergence of scientific thought.

The most important of these historical classifications is perhaps that of Charles Linnaeus in 1735 or 1758, since it laid the foundations for future classifications such as those of Leuckart, Lankester, Grobben, Bütschli, Hyman or Nielsen, to name a few.

Many of the traditional precepts of these classifications have been abandoned over time, although many others inspired more modern and comprehensive versions, adapted to contemporary technology and its amazing discoveries.

Thus, for example, traditionally groups of animals were organized (called phyla or phylum), in two large initial categories, which were vertebrates (those endowed with a backbone) and invertebrates (those who do not have it).

This order has today been transformed into a similar one, which distinguishes between bilateral and non-bilateral phyla, that is, those whose animals present or do not present bilateral symmetry respectively (their body can be divided into two identical longitudinal halves).

From this first distinction, we can advance in the classification of the almost 1,454,000 known animal species, as follows:

Non-bilateral edges, whose animals do not show bilateral but radial symmetry, or none. They are all invertebrates and comprise the following phyla:

• Phylum porífera ("Pore carriers"), immobile and asymmetric animals, whose body has pores to inhale the Water surrounding. They are traditionally known as sponges, and there are some 9,000 described species.
• Phylum cnidaria (“Nettles”), simple aquatic animals that possess cells stinging or poisons called cnidocytes, such as jellyfish and anemones. They are an evolutionarily extremely ancient group, of which some 10,000 different species are known.
• Phylum ctenophora (“Comb bearers”), exclusively marine, bioluminescent animals that capture their microscopic prey (plankton) by sticky comb-like filaments (cilia). Only 166 of its species are known.
• Phylum placozoa ("Plate animals"), aquatic animalsflattened and crawling, they have the simplest body structure known on the planet, barely above the protozoa. Only one species of them is known, but it is estimated that there could be as many as 100 still unknown.

Bilateral edges, whose animals have bilateral symmetry, that is, symmetrical bodies from a vertical axis that crosses them in the middle. They are the most diverse of this classification, which includes the edges:

