Invertebrate animals are those animals that do not have a backbone, unlike the Vertebrate animals. All are oviparous and are usually small. We can find them in all habitats and they comprise 95% of the animal species.

Snails - Invertebrate Animals

Snails are invertebrate animals as they lack a skeleton.

Classification of invertebrate animals

We can differentiate two main types of invertebrate animals, those that have an exoskeleton that covers their body (Mollusks, arthropods and echinoderms) and those that do not (Worms, coelenterates and porifers).


They are mainly insects and can be found in almost any type of habitat, they have small jointed legs. They are divided into 4 distinct groups that are insects, arachnids, myriapods and crustaceans.


The insects They are the most varied group of invertebrate animals, there are many species and they have large colonies. It is believed that 90% of the species are insects. They have 3 pairs of legs, their body is distributed in 3 differentiated parts (Head, thorax and abdomen) and antennas that they use to position themselves, guide themselves or eat, among other things. Some may have wings, making them the only invertebrate animals with the ability to fly.


The body of arachnids It is divided into two parts, the cephalothorax (Head and thorax) and the abdomen. Unlike insects, they do not have antennae and have 4 pairs of legs. They are the second largest species on earth.


They have a long, highly segmented body with many pairs of legs and a head with antennae and jaw, like centipedes.


They are almost all aquatic invertebrate animals and are the only arthropods with antennae. Some have front claws, like crabs, and generally have 5 to 10 pairs of legs.


They are the most numerous invertebrate animals after arthropods, their body is soft and many are covered by an exoskeleton or shell. There are three main groups:


They are all aquatic animals and are not covered by a shell, the legs are next to their skull and they have at least 4 pairs of legs. They are the invertebrate animals with the most developed vision. Some, like squid, can spit ink to defend themselves.


They have a two-part shell called the valve (hence their name), they are all aquatic invertebrate animals and do not have a recognized head. Their valves are generally symmetrical, like that of oysters.


Slightly more than half of gastropods are aquatic, their body is made up of a muscular head and trunk with one or two pairs of sensitive tentacles and they have a spiral-shaped shell.


All echinoderms have their habitat in salt water. Its skin is rough and rough, its symmetry is different on the top and on the bottom. Its lower part is where its mouth is located and the upper part is the one that is hardest (Like starfish), some have spikes like sea urchins.


Made up of a long, soft body, worms crawl around. We have 3 groups of worms divided into:


They differ thanks to their ringed body and their bilateral body. Its habitat is humid areas, such as swamps or seas.


Better known as roundworms, their body is cylinder-shaped and elongated. The best known nematode is the anisaki.


They have the shape of a flattened ribbon and their body is bilateral. They are mostly parasitic, although some inhabit humid areas. The most famous flatworm is the tapeworm.


They have tentacles around their mouth. We can distinguish the following two groups:


Jellyfish are almost transparent, they float and are shaped like umbrellas. Its tentacles are dangerous as they can injure or paralyze.


Its shape resembles that of a bag, they have one end that they use to stick to a sea rock and another end with a hole that it uses to hunt and feed. The best known polyps are the anemone and the coral.


Commonly called sponges, they live on the rocks of the sea. They are shaped like a plant and its body is made up of holes and small pores that it uses to feed itself and they are totally asymmetrical. They have the simplest organism of invertebrate animals (They do not have organs or nervous system, they only have cells that they use for food).


Invertebrate feeding methods are as diverse as invertebrates themselves, which are adapted to all kinds of habitats, in fresh water, in the sea and on land. The feeding mechanisms are best classified by the method used: navigation, suspension feeding, tank feeding, carnivores and phytophages (plant eaters).

An alternative classification often adopted, but perhaps less satisfactory, may be based on the size of the ingested particles. Thus, the same invertebrate can be described as microfagus (feeding on tiny organisms) or dependent on substances in solution.

Both classification systems can be subdivided. Carnivorous feeders, for example, include animal predators and parasites; both share dependence on other (live) animals as a source of food. Some methods will be limited to particular habitats. Suspension feeders, for example, can only be aquatic, while the phytophagous habit can be found wherever there are edible plants.


