Invertebrate animals: characteristics and classification


It is known as Invertebrate animals to all the species of the animal kingdom that do not possess a dorsal cord or cord, or a spine, or an articulated internal skeleton. In this set, 95% of the known living species are found, between 1.7 and 1.8 million species (2005 figures).

Invertebrates are also usually smaller, compared to large terrestrial or aquatic vertebrates, and although they lack an articulated skeleton, they often have exoskeleton (such as insects) or shells and shells of resistant substances (such as mollusks).

This is not a homogeneous group, much less: since the beginning of the term "invertebrate" (from French animauxsans vertèbres, animals without vertebrae), its creator, the naturalist Jean-Baptiste Lamarck, subdivided them into ten different classes: mollusks, cirripeda, annelids, crabs, spiders, insects, worms, echinoderms, polyps and infusoria. This classification is no longer used, of course.

The invertebrate group has been difficult to classify and study, given its Small size and diversity of environments that inhabit. In the current classification of zoology, it is estimated that the group is composed of animals of the following edges:

  • Arthropods, equipped with exoskeleton and articulated limbs, such as insects, arachnids, crustaceans and myriapods.
  • Mollusks, with a soft and non-segmented body, often covered in shell or shell to defend it.
  • Porifers, that is, marine sponges, with bodies endowed with radial symmetry.
  • Cnidarians, simple aquatic animals, such as corals and jellyfish.
  • Echinoderms, mostly marine, such as sea urchins and starfish.
  • Platelmintos, that is, flat worms (in some cases parasites).
  • Nematodes, or cylindrical worms.
  • Annelids, or earthworms and leeches.

9 characteristics of invertebrate animals

Invertebrates are all those animals that do not belong to vertebrates, that is, that they don't have a backbone Not an articulated internal skeleton. Invertebrate animals with a majority in the animal world, being 95% of animal species.

There are invertebrates of many kinds, but most have a series of common features that differentiate them from vertebrates. Some of these features are:

  1. As a general rule they are usually very small animals compared to vertebrate animals.
  2. They do not have an articulated internal skeleton, although some do have an external skeleton called exoskeleton, such as arthropods.
  3. It div>


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

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

The body of arachnids is divided into two parts, the cephalothorax (Head and thorax) and abdomen. Unlike insects, they have no antennae and have 4 pairs of legs. They are the second most voluminous species on earth.


Arthropods have articulated legs and a body divided into distinct parts such as a head, thorax and abdomen. They live in all media.

Arthropods can be divided into 4 groups:

The insects
Insects are the most diverse animals on our planet, with millions of species and appear in large numbers. It is estimated that more than 90% of the life forms of planet Earth are insects.
Its body is divided into three parts: the head, the thorax and the abdomen.
They have three pairs of legs and a pair of antennas.
Many times insects have two pairs of wings and are the only invertebrates capable of flying.
Examples of insects: fly, ant, mosquito, beetle, butterfly, bee.

The arachnids
Arachnids are the second largest group in the animal kingdom.
Its body is divided into two parts: the cephalottax (the union of the head and the thorax) and the abdomen.
Arachnids have four pairs of legs and have no antennae.
Examples of arachnids: spider, scorpion, tick.

The mypopods
Myriods have a head and a long trunk formed by many segments.
They have a multitude of pairs of legs and also have antennas and jaws.
Examples of mypopods: centipede, millipede.

Crustaceans are almost all aquatic.
In general they have from 5 to 10 pairs of legs. Some crustaceans have the front legs transformed into tweezers.
They are the only arthropods with two pairs of antennas.
Examples of crustaceans: crab, lobster, shrimp.

Mollusks are the most numerous invertebrates after arthropods.
They have a soft and many body protected by a hard calcific shell of bilateral symmetry. They are the only animals with a muscular foot.

Mollusks can be divided into 3 main groups:

All cephalopods are aquatic and do not have an outer shell.
The feet appear next to the head. Octopuses have 8 feet but other cephalopods can have many more.
They have the most developed eyes of all invertebrates.
Some cephalopods can secrete a black ink to hide.
Examples of cephalopods: octopus, squid.

