The kingdom animalia or metazoa, known as the animal kingdom, includes many different organisms. There are animals that measure less than a millimeter, such as many rotifers; but also animals that can reach 30 meters, such as the blue whale. Some only live in very specific habitats, while others can survive even in the most extreme conditions. This is the case of the seahorse and the tardigrades, respectively.
Also, animals can be as simple as a sponge or as complex as human beings. However, all types of animals are very well adapted to their habitat and, thanks to this, have survived to this day. Do you want to meet them? Don't miss this article on our site about the animal kingdom: classification, characteristics and examples
Animal classification
The classification of animals is very complex and includes many small animals not visible to the naked eye and very unknown. Due to the enormous diversity of these groups, we are going to talk only about the most abundant and well-known phyla or types of animals They are the following:
- Porifera (Phylum Porifera).
- Cnidarians (Phylum Cnidaria).
- Platyhelminthes (Phylum Platyhelminthes).
- Mollusks (Phylum Mollusca).
- Annelids (Phylum Anellida).
- Nematodes (Phylum Nematoda).
- Arthropods (Phylum Arthropoda).
- Echinoderms (Phylum Echinodermata).
- Chordates (Phylum Chordata).
Later, we will leave a list with the most unknown organisms of the animal kingdom.
Porifera (Phylum Porifera)
The Phylum Porifera includes more than 9,000 known species. Most are marine, although there are about 50 freshwater species. They are sponges, sessile animals that live attached to a substrate and feed by filtering the water around them. Their larvae, however, are mobile and pelagic, so they are part of the plankton.
Examples of porifera
Here are some interesting examples of porifera:
- Glass sponge (Euplectella aspergillum): it houses a couple of crustaceans of the genus Spongicola that get trapped in it.
- Hermit sponge (Suberites domuncula): grows on the shells occupied by hermit crabs and takes advantage of their movement to capture nutrients.
Cnidarians (Phylum Cnidaria)
The cnidarian group is one of the most interesting phyla within the animal kingdom. It is made up of more than 9.000 aquatic species, most of them marine. They are characterized by the fact that, throughout their development, they can have two forms of life: polyps and jellyfish
Polyps are benthic and remain attached to a substrate on the seabed. They often form colonies known as corals When it's time to reproduce, many species become pelagic creatures that float in the water. They are known as jellyfish.
If you want to know more about the life cycle of cnidarians, we recommend this article on Reproduction of jellyfish.
Examples of cnidarians
- Portuguese Skull (Physalia physalis): This is not a jellyfish, but a floating colony of small jellyfish.
- Magnificent Anemone (Heteractis magnifica): This is a polyp with stinging tentacles that some clownfish live among.
Platyhelminthes (Phylum Platyhelminthes)
The flatworm phylum contains more than 20,000 species known as flatworms It is one of the most feared groups in the animal kingdom due to its their frequent condition of parasites. However, many flatworms are free-living predators. Most are hermaphrodites and their size varies between a millimeter and several meters.
Examples of flatworms
Here are some examples of flatworms:
- Taenia (Taenia solium): huge flatworm that parasitizes pigs and humans.
- Planaria (Pseudoceros spp.): flatworms that glide over the seabed. They are predators and stand out for their great beauty.
Mollusks (Phylum Mollusca)
The Phyllum Mollusca is one of the most diverse in the animal kingdom and includes more than 75,000 known species. These include marine, freshwater and terrestrial species. They are characterized by having a soft body and the ability to make their own shells or skeletons
The best-known types of molluscs are gastropods (snails and slugs), cephalopods (octopuses, cuttlefish and nautiluses) and bivalves (mussels and clams).
Examples of molluscs
These are some curious examples of molluscs:
- Sea slugs (Discodoris spp.): marine gastropods of great beauty.
- Nautilus (Nautilus spp.): These are shelled cephalopods that are considered living fossils.
- Giant clams (Tridacna spp.): they are the largest bivalves that exist and can reach a size of two meters
Annelids (Phylum Annelida)
The group of annelids includes some 13,000 known species and, like the previous group, includes species from the sea, freshwater and land. They are segmented animals and very diverse. There are three classes or types of annelids: the polychaetes (marine worms), the oligochaetes (earthworms) and the hirudinomorphs (leeches and other parasites.)
Examples of annelids
These are some curious examples of annelids:
- Feather worms (Sabellidae family): Often mistaken for corals, they are one of the prettiest annelids around.
- Giant Amazon Leech (Haementeria ghilianii): It is one of the largest leeches in the world.
Second photo taken from YouTube.
Nematodes (Phylum Nematoda)
The phylum of nematodes is, despite appearances, one of the most diverse within the classification of animals. Includes more than 25,000 species of roundworms These worms have colonized all media and are found at all levels of the food chain. This means that they can be phytophagous, predatory or parasitic, the latter being the best known.
