Animal is a friends

Animal kingdom classification is not a job for the faint-hearted, for it is a really long list of classes and subclasses, many of which are named in Latin. There are millions of animal species on the Earth, and they are segregated under the kingdom Animalia, which in turn is divided into subkingdoms, infrakingdoms, superphyla, and phyla. Each phylum is further divided into class, order, family, genus, and species.

So, all animals ranging from single-celled microorganisms to highly complex, multi-celled organisms are classified into different phyla. It will definitely not be easy to capture 3.5 billion years of evolution in one small article, but let's give it a shot. My attempt at this categorization should help you with the basics.

Please note that this article will only explain the main phyla, and not the class, order, family, genus, and species. The term 'phylum' is defined as a taxonomic category that ranks above class and below kingdom. In general, it can be said that animals with similar body plans are classified in a particular phylum.

The kingdom Animalia is divided into three main sections - Subkingdoms Radiata and Bilateria, and Phylum Myxozoa. The Subkingdom Radiata has four phyla, whereas Subkingdom Bilateria is divided into two infrakingdoms - Deuterostomia and Protostomia. Infrakingdom Deuterostomia consists of four phyla. Infrakingdom Protostomia has four superphyla - Ecdysozoa (with 8 phyla); Lophozoa (with 8 phyla); Platyzoa (with 8 phyla); and Chaetognathi (single Phylum Chaetognatha).

Phylum Myxozoa - Ciliated Protozoans

This phylum has a group of minute, but multicellular, parasitic, aquatic animals. It is further divided into two classes - Malacosporea (with a spore size of 10 μm to 20 μm) and Myxosporea (with a maximum size of 2 mm). Some of the common parasites that infect fish such as salmon, catfish, carp, and trout, are actually myxozoans. Some myxozoans infect amphibians, reptiles, and mammals. Even humans can be hosts of certain myxospores.

Phylum Chordata - Chordates

This phylum has some of the most familiar animals, including mammals such as humans. It has vertebrates such as reptiles, birds, and mammals; marine invertebrates such as sea squirts and sea tulips; and segmented marine animals (such as lancelets) with elongated bodies. Along with hagfish, vertebrates comprise the subdivision Craniata. Others include tunicates (Urochordata) and lancelets (Cephalochordata). Characteristic features of chordates include a notochord, a hollow dorsal nerve cord, pharyngeal slits, and a post-anal tail. Most of these animals are found to have one or more of these features, at least for a short period during their life cycles.

Class Mammalia - Mammals

These animals have hair or fur, and are capable of regulating body temperature, give birth to fully-formed babies, and produce milk to feed their young ones. Examples of mammals are monkeys, dogs, elephants, hippopotamuses, squirrels and other rodents, koalas, kangaroos, and humans. Marine mammals include whales and dolphins.

Class Amphibia - Amphibians


Amphibians lay eggs, usually in water. Most of them breathe through the lungs and skin. While some have gills, there are certain amphibians that breathe only through the skin as they lack lungs. These animals cannot regulate their body temperatures and can live both on land and in water. While some species are arboreal, there are some fossorial amphibians too.

Class Reptilia - Reptiles

Reptiles have roamed the Earth before the age of dinosaurs, and these animals adapt well to changes in the environment. Dinosaurs were actually a clade of reptiles known as Dinosauria. Examples of extant reptiles are alligators, crocodiles, lizards, snakes, tortoises, etc. These animals cannot regulate body temperature on their own. Other features include dry skin covered with scales and amniotic eggs.

Class Aves - Birds


There are over 8000 different species of birds that include parrots, sparrows, pigeons, geese, sea gulls, etc. They have feathered wings, which enable them to fly; beaks without teeth; and light bones that make their flight easy. Birds can breathe even at high altitudes, and they lay hard-shelled eggs.

Fish


Water is home to more than 20,000 different types of fish. They have gills and lack limbs. Earlier fish belonged to the class Pisces, but this classification is not in use anymore. Some examples of fish are sharks, goldfish, and herring.

Phylum Echinodermata - Echinoderms

The echinodermata phylum consists of animals such as star fish, sea urchin, sea cucumber, etc. The phylum has around 7000 species of marine animals with pentamerous (five-point) symmetry. Most of these animals are star-shaped, spherical, or elongated. Most of them have a spiny exoskeleton and lack a distinct head. Locomotion is achieved with the help of tubed feet.

Phylum Hemichordata - Hemichordates

The phylum has around 100 species that possess some of the characteristics of chordates. This is why the phylum is named Hemichordata. The characteristic feature of these marine animals is that their bodies are divided into three parts - the anterior part with a preoral lobe, a collar, and the trunk. Acorn worms are among the prominent members of this phylum. They are solitary, worm-like organisms that live in burrows. Others include pterobranchs (seen in colonies) and graptolites.

Phylum Xenacoelomorpha

This is a new phylum that has xenoturbellids and acoelomorphs. It has been discovered that these simple marine worms are closely related to sea urchins and even humans. So, it was decided to classify them under the infrakingdom Deuterostomia. Though xenoturbellids and acoelomorphs have a simple body structure, scientists believe that they have a common ancestor. It is also believed that the other complex deuterostomes descended from this ancestor.

Phylum Cnidaria - Jellyfish, Sea Anemones ...

Commonly known as Cnidarians, there are around 9000 species of aquatic animals in this phylum. Most of them are found in marine water, and many have specialized stinging organelles. The most popular Cnidarians are jellyfish, sea anemones, corals, and Portuguese man-of-war. This group usually consists of two types of animals, namely the polyps (fixed, solitary, or colonial life) and medusae (free-swimming). Reproduction is asexual (budding) in polyps and sexual in medusae.

Phylum Ctenophora - Comb Jellies


Otherwise known as comb jellies or sea walnuts, Ctenophores are marine animals with gelatinous bodies. They have hair-like cilia that help them swim. These tufts of cilia have a comb-like appearance, and so, the animal got the name comb jelly. There are around 100 to 150 species of Ctenophores, and most of them are hermaphrodites.

Phylum Placozoa

Placozoans are non-parasitic, multicellular animals that are considered as the most simplest form of invertebrates. The body of a Placozoan is made of thousands of cells that are arranged as a plate with two layers. The flattened body of a Placozoan has a thickness of around a millimeter, and it moves with the help of cilia. Though it is believed that the phylum has a single genus Trichoplax, with a single species Trichoplax adhaerens, it has been speculated that there are more placozoans in the world. Treptoplax reptans is another species that was described in 1896. However, the organism was not seen after that year.