Classification of invertebrate animals

Author: Laura McKinney
Date Of Creation: 3 August 2021
Update Date: 1 November 2024
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Invertebrate Animals | Educational Video for Kids
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Invertebrate animals are those that, as a common feature, share the absence of a spinal column and an internal articulated skeleton. In this group are most animals in the world, representing 95% of existing species. As the most diverse group within this realm, its categorization has become very difficult, so there are no definitive classifications.

In this article by PeritoAnimal, we talk about classification of invertebrate animals which, as you can see, is a vast group within the fascinating worlds of living beings.

The use of the term invertebrate

The term invertebrate does not correspond to a formal category in scientific classification systems, as it is a generic term which refers to the absence of a common feature (vertebral column), but not to the presence of a feature shared by everyone in the group, as in the case of vertebrates.


This does not mean that the use of the word invertebrate is invalid, on the contrary, it is commonly used to refer to these animals, it only means that it is applied to express a more general meaning.

How is the classification of invertebrate animals

Like other animals, in the classification of invertebrates there are no absolute results, however, there is a certain consensus that the main groups of invertebrates can be classified into the following lines:

  • arthropods
  • molluscs
  • annelids
  • platyhelminths
  • nematodes
  • echinoderms
  • Cnidarians
  • porifers

In addition to knowing invertebrate groups, you may be interested in knowing examples of invertebrate and vertebrate animals.

Classification of Arthropods

They are animals with a well-developed organ system, characterized by the presence of a chitinous exoskeleton. In addition, they have differentiated and specialized appendices for different functions according to the group of invertebrates they are part of.


the arthropod phylum corresponds to the largest group in the animal kingdom and it is classified into four subphyla: trilobites (all extinct), chelicerates, crustaceans and unirámeos. Let's know how the subphyla that currently exist and several examples of invertebrate animals are divided:

chelicerates

In these, the first two appendages were modified to form the chelicerae. In addition, they can have pedipalps, at least four pairs of legs, and they don't have antennas. They are made up of the following classes:

  • Merostomates: they have no pedipalps, but the presence of five pairs of legs, such as the horseshoe crab (limulus polyphemus).
  • Pychnogonids: 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. Some examples of vertebrate animals in this class are spiders, scorpions, ticks and mites.

Crustaceans

Generally aquatic and with the presence of gills, antennae and mandibles. They are defined by five representative classes, among which are:


  • Remedies: are blind and live in deep sea caves, like the species Speleonectes tanumekes.
  • Cephalocarids: they are marine, small in size and simple anatomy.
  • Branchiopods: Small to medium in size, mainly living in fresh water, although they also live in salt water. They have later appendices. In turn, they are defined by four orders: Anostraceans (where we can find goblin shrimp like the Streptocephalus mackini), notostraceans (called tadpole shrimp like the Franciscan Artemia), cladocerans (which are water fleas) and concostraceans (mussel shrimp like the Lynceus brachyurus).
  • Maxillopods: Usually small in size and with reduced abdomen and appendages. They are subdivided into ostracods, mistacocarids, copepods, tantulocarids and cirripedes.
  • Malacostraceans: the crustaceans best known to humans are found, they have an articulated exoskeleton that is relatively smoother and they are defined by four orders, among which are the isopods (Ex. Armadillium granulatum), amphipods (Ex. giant Alicella), the eufausiaceans, which are generally known as krill (Ex. Meganyctiphanes norvegica) and decapods, including crabs, shrimp and lobsters.

Unirámeos

They are characterized by having only one axis in all appendices (without branching) and having antennae, mandibles and jaws. This subphylum is structured into five classes.