• Phylum annelida (“Little rings”), invertebrate animals with a worm shape, ringed body and habitats wet, such as earthworms, leeches, or polychaete marine worms. 17,200 different species of annelids are known.
• Phylum arthropoda (“Jointed legs”), is one of the most evolutionarily successful phyla of all, in which there is the largest number of known species, that is, the largest biodiversity that exists on the planet. They are invertebrate animals with a rigid chitinous shell and jointed limbs, such as insects, crustaceans, arachnids and myriapods. They are present in all ecosystems of the world and play all possible ecological roles.
• Phylum brachiopoda (“Short legs”), composed of animals with two valves (rigid shells) joined in their posterior region of the body, inhabitants of the ocean floor, where they are attached to hard materials through a single foot they have, or they are they bury themselves in sand or other soft substrates. Only 335 current species are known, although there are fossil records of more than 16,000 extinct species.
• Phylum bryozoa ("Moss-animals"), comprises small colonial animals, similar to brachiopods, that lead a fixed life, feeding through a crown of tentacles that they use to filter water and capture microorganisms. About 5,700 species are known, of which only 50 live in fresh water.
• Phylum chaetognatha ("Spiny jaws"), animals better known as "arrow worms", which make up marine zooplankton throughout the planet. Are predators, with almost transparent bodies and torpedo shape, measuring between 2 mm. and 12 cm. Some 121 different species are known.
• Phylum chordata ("Endowed with rope"), another of the highly biodiverse animal phyla, adapted to almost all ecological niches. Chordates are characterized by presenting during their embryonic development: a dorsal or notochord cord, a hollow nerve cord that will later give rise to the spinal cord, and a postanal tail at some point in their development. In addition, they have a complete digestive system (with a differentiated mouth and anus). This phylum encompasses three different subphiles of immense biological diversity, for which they deserve to be named:
  • Subphylum urochordata or tunicata, a group of more than 2,000 marine species with different types of colonial, solitary or benthic life, of sexuality hermaphrodite Y life cycle very inactive. They are often considered the "lower" chordates.
  • Subphylum cephalochordata ("Rope-headed"), a group of just 33 species that inhabit coastal areas and sandy bottoms, considered the evolutionary link between the lower and higher chordates or vertebrates.
  • Subphylum vertebrata or craniata (“With vertebrae” or “with skull”), the most diverse and varied group in this entire segment, in which there are around 73,000 different species of animals endowed with a vertebral column and a skull, or at least sketches of them , and found in all possible habitats. The most evolutionarily complex animals are found in this line, organized into different classes: myxini (hagfish), hyperoartia (lampreys), chondrichthyes (cartilaginous fish), actinopterygii (ray-finned fish), sarcopterygii (fish with lobe fins), amphibia (amphibians), reptilia (reptiles), mammalia (mammals) Y birds (birds).
• Phylum Cycliophora ("Carrier of wheels"), edge of a single genus of known animals, of habits symbiotic, discovered in 1995 in the mouth of a marine crustacean. They have a suction cup to fixate on and a tentacular crown with which to feed.
• Phylum echinodermata (“Spiny skin”), this is the only case of animals with secondary pentaradial symmetry, and an aquifer vascular system, which makes them an exception within the general tendency of the classification. They have a calcareous internal skeleton, often comprising spines and quills, like sea urchins and starfish. Around 7,000 current species are known.
• Phylum entoprocta (“Inner anus”), small aquatic animals of which about 170 species are known, shaped like a chalice and a tentacular crown used to filter the water and in which the anal cone is also found. They do not have a respiratory or circulatory system.
• Phylum gastrotricha ("Hair stomach"), tiny aquatic animals (less than 4 mm.) That feed on organic material from bacteria, mushrooms and protozoa, and serve as food for other species such as annelids, nematodes and arthropods marine. Around 400 different species are known.
• Phylum gnathostomulida (“Small jaw-bearing mouth”), worm-shaped animals and marine habitats, which feed on fungi and other microorganisms in anaerobic environments. Its nearly 100 known species have jawed mouths, despite their diminutive size.
• Phylum hemichordata (“With half a rope”), vermiform-looking animals that do not have a true notochord during their embryonic formation (therefore, they are not chordates), but something very similar, so they could be evolutionarily related. Currently only about 100 species are known.
• Phylum kinorhyncha ("Moving trunk"), these small marine invertebrates inhabit the ocean floor of the entire planet, and move through a series of special spines that they have on the back of the body. There are about 250 known species.
• Phylum loricifera (“Coat-bearers”), is an edge of just 28 marine species discovered in 1983, whose bodies are covered by an external skeleton in the form of armor. They are one of the very few cases of animals that can live without oxygen.
• Phylum micrognathozoa ("Small animals with jaws"), microscopic invertebrates whose only species was discovered in 2000. They have very complex jaws made up of 32 moving parts, an exception among invertebrates.
• Phylum mollusca ("Soft"), another of the most extensive phyla in the animal kingdom, with around 93,000 known species. In general, they are non-segmented invertebrates, with a soft body sometimes protected by a calcareous shell, and which can have various tentacular extremities. They can be found in almost all aquatic habitats and even on land, and they present a great diversity of species, among which are clams, squid, octopus, slugs, oysters, snails, etc.
• Phylum nematoda ("Similar to a thread"), composed of animals shaped like round or cylindrical worms, of which 25,000 species are known but it is estimated that there could be around 500,000, thus being the fourth most numerous phylum in the classification. Essentially aquatic life, although many species have a life parasitic, both from the body of plants and animals, including the human being. Many nematodes are the cause of gastrointestinal diseases.
• Phylum nematomorpha ("Thread-like"), a phylum of 320 species of parasitoid worms very similar to nematodes, whose adults live freely in streams or ponds, but their larvae parasitize arthropods or leeches. Its presence in certain environments is understood as a symptom of water pollution.
• Phylum nemertea ("Marine nymph"), a group of non-segmented worms, somewhat flattened and less than 20 cm long, whose bodies have a characteristic proboscis or trunk. Free-living and aquatic habitat, some 1,200 different species are known.
• Phylum onychophora ("Claw carrier"), known as velvety worms, it is about 180 known species of worms similar to the caterpillars of insects, since they have legs ending in small nails or claws.
• Phylum phoronida (“Descendant of foroneum”), phylum composed of 20 species of tentaculated animals, with a “U” -shaped body, which inhabit the benthic regions of the sea, creating colonies on dry substrates or soft sediments.
• Phylum platyhelminthes (“Flat worms”), composed of flattened and hermaphroditic worms that inhabit aquatic, terrestrial and even aerial environments, and many of which lead parasitic lives. They are simple animals that present interneurons, especially concentrated in a specific region of the body, which suggests that they are an intermediate step towards the evolution of a nervous system. Some 20,000 different species are known.
• Phylum priapulida (from Priapus, a Greek deity with a huge phallus), worm-shaped marine animals that dig galleries with their trunks in sandy or muddy bottoms. Its size varies between 5 mm. and 40 cm., and only 18 species are known.
• Phylum rhombozoa (“Rhombus-animals”), microscopic invertebrates with parasitic life, which infect fish, mollusks and especially cephalopods, whose bodies are made up of only 30 to 50 cells. 75 different species are known.
• Phylum rotifera (“Wheel carriers”), a phylum of microscopic animals that inhabit fresh waters, humid earth or even mosses and lichens, of which 2,200 species are known.
• Phylum tardigrada (“Slow steps”), composed of around 1000 different species of tardigrades, also called “water bears”, invertebrate, segmented and microscopic animals that could have been evolutionary precursors of arthropods. They have an extremophilic life, that is, they are capable of withstanding conditions incompatible with life, such as outer space, pressures of up to 6000 atm, temperatures of up to -200 ° C or 150 ° C, strong doses of ionizing radiation or even dehydration. for very long periods (up to 10 years without water).

Each phylum groups together a set of species that have similar bodily, embryonic and physiological characteristics, and that therefore can be understood as variants of the same general theme. However, there are intermediate levels of taxonomic classification: class, order, family and genus, in that same hierarchical order. Finally, within each animal species there may be subspecies, varieties or races.