All bivalves are aquatic.
They have a shell (shell) of two pieces that are called leaflets. The leaflets are normally symmetrical and are joined by a hinge and ligaments. They do not have a differentiated head.
Examples of bivalves: oyster, mussel, clam.

The gasterpods
Two thirds of the species of gastropods live in the sea.
The gastropods have a head, a muscular foot and usually a spirally wound dorsal shell. It has 2 Гі 4 sensory tentacles.
Examples of gastropods: snail, slug, barnacle.


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

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


All echinoderms live in the sea (they do not live in fresh water).
They have a rough body with radial symmetry. It has two well-defined sides, one at the bottom where its mouth is, and the other the hardest top.
The body of a starfish is divided into five regions that are arranged around a central disk. If one of your Arms breaks, easily regenerates.
Hedgehogs have a more round body and are covered with thorns or spines.

Examples of echinoderms: starfish, hedgehog.

They have a soft and elongated body. They move crawling.

The worms can be divided into 3 main groups:

The main feature is that its body is segmented into rings and with bilateral symmetry. They usually live in the sea or in damp places.
Examples of Annelids: earthworm, leech.

Sometimes they are known as roundworms and do not have a segmented body. They have an elongated, cylindrical body with bilateral symmetry.
Examples of Nematodes: anisakis, triquina, oxiuro (pidulle).

They are of bilateral symmetry and are usually flattened like a tape.
Many of the flatworms are parasites that need a guest while others live in humid marine or terrestrial environments.
Examples of Platelmintos: planaria, had (solitary).

Celentà © reos

There are two forms of celentà © reos, the jellyfish that can move freely and the polyp They are fixed in one place.

Jellyfish have an almost transparent body, float in water and have a radial shape resembling an umbrella. They have tentacles, which produce hives or paralyze.

The polyp are shaped like a sack, with one end that is fixed to a rock (or marine object) and the other side with a hole with tentacles to catch their prey.

Examples of celentà © reos: sea anemone, coral.

Porphers / Sponges

Porphors are better known as sponges. They look like a plant and live in the sea subject to rocks or other submerged objects.
Your body has many holes or small pores.
They have an irregular body without symmetry.
Porphors are the simplest invertebrates, they have no organs, no brain, no nervous system. They use certain cells to capture food particles that are in the water that enters through their pores.

Examples of porphors: Tubular sponge.

General characteristics of invertebrate or boneless animals

What are the characteristics of invertebrate animals? The truth is that there are many, but these are some of the main ones. To begin with, his name refers to they lack a spine (or notochord) and an internal skeleton, as in the case of vertebrates, so we can say that they are animals that have no bones. In addition, they are usually small animals, although variable, and many of them have protective structures such as shells, shells or covers.

These invertebrates are found in almost all habitats found on Earth, from waters at high temperatures in hydrothermal sources to the depths of the seas or the frozen surface of the Antarctic.

For a long time his research was scarce due to its complex description and the dismissal of its potential in various fields such as economic, scientific and, although it may not be shared in all cultures, the food field. In addition, to all this it was added that this group are often small animals and that live discreetly, in the seas, sediments, soils, etc.

Despite all this, they constitute the most of the known biodiversity in the planet. In 2005, between 1.7 and 1.8 million species were censored, of which about 990,000 were from invertebrate animals or boneless animals. However, these figures are constantly changing.

Great freshwater invertebrates

This term is frequently used to refer to invertebrates that inhabit freshwater, as riverbeds, ponds, lakes and lagoons. It includes the following groups of aquatic macroinvertebrates:

  • Insects
  • Crustaceans
  • Annelids
  • Mollusks
  • Platelmintos

The abundance and diversity of these macroinvertebrates are factors used as bioindicators, that is, they are indicators of the state of the ecosystem and of the local biodiversity. They are essential to serve as food for other organisms and be transformers of organic matter and, in addition, they function as predators in small ponds in which the absence of fish makes them the main responsible for the predation of zooplankton.

Invertebrates or animals that have no bones: the groups

There is a lot of invertebrate groups, but the largest of them is that of arthropods. Thus, of the immense variety of invertebrate edges (about 34) the vast majority is in the group of arthropods (80%) of animals. Some of the most common edges of invertebrates They are (8 edges):

  • Porifers (9,000 species or sp).
  • Cnidarians (10,000 sp)
  • Platelmintos (20,000 sp)
  • Mollusks (100,000 sp)
  • Annelids (16,500 sp)
  • Nematodes (25,000 sp)
  • Arthropods (more than 1,000,000 sp).
  • Echinoderms (7,000 sp).

