Classification of the Animal Kingdom

We explain what the classification of the animal kingdom is, its history and what are the phyla and subphyla that are currently used.

Organization of the animal kingdom

Biology classifies animals as members of the Animal Kingdom or Animalsone of the great chapters in which known life is classified, different from the kingdom of plants, fungi or microorganisms. However, within this kingdom, numerous different species of animals coexist, which merit extensive classification by specialists.

This classification It is always changing, as the nature of life is understood more and better. It had its beginnings in ancient times itself, since the desire of human beings to understand and categorize what surrounds them does not exclude, by any means, the animals with which they share the Earth.

The first animal classification systems date back to the 4th century BC. c and they remained more or less in vogue until practically the 18th century, when the Scientific Revolutions and 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 (1848), Lankester (1877), Grobben (1908), Bütschli (1910), Hyman (1940) or Nielsen (2012), to mention a few.

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

Thus, for example, groups of animals (called phyla or phyla) were traditionally organized phylum), into two large initial categories, which were vertebrates (those with a backbone) and invertebrates (those without one).

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 present bilateral but radial symmetry, or none at all. They are all invertebrates and include the following phyla:

• Phylum porifera (“pore-bearers”), immobile and asymmetrical animals, whose body has pores to inhale the surrounding water. They are traditionally known as sponges, and there are about 9,000 described species.
• Phylum cnidaria (“nettles”), simple animals of an aquatic environment, which have stinging or toxic cells called cnidocytes, such as jellyfish and anemones. They are an extremely ancient group evolutionarily, of which about 10,000 different species are known.
• Phylum ctenophora (“comb bearers”), exclusively marine, bioluminescent animals that capture their microscopic prey (plankton) using sticky comb-like filaments (cilia). Only 166 of its species are known.
• Phylum placozoa (“plate animals”), aquatic animals, flattened and crawling, have the simplest body structure known on the planet, just above the protozoa. Only one species of them is known, but it is estimated that there could be up to 100 still unknown.

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

• Phylum annelida (“little rings”), worm-shaped invertebrate animals with a ringed body and humid habitats, such as earthworms, leeches or polychaete marine worms. There are 17,200 different species of annelids known.
• Phylum arthropoda (“articulated legs”), is one of the most evolutionarily successful phyla of all, in which the largest number of known species is found, that is, the greatest biodiversity that exists on the planet. These are invertebrate animals with a rigid chitinous shell and articulated limbs, such as insects, crustaceans, arachnids and myriapods. They are present in all ecosystems in the world and play all possible ecological roles.
• Phylum brachiopoda (“short legs”), composed of animals with two valves (rigid shells) joined in the posterior region of the body, inhabitants of the ocean floors, where they are attached to hard materials through a single foot that they have, or they are They bury 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”), includes 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. There are about 5,700 species 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. They are predators, with almost transparent, torpedo-shaped bodies, measuring between 2 mm and 12 cm. There are about 121 different species 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 chord or notochord, 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 subphylums of immense biological diversity, which is why 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, hermaphrodite sexuality and a very inactive life cycle. They are often considered the “inferior” 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 chordates and the higher ones or vertebrates.
  • Subphylum vertebrata or craniata (“with vertebrae” or “with skull”), the most diverse and varied group of this entire segment, in which there are around 73,000 different species of animals equipped with a vertebral column and a skull, or at least sketches of them , and they are found in all possible habitats. The most evolutionarily complex animals are found in this line, organized into different classes: myxini (hagfish), hyperarthia (lampreys), chondrichthyes (cartilaginous fish), actinopterygii (ray-finned fish), sarcopterygii (lobe-finned fish), amphibia (amphibians), reptile (reptiles), mammalia (mammals) and birds (birds).
• Phylum Cycliophora (“wheel-bearer”), phylum of a single genus of known animals, with symbiotic habits, discovered in 1995 in the mouth of a marine crustacean. They have a suction cup to attach themselves 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 aquiferous 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 urchins and sea stars. Around 7,000 current species are known.
• Phylum entoprocta (“inner anus”), small aquatic animals of which about 170 species are known, shaped like a calyx and a tentacular crown used to filter 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 matter from bacteria, fungi and protozoa, and serve as food for other species such as annelids, nematodes and marine arthropods. Around 400 different species are known.
• Phylum gnathostomulida (“small mouth with jaw”), worm-shaped animals with a marine habitat, which feed on fungi and other microorganisms in anaerobic environments. Its almost 100 known species have a jawed mouth, despite their tiny size.
• Phylum hemichordata (“with half a chord”), vermiform-looking animals that do not have a true notochord during their embryonic formation (therefore, they are not chordates), but do have something very similar, so they could be evolutionarily related. Currently only about 100 species are known.
• Phylum kinorhyncha (“moving proboscis”), these small marine invertebrates live on the ocean floor of the entire planet, and move through a series of special spines that they have on the back of their body. There are about 250 known species.
• Phylum loricifera (“mail carriers”), is a phylum 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 a jaw”), microscopic invertebrate animals whose only species was discovered in 2000. They have very complex jaws made up of 32 mobile parts, quite an exception among invertebrates.
• Phylum mollusca (“soft”), another of the most extensive phyla of 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 may 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, including clams, squid, octopuses, slugs, oysters, snails, etc.
• Phylum nematoda (“thread-like”), 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 in life, although many species have parasitic life, both in the body of plants and animals, including humans. Many nematodes cause gastrointestinal diseases.
• Phylum nematomorpha (“thread-shaped”), 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 aquatic pollution.
• Phylum nemertea (“sea nymph”), a group of non-segmented worms, somewhat flattened and less than 20 cm long, whose bodies have a characteristic proboscis or proboscis. Free-living and aquatic habitat, there are about 1,200 different species known.
• Phylum onychophora (“claw bearer”), known as velvet worms, are about 180 known species of worms similar to insect caterpillars, as they have legs ending in small nails or claws.
• Phylum phoronida (“descendant of phoroneus”), a phylum composed of 20 species of tentacled animals, with a “U”-shaped body, that inhabit the benthic regions of the sea, creating colonies in diros substrates or soft sediments.
• Phylum platyhelminthes (“flatworms”), composed of flattened, hermaphroditic worms that inhabit aquatic, terrestrial and even aerial environments, and many of which lead parasitic lives. They are simple animals that have 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. About 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 composed of only 30 to 50 cells. 75 different species are known.
• Phylum rotifera (“wheel-bearers”), a phylum of microscopic animals that inhabit fresh waters, humid land 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 extremist 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 a set of species that present similar bodily, embryonic and physiological characteristics and which 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.

Continue with: Classification of animals

References

• “Animalia” on Wikipedia.
• “Classification of animals” in Digital Contents of the University of La Punta (Argentina).
• “Animals: classification” in INTAchicos (Argentina).
• “Classification of Animals” (video) in Make It Easy Education.
• “Animal” in The Encyclopaedia Britannica.