Reproduction in invertebrates differs by species. Asexual reproduction (not having sex or sexual organs) is quite common, however sexual reproduction is more typical. Hermaphrodites are common in invertebrates, this means that both male and female sexual organs are present in an individual. In single-sex species, where only one sex organ is present, males and females do not have to make contact to reproduce as fertilization can occur externally. After reproduction, most invertebrates change shape and appearance by going through a process called metamorphosis in which adults and young have different lifestyles, including how and what they eat.


The two common respiratory organs of invertebrates are the windpipe and gills. The diffusion lungs, in contrast to the ventilating lungs of vertebrates, are limited to small animals, such as lung snails and scorpions.


This respiratory organ is a hallmark of the insects. It is formed by a system of branched tubes that supply oxygen to the tissues and remove carbon dioxide from them, thus avoiding the need for a circulatory system to transport respiratory gases (although the circulatory system serves other vital functions, such as the supply of energy-containing molecules derived from food).

The outward pores, called spiracles, are typically paired structures, two in the thorax and eight in the abdomen. The periodic opening and closing of the spiracles prevents the loss of water through evaporation, a serious threat to insects that live in dry environments. Muscle pumping movements of the abdomen, especially in large animals, can promote ventilation of the tracheal system.

Although the tracheal systems are designed primarily for life in the air, in some insects the modifications allow the tracheae to serve for the exchange of gases under water. Of special interest are the insects that might be called bubble breathers, which, as in the case of water beetle (Dytiscus), receive a supply of gas in the form of an air bubble under the surfaces of their wings next to the blowholes before submerging. Tracheal gas exchange continues after the beetle dives and anchors below the surface. As oxygen is consumed from the bubble, the partial pressure of oxygen within the bubble drops below that of water; consequently, oxygen diffuses from the water into the bubble to replace the one that is consumed. The carbon dioxide produced by the insect diffuses through the tracheal system to the bubble and from there to the water. The bubble behaves like a gill. There is an important limitation to this adaptation: As oxygen is removed from the bubble, the partial pressure of nitrogen increases, and this gas diffuses out into the water. The consequence of external nitrogen diffusion is that the bubble contracts and its oxygen content must be replaced by another trip to the surface. A partial solution to the problem of renewal of bubbles has been found by small aquatic beetles of the family Elmidae, which capture bubbles containing oxygen produced by the algae and incorporate this gas into the gill of the bubble. Several species of aquatic beetles also increase gas exchange by agitating the surrounding water with their hind legs.

An elegant solution to the problem of depletion of bubbles during immersion has been found by certain beetles that have a high density of skin hair over much of the surface of the abdomen and thorax. The pile of hair is so dense that it resists moisture, and an air gap forms underneath it, creating a plastron, or layer of air, in which the tracheae open. As respiration proceeds, the outward diffusion of nitrogen and the consequent contraction of the gas space are prevented by surface tension - a condition manifested by properties that resemble those of elastic skin under tension - between tightly fitting hair and Water. The plastron becomes "permanent" in the sense that it is no longer necessary to trap any more bubbles on the surface, and the beetles can remain submerged indefinitely. Since plastron hairs tend to resist deformation, beetles can live at considerable depths without compression from plastron gas.

An extraordinary strategy used by insects hemipteranos Good y Anisops it is an internal oxygen store that allows them to lurk for minutes without surfacing while waiting for food in midwater areas relatively free of predators but poor in oxygen. The internal oxygen reserve occurs in the form of cells filled with hemoglobin that constitute the first line of oxygen supply to actively metabolize the cells, sparing the small air mass in the tracheal system while the hemoglobin reserve is being depleted.

The respiratory structures of spiders consist of peculiar «book lungs', Leaf-shaped plates over which air circulates through openings in the abdomen. They contain blood vessels that bring the blood in close contact with the surface exposed to the air and where the exchange of gases between the blood and the air occurs. In addition to these structures, there may also be abdominal spiracles and an insect-like tracheal system.