Porifers or sponges, marine invertebrate animals

Porifers or sponges are the most primitive animals and simple. They are aquatic organisms and sessile life, that is, they grow and live rooted in the ground or rocks. Its body is formed by numerous small holes as pores (which is why they are called porifers). They have a larger hole called the osculus.

Sea sponges They feed on the particles that are in the water, which passes through currents through the internal channels of your body. These currents are created by cells that have a scourge surrounded by a fold called co-ocytes. Learn more about them in this other Green Ecology article about the Starfish: what it is and its characteristics.

All of them they are aquatic, the cnidarians encompass corals, jellyfish and hydras. They are animals whose body is shaped like a sac with a single opening that constitutes the mouth and connects with the gastrovascular cavity (similar to a primitive stomach). Around the mouth they have tentacles with specialized cells called cnidoblasts with stinging substances that serve as both defense and attack on their prey.

It is a kind of invertebrates that do not move For example, jellyfish are carried along by the current and corals remain anchored to the ground or rocks.

Platelmintos or flat worms, other animals without bones

Also called "flat worms", they are the group of tapeworms, planarians and trematodes. They present free life forms and parasitic life forms (had or lonely). The novelty that they present with respect to the previous groups is the presence of a Central Nervous System, which is able to process the information collected by the eyes and other sensitive structures. Its body can be divided into two identical halves lengthwise.

Mollusks, another well-known group>

The mollusks, the group of snails, clams, squids or octopusesThey have a soft body, with species in which it is protected by a hard shell.

Its internal organs form a visceral mass and its body consists of a muscular foot, the mantle, the paleal cavity that functions as a respiratory system and, finally, a shell of limestone materials that can be formed by a single piece or leaflet or by two pieces. This may be found internally in some groups or may be non-existent.

The mollusks They consist of 8 classes, of which the largest number of species present are:

  • Gastropods (75,000 species): terrestrial or aquatic, they have a widened foot and eye tentacles. They have a shell formed by a single piece. Example: snails and barnacles.
  • Bivalves (13,000 species): all of them aquatic organisms that are buried in the seabed. They have an ax-shaped foot and lack a differentiated head. They feed on the organic particles that are floating in the water. Example: clams, mussels, oysters ...
  • Cephalopods (800 species): All of them marine. They are the group of octopus, squid and cuttlefish. They have a very developed foot from which the tentacles leave with suction cups so characteristic. The shell is internal or non-existent, depending on the species. To move consist of a propellant siphon.

Animals without skeleton: anél>

This group of animals that have no bones is the famous group earthworm. It is cylindrical body animals and divided into rings. They present metamería, that is to say, certain organs are repeated in each one of the rings. Your body is covered by mucus, a viscous substance secreted by certain glands, which facilitates its movement. Most are free-living, that is, they are not parasites.

Round nematodes or worms, animals without bones and parasites

The nematodes or “round worms”, Are organisms of great abundance and diverse, found in the soil and aquatic habitats. Its body is without segmenting and it has characteristic muscles for its displacement. Can be of free life or parasites. From the appearance of the nematodes, a new body cavity called celoma appears in the invertebrates, which makes the existence of hollow internal organs possible.

Other large groups of boneless animals: arthropods or insects, arachn>

It is the most diverse and heterogeneous group, being found in almost all existing habitats. It is very important because they encompass the great most known animal species (80% of animals are arthropods), such as insects, crustaceans or arachnids.

They have a very elaborate body design, with an exoskeleton chitin divided into segments and appendages (legs, antennae and mouthparts).

When the growth of the individual occurs, it emerges from its exoskeleton through the change, after which it produces a new one. This is repeated several times throughout his life. In addition, it is common for the juvenile to be very different from the adult animal, so it undergoes a process of metamorphosis until reaching its final adult appearance.