  • Diplopods: characterized by having generally two pairs of legs in each of the segments that form the body. In this group of invertebrates we find the millipedes, as the species Oxidus gracilis.
  • Chilopods: they have twenty-one segments, where there are a pair of legs in each one. Animals in this group are commonly called centipedes (Lithobius forficatus, among others).
  • pauropods: Small size, soft body and even with eleven pairs of legs.
  • symphiles: off-white, small and fragile.
  • insect class: have a pair of antennas, three pairs of legs and usually wings. It is an abundant class of animals that groups together nearly 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 has a scraping function. They have a structure called a foot that can be used for locomotion or fixation. Its circulatory system is open in almost all animals, gas exchange takes place through the gills, lungs or the surface of the body, and the nervous system varies by group. They are divided into eight classes, which we will now know more examples of these invertebrate animals:

  • Caudofoveados:marine animals that dig the soft soil. They do not have a shell, but they have calcareous spikes, such as the crossotus sickles.
  • Solenogastros: similar to the previous class, they are marine, excavators and with limestone structures, however they do not have radula and gills (e.g. Neomenia carinata).
  • Monoplacophores: they are small, with a rounded shell and the ability to crawl, thanks to the foot (ex. Neopilin rebainsi).
  • Polyplacophores: with elongated, flat bodies and the presence of a shell. Understand the quitons, like the species Acanthochiton garnoti.
  • Scaphopods: its body is enclosed in a tubular shell with an opening at both ends. They are also called dentali or elephant tusk. An example is the species Antalis vulgaris.
  • gastropods: with asymmetrical shapes and presence of shell, which suffered torsion effects, but which may be absent in some species. The class consists of snails and slug, like 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 verrucous venus.
  • Cephalopods: its shell is quite small or absent, with a defined head and eyes and presence of tentacles or arms. In this class we find squid and octopuses.

Classification of annelids

Are metameric worms, that is, with segmentation of the body, with a moist external cuticle, closed circulatory system and complete digestive system, the gas exchange takes place through the gills or through the skin and can be hermaphrodites or with separate sexes.

The top ranking of annelids is defined by three classes that you can now check with more examples of invertebrate animals:

  • 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 succinic nereis and Phyllodoce lineata.
  • oligochetes: are characterized by having variable segments and without a defined head. We have, for example, the earthworm (lumbricus terrestris).
  • Hirudine: as an example of hirudine we find the leeches (ex. Hirudo medicinalis), with fixed number of segments, presence of many rings and suction cups.

Platyhelminths Classification

The flatworms are flat animals dorsoventrally, with oral and genital opening and primitive or simple nervous and sensory system. Furthermore, animals from this group of invertebrates do not have a respiratory and circulatory system.

They are divided into four classes:

  • whirlwinds: they are free-living animals, measuring up to 50cm, with an epidermis covered by eyelashes and with the ability to crawl. They are commonly known as planarians (eg. Temnocephala digitata).
  • Monogenes:They are mainly parasitic forms of fish and some of frogs or turtles. They are characterized by having a direct biological cycle, with only one host (e.g. Haliotrema sp.).
  • Trematodes:Their body has a leaf shape, being characterized by being parasites. In fact, most are vertebrate endoparasites (Ej. Fasciola hepatica).
  • Baskets: with characteristics that differ from the previous classes, they have long and flat bodies, without cilia in the adult form and without digestive tract. However, it is covered with microvilli that thicken the integument or outer covering of the animal (e.g. Taenia solium).

Classification of Nematodes

little parasites that occupy marine, freshwater and soil ecosystems, both in polar and tropical regions, and can parasitize other animals and plants. There are thousands of species of nematodes identified and they have a characteristic cylindrical shape, with a flexible cuticle and absence of cilia and flagella.

The following classification is based on the morphological characteristics of the group and corresponds to two classes:

  • Adenophorea: Your sensory organs are circular, spiral, or pore-shaped. Within this class we can find the parasite form Trichuris Trichiura.
  • Secernte:with dorsal lateral sensory organs and cuticle formed by several layers. In this group we find the parasitic species lumbricoid ascaris.

Classification of Echinoderms

They are marine animals that do not have segmentation. Its body is rounded, cylindrical or star-shaped, headless and with a varied sensory system. They have calcareous spikes, with locomotion through different routes.

