Invertebrate animals are those that, as a common feature, share the absence of a vertebral column and an internal articulated skeleton. The majority of the world's animals are found in this group, representing 95% of existing species Being the most diverse group within this kingdom, its categorization is has made it very difficult, therefore there are no definitive classifications, since the scientific community manages to make new identifications on a regular basis, which are included in the respective lists.
In the following article on our site, we bring you information on the classification of invertebrate animals which, as you can see, is a vast group within the fascinating world of living beings.
On the use of the term invertebrate
The term invertebrate does not correspond to a formal category in scientific classification systems, since it is a generic term that refers to the absence of a common trait (backbone), but not to the presence of a trait shared by groups, as in the case of vertebrates.
The aforementioned does not mean that the use of the word invertebrate is invalidated, on the contrary, it is commonly used to mention these animals, only that it is applied to express amore general meaning.
How are invertebrate animals classified?
As with other animals, in the classification of invertebrates there are no absolute results, however, there is some consensus that the main groups of invertebratescan be classified into the following phyla:
- Arthropods.
- Mollusks.
- Annelids.
- Flathelminths.
- Nematodes.
- Echinoderms.
- Cnidarians.
- Porifera.
Classification of arthropods
They are animals with a well-developed organ system, characterized by the presence of an exoskeleton made of chitin. In addition, they have differentiated and specialized appendages for various functions depending on the group.
The arthropod phylum corresponds to the largest group in the animal kingdom and is classified into four subphyla: Trilobites (all extinct), chelicerates, crustaceans and unirameans. Let's find out how the edges that exist today are divided.
Chelicerates
In these, the first two appendages have been modified to form the chelicerae. In addition, they have pedipalps, four pairs of legs and no antennae. They are made up of the classes:
- Merostomates: they have no pedipalps, but the presence of five pairs of legs, such as the pot crab (Limulus polyphemus).
- Pycnogonids: Marine animals with five pairs of legs that are commonly known as sea spiders.
- Arachnids: they have two regions or tagmas, chelicerae, pedipalps that are not always well developed, and four pairs of legs. Includes spiders, scorpions, ticks, and mites.
Crustaceans
Generally aquatic and with the presence of gills, antennae and jaws. They are made up of five representative classes, among which are:
- Remipedios: they are blind and live in deep sea caves, like the species Speleonectes tanumekes.
- Cephalocarids: they are marine, small in size and simple in anatomy.
- Branchiopods: Small to medium in size, they inhabit mainly freshwater, but also s altwater. They have posterior appendages. In turn, they are made up of four orders: anostracea (where we can locate the goblin shrimp such as Streptocephalus mackini), notostracea (called tadpole shrimp, such as Artemia franciscana), cladocerans (which are water fleas) and concrustaceans (the shrimp clams, such as Lynceus brachyurus).
- Maxillopods: generally small in size and with reduced abdomen and appendages. They are subdivided into ostracods, mystacocarids, copepods, tantulocarids, branchiers and barnacles.
- Malacostracea: These are the crustaceans best known to humans. They have an articulated exoskeleton that is relatively softer and are made up of four orders, among which are the isopods (eg Armadillium granulatum), the amphipods (eg Alicella gigantea), the euphausiaceans, which are generally known as krill (eg Meganyctiphanes norvegica) and the decapods, among which we find to crabs, shrimp and lobster.
Unirame
They are characterized by the fact that all the appendages they have are of a single branch or axis and have antennae, mandibles and maxillae. This subphylum is made up of five classes:
- Diplopoda: characterized by generally having two pairs of legs in each of the segments that make up the body. In this group we find millipedes, such as the species Oxidus gracilis.
- Chilopods: they have twenty-one segments, each with a pair of legs. This group is commonly called centipedes (Lithobius forficatus, among others).
- Pauropods: small in size, with soft bodies and up to eleven pairs of legs.
- Symphylls: whitish, small and fragile.