Arthropods are divided into several groups. Depending on the type of appendices and the number, some of the most common arthropods They are:

  • Myriapods: Its body consists of a head with a pair of antennae and an elongated trunk that is segmented. One or two pairs of legs are born from each of these segments. Example: centipede and scolopendras.
  • Arachnids: The body is divided into cephalothorax (the head and abdomen are fused) and abdomen. They have four pairs of legs, lack antennas or jaws and in the mouth they have clamp-shaped structures, called chelyces, and a pair of appendages (non-legs) called pedipalps. Example: spiders, scorpions and mites.
  • Crustaceans: The body consists of cephalothorax and abdomen. They have two pairs of antennas and all of them are aquatic. Within the crustaceans we find the inferior crustaceans, such as barnacles or other species that make up the zooplankton, and the decapods, such as prawns, crabs or shrimp. They have 10 legs in the cephalothorax and a pair of appendages per segment of the abdomen.
  • Insects: divided into head, thorax and abdomen, they have a pair of antennas, a pair of compound eyes and several simple eyes. Its distinctive feature is the presence of 6 legs and a pair of wings (except in some groups). There are a large number of orders of insects that are classified based on the type of wings, mouthparts, according to the type of feeding or metamorphosis they suffer. Example: butterflies, grasshoppers, beetles, ants ...


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

They have a two-part conformed shell called valva (hence its name), they are all aquatic invertebrate animals and do not have a recognized head. Its leaflets are usually symmetrical, like that of oysters.


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

Formed by a long and soft body, the worms move crawling. 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, its body is shaped like a cylinder and is elongated. The best known nematode is anisaki.


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

Jellyfish are almost transparent, float and have an umbrella shape. Their tentacles are dangerous, as they can hurt or paralyze.

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

Commonly called sponges, they live on the rocks of the sea. They have a plant shape and its body is formed by holes and small pores that it uses to feed itself and they are totally asymmetrical. They have the simplest organism of invertebrate animals (they don't have organs or nervous system, they only have cells that they use to feed).


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

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

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


Reproduction in invertebrates differs depending on the 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 sex organs are present in an individual. In single-sex species, where only one sexual organ is present, males and females do not have to make contact to reproduce since fertilization can occur externally. After reproduction, most invertebrates change shape and appearance by going through a process called metamorphosis in which adults and young people have different lifestyles, including how and what they feed on.


The two common respiratory organs of invertebrates are the windpipe and the gills. The diffusion lungs, in contrast to the ventilation 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 eliminate 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 supply of molecules that contain energy derived from food).

The pores outward, called spiraclesThey 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 by 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 tracheal systems are designed primarily for life in the air, in some insects the modifications allow the tracheas to be used for the exchange of gases under water. Of special interest are insects that could be called bubble respirators, which, as in the case of water beetle (Dytiscus), they receive a gas supply in the form of an air bubble under the surfaces of their wings next to the spiracles before diving. The exchange of tracheal gases continues after the beetle submerges and anchors beneath the surface. As the oxygen is consumed from the bubble, the partial pressure of oxygen inside the bubble falls below that of the water, consequently, the oxygen diffuses from the water to the bubble to replace the one consumed. The carbon dioxide produced by the insect is diffused 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 the oxygen is removed from the bubble, the partial pressure of the nitrogen increases, and this gas diffuses outward into the water. The consequence of the external diffusion of nitrogen 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 bubble renewal has been found by small family water beetles Elmidae, which capture bubbles that contain oxygen produced by the algae and incorporate this gas into the gill of the bubble. Several species of water beetles also increase gas exchange by stirring the surrounding water with their hind legs.

An elegant solution to the problem of bubble depletion during immersion has been found by certain beetles that have a high density of cutaneous hair on much of the surface of the abdomen and thorax. The pile of hair is so dense that it resists moisture, and an air gap is formed beneath it, creating a plastron, or layer of air, in which the tracheas open. As breathing progresses, diffusion out of nitrogen and the consequent contraction of the gas space are prevented by surface tension - a condition manifested by properties that resemble those of an elastic skin under tension - between tight hair and Water. The plastron becomes "permanent" in the sense that it is no longer necessary to trap 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 of plastron gas.

An extraordinary strategy used by insects hemipteranos Good toY AnisopsIt is an internal oxygen store that allows them to stalk for minutes without surfacing while they wait for food in areas of medium waters relatively free of predators but poor in oxygen. The internal oxygen reserve is presented in the form of hemoglobin-filled cells that constitute the first oxygen supply line to actively metabolize the cells, saving the small mass of air in the tracheal system while the hemoglobin reserve is running low.