Examples of nematodes
Here are some examples of nematodes:
- Soybean nematode (Heterodera glycines): parasitizes soybean roots, causing serious problems in crops.
- Heartworms (Dirofilaria immitis): these are worms that parasitize the heart and lungs of canids (dogs, wolves, etc.).
Arthropods (Phylum Arthropoda)
The Phylum Arthropoda is the most diverse and abundant group in the animal kingdom. The classification of these animals includes arachnids, crustaceans, myriapods and hexapods, among which are all types of insects.
All these animals have articulated appendages (legs, antennae, cerci, etc.) and an exoskeleton known as a cuticle. During their life cycle, they shed their cuticle several times and many present larvae and/or nymphs. When they are very different from adults, they go through a process of metamorphosis.
Examples of arthropods
To demonstrate the diversity of this type of animal, we leave you some curious examples of arthropods:
- Sea spiders (Pycnogonum spp.): they are species of the Pycnogonidae family, the only sea spiders that exist.
- Barnacle (Pollicipes pollicipes): Few people know that barnacles are crustaceans, like crabs.
- European scolopendra (Scolopendra cingulata): it is the largest scolopendra in Europe. Its sting is very powerful, but very rarely becomes fatal.
- Antlion (Myrmeleon formicarius): these are neuropterous insects whose larvae live buried in the ground under a cone-shaped hole. There, they wait for their prey to fall on their mouths.
Echinoderms (Phylum Echinodermata)
The phylum of the echinoderms includes more than 7,000 species characterized by having pentaradial symmetry This means that their body is divided into five equal parts. It's easy to imagine when we know what kind of animals it is: brittle stars, lilies, cucumbers, stars and sea urchins.
Other characteristics of echinoderms are their calcareous skeleton and their system of internal channels through which seawater flows. Their larvae, which have bilateral symmetry and lose it as their life cycle progresses, are also very particular. You can learn more about them in this article about the Starfish Life Cycle.
Examples of echinoderms
These are some members of the animal kingdom that belong to the group of echinoderms:
- Indo-Pacific sea lily (Lamprometra palmata): like all sea lilies, they live attached to a substrate and present the mouth in a superior position, next to the anus.
- Swimming cucumber (Pelagothuria natatrix): This is one of the best swimmers of the sea cucumber group. Its appearance is similar to that of a jellyfish.
- Crown of Thorns (Acanthaster planci): This voracious starfish feeds on cnidarian (coral) polyps.
Chordates (Phylum Chordata)
The group of chordates includes the best-known organisms of the animal kingdom, since it is the phylum to which human beings and their similars belong. They are characterized by the possession of an internal skeleton that runs the entire length of the animal. This may be a flexible notochord, in the most primitive chordates; or a backbone, in vertebrates.
In addition, all of these animals possess a dorsal nerve cord (spinal cord), pharyngeal slits, and a posterior tail, at least in some moment of fetal development.
Classification of chordate animals
The chordates are further divided into the following subphyla or types of animals:
- Urochordates: they are aquatic animals. Most live fixed to a substrate and have free-living larvae. They all have a protective covering known as a robe.
- Cephalochordates: they are very small, elongated animals with transparent bodies that live semi-buried on the seabed.
- Vertebrates: includes the best-known organisms within the classification of animals: fish and tetrapods (amphibians, reptiles, birds and mammals).
Other types of animals
In addition to the named phyla, in the classification of the animal kingdom there are many other less numerous and well-known groups. In order not to forget them, we have brought them together in this section, highlighting the most abundant and interesting ones in bold.
These are the types of animals we haven't named:
- Loricifera (Phylum Loricifera).
- Quinorhynchus (Phylum Kinorhyncha).
- Priapulids (Phylum Priapulida).
- Nematomorphs (Phylum Nematomorpha).
- Gastrotricos (Phylum Gastrotricha).
- Tardigrades (Phylum Tardigrada).
- Onychophora (Phylum Onychophora).
- Chaetognathus (Phylum Chaetognatha).
- Acanthocephalans (Phylum Acanthocephala).
- Rotifers (Phylum Rotifera).
- Micrognatozoa (Phylum Micrognathozoa).
- Gnatostomulids (Phylum Gnatostomulida).
- Equiuros (Phylum Echiura).
- Sipunculi (Phylum Sipuncula).
- Cyclophores (Phylum Cycliophora).
- Entoprocta (Phylum Entoprocta).
- Nemertines (Phylum Nemertea).
- Bryozoa (Phylum Bryozoa).
- Phoronids (Phylum Phoronida).
- Brachiopods (Phylum Brachiopoda).