- Class insecta: they have a pair of antennae, three pairs of legs and generally wings. It is an abundant class of animals that groups almost thirty different orders.
Classification of molluscs
This phylum is characterized by having a complete digestive system, with the presence of an organ called the radula, which is located in the mouth And it has scraper function. They have a structure called a foot that can be used for locomotion or fixation. Their circulatory system is open in almost all of them, gaseous exchange is carried out through gills, lungs or the surface of the body and the nervous system varies according to the group. They are divided into eight classes:
- Caudofoveados: marine animals that dig soft soil. They lack a shell but have calcareous spicules, such as Falcidens crossotus.
- Solenogastros: like the previous class, they are marine, burrowers and have calcareous structures, however, they lack radula and gills, (eg Neomenia carinata).
- Monoplacophores: they are small, with a rounded shell and the ability to crawl thanks to the foot, (eg Neopilina rebainsi).
- Polyplacophores: with elongated, flattened bodies and the presence of a shell. It conforms to chitons, such as the species Acanthochiton garnoti.
- Scaphopods: its body is enclosed in a tubular shell with an opening at both ends. They are also called dentalia or fang shells. An example is the species Antalis vulgaris.
- Gastropods: with asymmetric shapes and the presence of a shell, which has suffered the effects of torsion, but which may be absent in some species. The class includes snails and slugs, such as the snail species Cepaea nemoralis.
- Bivalves: the body is inside a shell with two valves that can have different sizes. An example is the species Venus verrucosa.
- Cephalopods: its shell is quite reduced or absent, with a well-defined head and eyes and the presence of tentacles or arms. In this class we find octopuses and squids.
Classification of annelids
They are metameric worms, that is, with body segmentation, external moist cuticle, closed circulatory system and complete digestive system, the gas exchange is through gills or skin and they can be hermaphrodites or have separate sexes.
The higher classification of annelids is made up of three classes:
- Polychaetes: mainly marine, with well-differentiated head, presence of eyes and tentacles. Most segments have lateral appendages. We can mention as an example the species Nereis succinea and Phyllodoce lineata.
- Oligochaetes: they are characterized by variable segments and no defined head. We have for example the earthworm (Lumbricus terrestris).
- Hirudineos: as an example of hirudineos we find leeches (eg Hirudo medicinalis), with fixed number of segments, presence of many rings and suction cups.
Classification of flatworms
They are flattened animals dorsoventrally, with oral and genital opening and primitive or simple nervous and sensory system. In addition, they lack a respiratory and circulatory system.
They are divided into four classes:
- Turbellarians: with a free-living form, which can measure up to 50 cm, with an epidermis made up of cilia and with the ability to crawl. They are commonly known as planaria (eg Temnocephala digitata).
- Monogeneans: they are mainly parasitic forms of fish and some of frogs or turtles. They are characterized by having a direct biological cycle, with a single host (eg Haliotrema sp.).
- Trematodes: its body is leaf-shaped, characterized by being parasitic forms. In fact, most are endoparasites of vertebrates (eg Fasciola hepatica).
- Cestodes: with characteristics that differ from the previous classes, they have long and flat bodies, lacking cilia in the adult form and digestive tube. However, it is covered with microvilli that thicken the integument or covering of the animal (eg Taenia solium).
Classification of nematodes
Small parasites that occupy marine, freshwater and soil ecosystems, both in polar and tropical regions, being able to parasitize other animals and plants. There are thousands of identified species and they have a characteristic cylindrical shape, with a flexible cuticle and no cilia or flagella.
The following is a classification based on the morphological characteristics of the group and corresponds to two classes:
- Adenophorea: Their sensory organs are circular, spiral, or pore-shaped. Within this class we can find the parasitic form Trichuris trichiura.
- secernentea: with dorsolateral sensory organs and cuticle formed by several layers. In this group we place the parasitic species Ascaris lumbricoides.