The respiratory structures of spiders consist of peculiar «book lungs«, Sheet-shaped plates on which air circulates through openings in the abdomen. They contain blood vessels that put the blood in close contact with the surface exposed to the air and where gas exchange occurs between the blood and the air. In addition to these structures, there may also be abdominal spiracles and a tracheal system such as insects.

Because spiders are air respirators, 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 along the water surface as easily as on land. . The water spider (or diving bell spider), (Argyroneta aquatica) Known for its underwater silk cloth, which resembles a kind of diving bell, it is the only spider species that spends its entire life underwater. Using fine hairs on her abdomen, where her respiratory openings are located, the water spider captures small 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 inwards, thus inflating the underwater house with air. Research has shown that the inflated network serves as a kind of gill, removing dissolved oxygen from water when oxygen concentrations within the network are low enough to extract oxygen from water. As the spider consumes oxygen, the concentrations of nitrogen in the inflated spider 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 approximately once a day. Most of the water spider's life cycle, including courtship and reproduction, the capture and feeding of prey and the development of eggs and embryos, occurs below the surface of the water. Many of these activities take place within the spider's diving bell.

Many immature insects have special adaptations for an aquatic existence. The thin-walled bumps of the tegument, which contain tracheal networks, form a series of gills (tracheal gills) that make water come into contact with the closed tracheal tubes. Mayfly nymphs and dragonflies have external tracheal gills attached to their abdominal segments, and some of the gill plates can be moved in such a way that they create water currents on the exchange surfaces. Nymphs dragonflies possess 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. Gill removal or plugging of the rectum results in lower oxygen consumption. En los insectos acuáticos inmaduros también se produce un intercambio de gases considerable en la superficie general del cuerpo.

El sistema traqueal del insecto tiene limitaciones inherentes. Los gases se difunden lentamente en tubos largos y estrechos, y el transporte efectivo de gas sólo puede ocurrir si los tubos no exceden una cierta longitud. Generalmente se piensa que esto ha impuesto un límite de tamaño a los insectos.

Muchos invertebrados utilizan las branquias como un medio importante de intercambio de gases, unos pocos, como el caracol pulmonado, utilizan los pulmones. Casi cualquier extensión de pared delgada de la superficie del cuerpo que entra en contacto con el medio ambiente y a través de la cual ocurre el intercambio de gases puede ser vista como una branquia.

Las branquias suelen tener una gran superficie en relación con su masa, a menudo se utilizan dispositivos de bombeo para renovar el medio externo. Aunque las branquias se utilizan generalmente para la respiración acuática y los pulmones para la respiración con aire, esta asociación no es invariable, como lo ejemplifican los pulmones de agua de los pepinos de mar.

Los gusanos marinos poliquetos utilizan no sólo la superficie general del cuerpo para el intercambio de gases, sino también una variedad de estructuras parecidas a las agallas: parapodia segmentaria en forma de colgajo (en Nereis) o mechones ramificados elaborados (entre las familias Terebellidae y Sabellidae). Los penachos, utilizados para crear corrientes de alimentación y respiratorias, ofrecen una gran superficie para el intercambio de gases.

En los equinodermos (estrellas de mar, erizos de mar, estrellas quebradizas), la mayor parte del intercambio respiratorio ocurre a través de los pies del tubo (una serie de extensiones de ventosas utilizadas para la locomoción). Sin embargo, este intercambio se complementa con extensiones de la cavidad celómica, o de los fluidos corporales, en «branquias» de paredes finas o ramificaciones dérmicas que hacen que el fluido celómico entre en contacto cercano con el agua de mar. The pepinos de mar (Holothuroidea), equinodermos de cuerpo blando y forma de salchicha que llevan cierta respiración a través de sus tentáculos orales, que corresponden a pies de tubo, también tienen un elaborado «árbol respiratorio» que consiste en sacos huecos ramificados de la cloaca (intestino posterior). El agua es bombeada dentro y fuera de este sistema por la acción de la cloaca muscular, y es probable que una gran fracción del gas respiratorio de los animales se intercambie a través de este sistema.