Because spiders are air breathers, they are mostly restricted to terrestrial situations, although some of them regularly hunt aquatic creatures at the edges of streams or ponds and can travel on the surface of the water as easily as on land. . The water spider (or diving bell spider), (Argyroneta aquatica) Known for its underwater silk web, which resembles a kind of diving bell, it is the only species of spider that spends its entire life under water. Using fine hairs on its abdomen, where its respiratory openings are located, the water spider captures tiny air bubbles on the surface of the water, transports them to its silk web, which is anchored to plants or other underwater objects, and expels them. inward, thus inflating the underwater house with air. Research has shown that the inflated net serves as a kind of gill, drawing dissolved oxygen out of the water when oxygen concentrations within the net are low enough to draw oxygen out of the water. As the spider consumes oxygen, nitrogen concentrations in the inflated web rise, causing it to slowly collapse. Therefore, the spider must travel to the surface of the water to renew the bubbles, which it does about once a day. Most of the water spider life cycle, including courtship and reproduction, prey trapping and feeding, and egg and embryo development, occurs below the surface of the water. Many of these activities take place inside the spider's diving bell.

Many immature insects have special adaptations for an aquatic existence. The thin-walled protrusions of the cover, which contain tracheal webs, form a series of gills (tracheal gills) that bring water into contact with the closed tracheal tubes. Nymphs of mayflies and dragonflies have external tracheal gills attached to their abdominal segments, and some of the gill plates can move in such a way as to create water currents over the exchange surfaces. Dragonfly nymphs have a series of tracheal gills enclosed in the rectum. Periodic pumping of the rectal chamber serves to renew the flow of water over the gills. Removal of the gills or plugging of the rectum results in less oxygen consumption. In immature aquatic insects there is also considerable gas exchange on the general surface of the body.

The insect's tracheal system has inherent limitations. Gases diffuse slowly in long, narrow tubes, and effective gas transport can only occur if the tubes do not exceed a certain length. This is generally thought to have imposed a size limit on the insects.


Many invertebrates use the gills as an important means of gas exchange; a few, like him lung snail, they use the lungs. Almost any thin-walled extension of the body's surface that comes into contact with the environment and through which gas exchange occurs can be seen as a gill.

The gills usually have a large surface in relation to their mass; pumping devices are often used to renew the external environment. Although the gills are generally used for aquatic respiration and the lungs for air respiration, this association is not invariable, as exemplified by the water lungs of sea cucumbers.

Polychaete marine worms use not only the general surface of the body for gas exchange, but also a variety of gill-like structures: flap-shaped segmental parapodia (in Nereis) or elaborate branching tufts (among families Terebellidae and Sabellidae ). The plumes, used to create feeding and respiratory streams, provide a large surface area for gas exchange.

In echinoderms (starfish, sea urchins, brittle stars), most of the respiratory exchange occurs through the feet of the tube (a series of suction cup extensions used for locomotion). However, this exchange is complemented by extensions of the coelomic cavity, or body fluids, into thin-walled "gills" or dermal branches that bring the coelomic fluid into close contact with seawater. The sea ​​cucumbers (Holothuroidea), soft-bodied, sausage-shaped echinoderms that carry some respiration through their oral tentacles, corresponding to tube feet, also have an elaborate "respiratory tree" consisting of hollow branching sacs from the cloaca (hindgut). Water is pumped in and out of this system by the action of the muscular cloaca, and it is likely that a large fraction of the respiratory gas of animals is exchanged through this system.

The gills of mollusks have a relatively elaborate blood supply, although respiration also occurs through the mantle, or general epidermis. Clams have gills through which water circulates, propelled by the movements of millions of microscopic whips called cilia. In the few forms studied, the extraction of oxygen from the water has been found to be low, on the order of 2 to 10 percent. The currents produced by the ciliary movement, which constitute ventilation, are also used to introduce and extract food. During low tide or during a dry period, clams and mussels close their shells and thus prevent dehydration. The metabolism then switches to oxygen-consuming pathways (aerobics) to oxygen-free pathways (anaerobic), which causes acidic products to accumulate; When normal conditions are restored, the animals increase their ventilation and oxygen extraction to get rid of acidic products. In snails, the feeding mechanism is independent of the respiratory surface. A part of the mantle cavity in the form of a gill or "lung" serves as a place for gas exchange. In air-breathing snails, the "lung" can be protected from drying out by contact with air by having only a pore in the mantle as an opening to the outside. Cephalopod mollusks, such as squid and octopus, actively ventilate a protected chamber lined with feathery gills that contain small blood vessels (capillaries); its gills are quite efficient, extracting between 60 and 80 percent of the oxygen that passes through the chamber. In oxygen-poor waters, the octopus can multiply its ventilation by 10, indicating a more active control of respiration than appears to be present in other classes of mollusks.