Classification of echinoderms
They are marine animals that do not present segmentation. Its body is round, cylindrical or star-shaped, without a head and with a varied sensory system. They present calcareous spicules, with locomotion by different routes.
This phylum is divided into two subphyla: Pelmatozoa (cup or calyx-shaped) and Eleutherozoa (stellate, discoid, globular or cucumber-shaped bodies).
Pelmatozoa
This group is made up of the class crinoids, where we find those commonly known as sea lilies, and among which are it can mention the species Antedon mediterranea, Davidaster rubiginosus and Himerometra robustipinna, among others.
Eleutherozoa
In the second subphylum there are five classes:
- Concentricicloideos: known as sea daisies (eg Xyloplax janetae).
- Asteroids: or starfish (eg Pisaster ochraceus).
- Ophyuroids: which includes brittle stars (eg Ophiocrossota multispina).
- Echinoids: commonly known as sea urchins (eg Strongylocentrotus franciscanus and Strongylocentrotus purpuratus).
- Holothuroidea: also called sea cucumbers (eg Holothuria cinerascens and Stichopus chloronotus).
Classification of cnidarians
They are characterized by being mainly marine and there are few freshwater species. There are two types of forms in these individuals: polyps and medusas They have a chitinous, calcareous or protein exoskeleton or endoskeleton, with asexual or sexual reproduction and lack a respiratory system and excretory. A characteristic feature of the group is the presence of stinging cells that they use to defend or attack prey.
The edge has been divided into four classes:
- Hydrozoans: which have an asexual life cycle in the polyp phase and a sexual life cycle in the medusa phase, however, some species they may lack one of the phases. Polyps form fixed colonies and jellyfish can move freely (eg Hydra vulgaris).
- Scyphozoans: this class generally includes the large jellyfish, with bodies of various shapes and different thicknesses, which is formed by a gelatinous layer. Its polyp phase is very reduced (eg Chrysaora quinquecirrha).
- Cubozoa: Predominantly shaped like a jellyfish, some reach great heights. They are very good swimmers and hunters, and certain species can be lethal to humans, while some have mild poisons (eg Carybdea marsupialis).
- Anthozoa: these are flower-shaped polyps, without a medusa phase. They are all marine, being able to live superficially or deeply and in polar or tropical waters. It is divided into three subclasses, which are aoantaria (the anemones), cerianantipatharies, and alcyanians.
Classification of porifera
This group includes sponges, whose main characteristic is that their bodies have a large number of pores and a system of internal channels that they filter the food. They are sessile and rely heavily on water flowing through them for food and oxygen. They lack true tissue and therefore organs. They are exclusively aquatic, mainly marine, although there are some species that inhabit freshwater. Another fundamental feature is that they are made of calcium carbonate or silica and collagen.
They are divided into the following classes:
- Calcareous: in which the spicules or units that form the skeleton are of calcareous origin, that is, calcium carbonate (Sycon raphanus).
- Hexactinélidas: also called vitreous, which have as a particular characteristic that their skeleton is rigid and is formed by silica spicules of six rays (eg Euplectella aspergillum).
- Demosponjas: class in which almost 100% of the species of sponges and the largest ones are located, presenting very bright colors. flashy. The spicules that form them are silica, but not six-rayed (eg Xestospongia testudinaria).
Other invertebrates
As we have mentioned, this group is very abundant and there are other phyla that are included in the classification of invertebrate animals. Some of them are:
- Placozoa.
- Ctenophores.
- Chaetognatha.
- Nemertines.
- Gnathostomulids.
- Rotifers.
- Gastrotricos.
- Kinorincos.
- Loriciferae.
- Priapulids.
- Nematomorphs.
- Endoprocts.
- Onychophora.
- Tardigrades.
- Ectoprocts.
- Brachiopods.
As we could see, the classification of invertebrate animals is highly abundant, and with the passage of time, the number of species that comprise it will surely continue to grow, which shows us once again how wonderful that the animal world is.