Las branquias de los moluscos tienen un suministro de sangre relativamente elaborado, aunque la respiración también ocurre a través del manto, o epidermis general. Las almejas poseen branquias por las que circula el agua, impulsadas por los movimientos de millones de látigos microscópicos llamados cilios. En las pocas formas estudiadas, se ha encontrado que la extracción de oxígeno del agua es baja, del orden del 2 al 10 por ciento. Las corrientes producidas por el movimiento ciliar, que constituyen la ventilación, también se utilizan para introducir y extraer alimentos. Durante la marea baja o durante un período seco, las almejas y los mejillones cierran sus conchas y previenen así la deshidratación. El metabolismo entonces cambia de vías que consumen oxígeno (aeróbicas) a vías libres de oxígeno (anaeróbicas), lo que hace que los productos ácidos se acumulen, cuando se restablecen las condiciones normales, los animales aumentan su ventilación y extracción de oxígeno para deshacerse de los productos ácidos. En los caracoles, el mecanismo de alimentación es independiente de la superficie respiratoria. Una parte de la cavidad del manto en forma de branquia o «pulmón» sirve como lugar de intercambio de gas. En los caracoles que respiran aire, el «pulmón» puede protegerse de la desecación por contacto con el aire al tener sólo un poro en el manto como abertura hacia el exterior. Los moluscos cefalópodos, como el calamar y el pulpo, ventilan activamente una cámara protegida recubierta de branquias plumosas que contienen pequeños vasos sanguíneos (capilares), sus branquias son bastante eficaces, extrayendo entre el 60 y el 80 por ciento del oxígeno que pasa por la cámara. En aguas pobres en oxígeno, el pulpo puede multiplicar por 10 su ventilación, lo que indica un control más activo de la respiración de lo que parece estar presente en otras clases de moluscos.

Muchos crustáceos (cangrejos, gambas, cangrejos de río) son muy dependientes de sus branquias. Como regla general, el área de las branquias es mayor en los cangrejos de movimiento rápido (Portunidos) que en los habitantes de fondo perezosos, disminuye progresivamente de especies totalmente acuáticas, a especies intermareales, a especies terrestres, y es mayor en los cangrejos jóvenes que en los cangrejos más viejos. A menudo las branquias están encerradas en cámaras de protección, y la ventilación es proporcionada por apéndices especializados que crean la corriente respiratoria. Al igual que en los moluscos cefalópodos, la utilización de oxígeno es relativamente alta: hasta el 70 por ciento del oxígeno se extrae del agua que pasa por las branquias en el cangrejo de río europeo (Astacus). Una disminución de la presión parcial de oxígeno en el agua provoca un notable aumento de la ventilación (el volumen de agua que pasa por las branquias), al mismo tiempo, la tasa de utilización de oxígeno disminuye ligeramente. Aunque se extrae más oxígeno por unidad de tiempo, el aumento de la ventilación aumenta el costo de oxígeno de la respiración. El aumento del coste del oxígeno, junto con la disminución de la extracción por unidad de volumen, probablemente limita las formas acuáticas de los crustáceos a niveles de metabolismo oxidativo inferiores a los que se encuentran en muchas formas de respiración por aire. Esto se debe en gran medida al menor contenido relativo de oxígeno en el agua y al mayor coste oxidativo de ventilar un medio denso y viscoso en comparación con el aire. No todos los crustáceos sufren una reducción del oxígeno con una mayor ventilación y metabolismo. The cangrejos de espalda cuadrada (Sesarma) se vuelven menos activos, reduciendo su metabolismo oxidativo hasta que prevalecen condiciones más favorables.

Sistema locomotor

El movimiento forma parte de la vida de los animales. La mayoría de los animales tienen maneras de moverse por su entorno para atrapar comida, escapar de los depredadores o encontrar pareja. Los animales sésiles tienen que mover el agua o el aire que los rodea para atrapar comida, generalmente usando sus tentáculos o usando cilios batidos para generar corrientes de agua y capturar pequeñas partículas de comida. La mayoría de los filamentos de los animales incluyen especies que nadan, pero ya sea que vivan en la tierra o en los sedimentos del fondo del mar y en los lagos, los animales se arrastran, caminan, corren, saltan o se quedan quietos. La locomoción requiere energía, y la mayoría de los animales gastan una cantidad considerable de su tiempo gastando energía para superar las fuerzas de fricción y gravedad que tienden a mantenerlos inmóviles.