Many crustaceans (crabs, prawns, crayfish) are very dependent on their gills. As a general rule, the gill area is larger in fast-moving crabs (Portunidos) than in lazy bottom dwellers; it progressively decreases from totally aquatic species, to intertidal species, to terrestrial species; and it is higher in young crabs than in older crabs. The gills are often encased in protective chambers, and ventilation is provided by specialized appendages that create the respiratory stream. As in cephalopod molluscs, oxygen utilization is relatively high: up to 70 percent of oxygen is extracted from the water that passes through the gills in the european crayfish (Astacus). A decrease in the partial pressure of oxygen in the water causes a marked increase in ventilation (the volume of water that passes through the gills); at the same time, the oxygen utilization rate decreases slightly. Although more oxygen is removed per unit time, increasing ventilation increases the oxygen cost of respiration. The increased cost of oxygen, together with the decrease in extraction per unit volume, probably limits the aquatic forms of crustaceans to levels of oxidative metabolism lower than those found in many forms of air respiration. This is largely due to the lower relative oxygen content in water and the higher oxidative cost of venting a dense, viscous medium compared to air. Not all crustaceans suffer oxygen depletion with increased ventilation and metabolism. The square back crabs (Sesarma) become less active, reducing their oxidative metabolism until more favorable conditions prevail.

Locomotor system

Movement is part of the life of animals. Most animals have ways of moving through their environment to catch food, escape predators, or find mates. Sessile animals have to move the water or air around them to catch food, usually using their tentacles or using cilia shakes to generate streams of water and capture small food particles. Most animal filaments include species that swim, but whether they live on land or in sediments at the bottom of the sea and in lakes, the animals crawl, walk, run, jump, or stand still. Locomotion requires energy, and most animals spend a considerable amount of their time expending energy to overcome the forces of friction and gravity that tend to keep them immobile.

The energy cost of transport or any type of movement is different depending on the environment that surrounds it. In the aquatic environment, most animals float and overcoming gravity is less of a problem. Because water is a much denser medium than air, the main problem is resistance / friction, so the most energy efficient means of locomotion for aquatic organisms is their adaptation to an elegant hydrodynamic shape. Most four-legged aquatic vertebrates use their legs like paddles to push against the water. Fish swim using their body and tail from side to side and aquatic mammals lift their bodies up and down. Invertebrates such as squid, scallops, and some cnidarians are jet-propelled with water that is expelled from certain parts of the body.

At the cellular level, all animal movement is based on two systems of cell motility: the microtubules and microfilaments. Microtubules are responsible for beating the cilia and the undulations of flagella and microfilaments are the contractile elements of muscle cells. But muscle contraction itself cannot translate into movement in the animal unless the muscle has some kind of support to work against and that is some kind of skeleton.

Skeletons support and protect the body of the animal and are essential for movement. There are three types of skeletons: the endoskeleton, the exoskeleton and hydrostatic skeleton. Most cnidarians, flatworms, nematodes, and annelids have a hydrostatic skeleton that consists of a fluid that is kept under pressure in a closed body compartment. These animals can control their body shape and movement by using muscles to change the shape of fluid-filled compartments. Hydrostatic skeletons are ideal for life in aquatic environments and can protect internal organs from shocks and provide support for crawling and digging, but they cannot withstand any form of land locomotion in which the body of an animal is kept off the ground. .

The exoskeleton is a hard coating that is deposited on the surface of an animal. Most mollusks are enclosed in calcium carbonate shells secreted by a lamina as an extension of the body wall, the mantle. Animals increase the diameter of the shell by adding to its outer shell. Arthropods have an articular exoskeleton, the cuticle. As the animal grows in size, an arthropod's exoskeleton must be periodically shed and replaced with a larger one.

An endoskeleton consists of hard support elements buried within the soft tissues of an animal. Sponges, for example, are reinforced with hard spicules or spicules consisting of inorganic material or soft fibers made of proteins. Echinoderms have a hard plate endoskeleton under the skin and sea urchins have a tightly attached ossicle skeleton. Starfish ossicles are looser, allowing the animal to change the shape of its arms. Chordates have endoskeletons that consist of cartilage, bone, or both.

How does an invertebrate defend itself?