This group of invertebrates (phylum) is divided into two subphyla: Pelmatozos (cup or goblet-shaped) and eleuterozoans (stellate, discoidal, globular or cucumber-shaped bodies).

Pelmatozos

This group is defined by the crinoid class where we find those commonly known as sea ​​lilies, and among which one can mention the species Mediterranean Antedon, davidaster rubiginosus and Himerometra robustipinna, among others.

Eleuterozoans

In this second subphylum there are five classes:

  • concentricicloids:known as sea daisies (eg. Xyloplax janetae).
  • asteroids:or sea stars (ex. Pisaster ochraceus).
  • Ophiuroides: which includes sea snakes (ex. Ophiocrossota multispina).
  • Equinoids: commonly known as sea urchins (eg Strongylocentrotus franciscanus and Strongylocentrotus purpuratus).
  • holoturoids: also called sea cucumbers (eg. Holothuria cinerascens and Stichopus chloronotus).

Classification of Cnidarians

They are characterized by being mainly marine with only a few freshwater species. There are two types of forms in these individuals: polyps and jellyfish. They have a chitinous, limestone or protein exoskeleton or endoskeleton, with sexual or asexual reproduction and do not have a respiratory and excretory system. A characteristic of the group is the presence of stinging cells which they use to defend or attack prey.

The phylum was divided into four classes:

  • Hydrozoa:They have an asexual life cycle in the polyp phase and a sexual one in the jellyfish phase, however, some species may not have one of the phases. Polyps form fixed colonies and jellyfish can move freely (eg.hydra vulgaris).
  • scifozoa: this class generally includes large jellyfish, with bodies of varying shape and different thickness, which are covered by a gelatinous layer. Your polyp phase is very low (eg. Chrysaora quinquecirrha).
  • Cubozoa:with a predominant form of jellyfish, some reach large sizes. They are very good swimmers and hunters and certain species can be lethal to humans, while some have mild poisons. (eg Carybdea marsupialis).
  • antozooa: they are flower-shaped polyps, without jellyfish phase. All are marine, and can live superficially or deeply and in polar or tropical waters. They are divided into three subclasses, which are zoantarios (anemones), ceriantipatarias and alcionarios.

Classification of Porifers

To this group belong the sponges, whose main characteristic is that their bodies have a large amount of pores and a system of internal channels that filter the food. They are sessile and depend largely on water circulating through them for food and oxygen. They have no real tissue and therefore no organs. They are exclusively aquatic, mainly marine, although there are some species that inhabit fresh waters. Another key feature is that they are formed by calcium carbonate or silica and collagen.

They are divided into the following classes:

  • limestone: those in which their spikes or units that form the skeleton are of calcareous origin, that is, calcium carbonate (ex. Sycon raphanus).
  • Hexactinylides: also called vitreous, which have as a peculiar characteristic a rigid skeleton formed by six-ray silica spikes (ex. Euplectella aspergillus).
  • demosponges: class in which almost 100% of the sponge species and the larger ones are located, with very striking colors. The spicules that form are of silica, but not of six rays (ex. Testudinary Xestospongia).

Other invertebrate animals

As we mentioned, invertebrate groups are very abundant and there are still other phyla that are included within the invertebrate animal classification. Some of them are:

  • Placozoa
  • Ctenophores
  • Chaetognath
  • Nemertinos
  • Gnatostomulid
  • Rotifers
  • Gastrotrics
  • Kinorhincos
  • Loricifers
  • Priapulides
  • nematomorphs
  • endoprocts
  • onychophores
  • tardigrades
  • ectoprocts
  • Brachiopods

As we could see, the classification of animals is quite diverse, and over time, the number of species that constitute it will certainly continue to grow, which shows us once again how wonderful the animal world is.

And now that you know the classification of vertebrate animals, their groups and countless examples of invertebrate animals, you may also be interested in this video about the rarest marine animals in the world:

If you want to read more articles similar to Classification of invertebrate animals, we recommend that you enter our Curiosities section of the animal world.