El coste energético del transporte o de cualquier tipo de movimiento es diferente en función del entorno que lo rodea. En el medio acuático, la mayoría de los animales flotan y la superación de la gravedad es un problema menor. Debido a que el agua es un medio mucho más denso que el aire, el principal problema es la resistencia/fricción, por lo que el medio de locomoción más eficiente energéticamente para los organismos acuáticos es su adaptación a una forma hidrodinámica elegante. La mayoría de los vertebrados acuáticos de cuatro patas usan sus patas como remos para empujar contra el agua. Los peces nadan usando su cuerpo y su cola de lado a lado y los mamíferos acuáticos levantan su cuerpo hacia arriba y hacia abajo. Invertebrados como calamares, vieiras y algunos cnidarios son propulsados a chorro con agua que se expulsa de ciertas partes del cuerpo.

A nivel celular, todo movimiento animal se basa en dos sistemas de motilidad celular: los microtúbulos y los microfilamentos. Los microtúbulos son responsables del batido de los cilios y las ondulaciones de flagelos y microfilamentos son los elementos contráctiles de las células musculares. Pero la contracción muscular en sí misma no puede traducirse en movimiento en el animal a menos que el músculo tenga algún tipo de apoyo contra el que trabajar y eso es algún tipo de esqueleto.

Los esqueletos sostienen y protegen el cuerpo del animal y son esenciales para el movimiento. Existen tres tipos de esqueletos: el endoesqueleto, he exoesqueleto y el esqueleto hidrostático. La mayoría de los cnidarios, gusanos planos, nemátodos y anélidos tienen un esqueleto hidrostático que consiste en un líquido que se mantiene bajo presión en un compartimiento corporal cerrado. Estos animales pueden controlar la forma y el movimiento de su cuerpo usando músculos para cambiar la forma de los compartimentos llenos de fluido. Los esqueletos hidrostáticos son ideales para la vida en ambientes acuáticos y pueden proteger los órganos internos de los choques y proporcionar apoyo para arrastrarse y excavar, pero no pueden soportar ninguna forma de locomoción terrestre en la que el cuerpo de un animal se mantenga alejado del suelo.

El exoesqueleto es un revestimiento duro que se deposita en la superficie de un animal. La mayoría de los moluscos están encerrados en conchas de carbonato de calcio secretadas por una lámina como extensión de la pared del cuerpo, el manto. Los animales aumentan el diámetro de la cáscara añadiendo a su capa exterior. Los artrópodos tienen un exoesqueleto articular, la cutícula. A medida que el animal crece en tamaño, el exoesqueleto de un artrópodo debe ser periódicamente mudado y reemplazado por uno más grande.

Un endoesqueleto consiste en elementos de soporte duros enterrados dentro de los tejidos blandos de un animal. Las esponjas, por ejemplo, se refuerzan con espículas duras o consistentes en material inorgánico o fibras blandas hechas de proteínas. Los equinodermos tienen un endoesqueleto de placas duras debajo de la piel y los erizos de mar tienen un esqueleto de osículos fuertemente unidos. Los osículos de las estrellas de mar están más sueltos, lo que permite al animal cambiar la forma de sus brazos. Los cordados tienen endoesqueletos que consisten en cartílago, hueso o ambos.

¿Cómo se defiende un invertebrado?

Los invertebrados tienen una variedad de estrategias defensivas contra los depredadores. Muchos de ellos son similares a los utilizados por otros animales, incluyendo humanos. He aquí una lista de ejemplos:

    Corriendo o saltando: Los saltamontes y las pulgas saltan largas distancias. The guérr >Hábitat

Los insectos en particular tienen éxito porque son muy adaptables. Son comedores oportunistas, se alimentan de plantas, animales y material orgánico en descomposición. Son capaces de sobrevivir en ambientes extremos, incluyendo hábitats muy calientes y secos. Y muchos pueden volar, ya sea para escapar de los depredadores o para encontrar nuevas fuentes de alimento, agua y refugio.