Invertebrates have a variety of defensive strategies against predators. Many of them are similar to those used by other animals, including humans. Here is a list of examples:

  • Running or jumping: Grasshoppers and fleas jump long distances. The guérrido (Gerridae) can jump about 20 inches to avoid fish. Many insects, for example cockroaches (which can move quite fast), will flee when threatened by a predator.
  • Flying: Moths, butterflies and almost any species with wings, will fly away.
  • Playing deadStick insects can fall off their perches and pretend to be dead.
  • Hiding: Cockroaches seek shelter. Some moths dive into vegetation when chased by bats. Lower sides of low leaves, stems, and litter, may hide insects and invertebrates from some predators.
  • Camouflaging: Bagworm caterpillars live in camouflaged tubes. When disturbed, they seal the opening. Spittlebug larvae foam up to hide underneath. Tent caterpillars weave a silk web for protection. The slugs are covered with a repellent mud.
  • In colonies: Social insects, animals such as bees, wasps and termites find a refuge in the colonies and defend themselves against each other or have "soldiers" to protect them by attacking intruders en masse.
  • Camouflage: This is probably the main defense of insects and other invertebrates. Many are colored to match their habitats. There are countless examples: stick insects look like stems or twigs or leaves. Many caterpillars match the leaves that they feed on.
    • A variation on this camouflage strategy is the masking. The larvae of geometric moths hold the bits of flowers or leaves as a disguise. Another approach is to look like an undesirable object. Some swallowtail caterpillars look like bird droppings.
  • First move: Scaring an attacker can give him time to escape. Peacock butterflies, with eye-like spots on their wings, suddenly open their wings to show spots to surprise birds. The moths may have red or black wings flashing to distract the birds. Green grasshoppers can display black and yellow hind wings as they wander away. Some insects make noise. Some cockroaches hiss. Tiger moths make clicking sounds.
  • With warning signs: These signs mean "I am dangerous" and are usually bright colors on an insect. Other warning signs come with the smell, stink bugs produce a noxious olfactory fluid that can be emitted when disturbed. Red and black velvet ants are warning examples of colored insects with strong stings or bites. Some colored warning insects obtain their harmful substances by feeding on plants that contain unpleasant or poisonous substances.


Insects in particular are successful because they are so adaptable. They are opportunistic eaters, feeding on decaying plants, animals, and organic material. They are able to survive in extreme environments, including very hot and dry habitats. And many can fly, either to escape predators or to find new sources of food, water, and shelter.

List of invertebrate animals

The bee is essential for pollination.

Honey bee

honey bee o honey bees, also known as apis in Latin, although it is one of the most popular bees, it represents only a small percentage of the bee species. This species is the only surviving group of bees in the Apini subclass, which belongs to the genus Apis. They are known for producing and storing honey, or liquefied sugar, as well as for building impressive nests using wax secreted by the workers of a particular colony.

The bumblebee is a friendly insect that lives almost everywhere on the planet.


bumblebee (Bombus), is a large type of insects, hairy and close relatives of the well-known honey bee. Most species of bumblebees live in colonies, but their colonies are much smaller than those of honey bees or wasps, which can have up to several thousand individuals, the bumblebee colony will only consist of about 50 to 150 individuals. It has a loud hum that you can hear from a distance.

The axolotl is a popular amphibian in fish tanks. Albinos are bred in captivity.


axolotl is axolote (Ambystoma Mexicanum) is a amphibian type of salamander which has the rare trait of retaining its larval characteristics throughout its adult life. This condition, called neotenia, means that it maintains its tadpole-shaped dorsal fin, which runs almost the length of its body, and its feathery external gills, which protrude from the back of its broad head.

Giant target

giant crave (Tridacna gigas) It is the largest of the mollusks that we know, it can measure up to one meter and weigh up to 230 kilograms, being an immobile animal, it does not move from where it sits and remains in that place the rest of its life in the sea. Therefore we can define it as a wild animal that belongs to the Marine animals.

Colony of sea squirts



wasp (Hymenoptera) constitutes an enormous diversity of insects, with some 30.000 identified species. We are most familiar with those that are cloaked in bright warning colors, the ones that buzz angrily in groups and threaten us with painful stings, but most wasps are actually solitary, non-sting varieties. And they all do humans far more good by controlling pest insect populations than harm. Unlike of bees, the body of a wasp is smooth and has no hair.