VERTEBRATES: CHARACTERISTICS, CLASSIFICATION, EXAMPLES, SYSTEMS - BIOLOGY - 2024

The vertebrates are animals with backbone, hence the name. They belong to a subphylum called Vertebrata, also known as Craniata, which is included in the Chordata phylum of the Animalia kingdom.

Many of the best known and studied animals in the animal kingdom are classified in this group, as it includes reptiles, birds, amphibians and mammals, to name a few.

Photograph of a crocodile (Image by S. Hermann & F. Richter at www.pixabay.com)

Vertebrates represent the most numerous subphylum of the three that make up the Chordata phylum: Cephalochordata, Urochordata and Vertebrata.

This edge, however, is neither the most diverse nor the most abundant group among animals, at least with respect to the number of species; although it could rank fourth after arthropods, nematodes and mollusks, all invertebrate animals.

Despite the above, we have to say that the group of vertebrates is the one that has the largest and most colorful animals on the planet and with which human beings are most familiar.

Characteristics of vertebrates

Vertebrate animals are distinguished from another large group of animals, invertebrates (much more abundant and diverse), by the development of a vertebral column and the vertebrae that accompany it. However, many other elements characterize these animals:

Composed of eukaryotic cells

Like all organisms that we consider "animals", vertebrates are composed of eukaryotic cells that have a membranous nucleus where DNA is enclosed and that have other essential internal organelles such as:

- Mitochondria

- Lysosomes

- Peroxisomes

- Endoplasmic reticulum

- Golgi complex

Heterotrophic organisms

They are heterotrophic organisms, that is, their cells are not capable of synthesizing their own food and must obtain it from organic matter extracted from other living organisms, be they of plant origin (herbivores) or animal (carnivores).

Body

All vertebrate animals have a well-defined head, a thoracic or "trunk" region, and a caudal or "tail" portion.

They generally reach large sizes, thanks to the presence of an endoskeleton (bone or cartilaginous) under the skin.

This endoskeleton allows the support of your internal organs and is associated with muscles and joints, which make movement and other motor actions possible, as well as the protection of delicate organs.

- In the cephalic region (the head) there is a brain and three sensory organs: that of smell, that of sight and that of hearing.

- The trunk or thoracic region consists of a bilateral cavity (which if cut in half produces two almost identical portions) that contains the viscera.

- Usually the caudal portion contains the exit orifices of the digestive and excretory systems (for feces and urine).

All vertebrates also have:

- a notochord or notochord (a rigid “rod” that extends throughout the body in embryos and is often replaced by the spine)

- pharyngeal clefts

- thyroid gland

- a hollow dorsal nerve cord that forms the central nervous system

- a postnatal tail, representing a posterior elongation that extends beyond the anus

Some of these characteristics are only briefly present during embryonic development and others last until the animal's adulthood, but they are common to all vertebrates and also to chordates in general.

Spine

The main differences of the vertebrate group with respect to the other groups of chordates and invertebrates, of course, correspond to the vertebral column and the development of the skull and head.

The spine consists of a series of bones separated by blocks of cartilage that are firmly attached to each other like a column, which defines the main axis of the body. Between each vertebra are discs or "compression pads" called intervertebral discs.

Each vertebra is actually a cylindrical body that "encapsulates" what we call the notochord, within which the spine and some blood vessels are enclosed.

Habitat

The subphylum Vertebrata is a diverse group of animals from the point of view of size, shape, nutrition, habits and life cycle. They occupy marine, freshwater, terrestrial and even aerial environments, thus exhibiting a wide range of lifestyles.

Reproduction

All vertebrates multiply by sexual reproduction, so it is not common to observe clonal populations of vertebrate animals, that is, organisms identical to their progenitors.

Diversity among vertebrates

It is estimated that this group contains approximately 45 thousand species of animals, many of which are distributed from the Arctic or the Antarctic to the tropical regions of the planet.

The only place where vertebrates have not been detected is in the interior of Antarctica, in the colder areas of Greenland and in the “ice pack” of the North Pole, but they are in practically all the ecosystems of the biosphere.

Classification (types) of vertebrates

Let's see what are the main classes of vertebrates:

- Bony fish (c lase Osteichthyes)

Koi fish photography (Image by Pexels on Pixabay.com)

This group contains most of the fish with which we are familiar. All have partially or fully ossified jaws and skeletons.

They have a swim bladder, even fins, gills covered by a bony operculum, scales, a “lateral line” system (a sensory organ) and are almost all oviparous with external fertilization, although there are ovoviviparous and viviparous.

This class is also subdivided into two: the Actinopterygii class and the Sarcopterygii class. Actinopterygians are the "ray-finned fish" and sarcopterygii are the lobe-finned fish.

- Amphibians (c lase Amphibia)

Photograph of a frog, a type of amphibian (Image by Chalupský on www.pixabay.com)

They are cold-blooded animals. They can breathe using lungs, gills, integuments (the skin), or the lining of the mouth. They are characterized by having an aquatic larval stage or inside an egg. Their skin is moist and has many mucous glands, they do not have scales.

They are tetrapods, that is, they have four limbs. They can inhabit bodies of fresh water or be of terrestrial life. They have separate sexes, external fertilization, some with internal development; they can be ovoviviparous or viviparous.

To this class belong the Aponda orders, which includes the caecilians, the Anura order, where the frogs and toads are, and the Caudata order, which contains the salamanders.

- Reptiles (Reptilia class)

Photograph of a chameleon, a type of reptile (Image by PublicDomainPictures at www.pixabay.com)

They are also cold-blooded organisms, but they do not have a larval stage during their development. They use lungs to breathe and have well ossified skeletons. Their skin is dry, with scales, but without glands.

Its limbs have 5 fingers and usually have claws. During their reproduction, internal fertilization occurs and they have direct development, being able to be oviparous and ovoviviparous.

The class is divided into subclasses Anapsida (tortoises and water turtles), Lepidosauria (lizards with scales), and Archosauria. It also includes the subclasses Synapsida, Ichthyopterygia, and Synaptosauria, but they are from species now extinct.

- Birds

Photograph of a type of pigeon, a bird (Image by Ray Miller www.pixabay.com)

They are warm-blooded animals, whose "front" limbs are specialized for flight. The hind limbs have 4 or fewer toes and their bodies are covered with feathers, except for the legs, which have scales.

Instead of teeth they have horny beaks, all are oviparous with internal fertilization. Two subclasses are recognized: the Archaeornithes subclass (of extinct birds) and the Neornithes subclass, also called “true birds”.

- Mammals (class Mammalia)

Photograph of a cow and her calf (Image by Francesco Pitarresi at www.pixabay.com)

They are warm-blooded animals that are characterized by the presence of mammary glands and a lower jaw made up of a single bone. They have hair, a well-developed brain, and skin that covers them with glands and hair.

Juveniles feed on the milk produced by the mammary glands and are formed by internal fertilization. With few exceptions, it is a group of viviparous animals.

It is divided into the Prototheria and Theria subclasses. The first is a "primitive" class of mammals that lay eggs, but have mammary glands (no nipples) and hair. The second represents mammals with mammary glands and nipples, with functional teeth, uterus and vagina, all viviparous.

- Agnatos (Agnatha class)

Photograph of a witch fish (Source: User: (WT-shared) Pbsouthwood at wts wikivoyage / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0) via Wikimedia Commons)

These are the jawless fish, better known as "witch fish" and lampreys. They are considered a "primitive" group, since they do not have bones. They inhabit exclusively marine environments, have a soft, glandular and slimy skin, and lack true gill arches.

- Chondrichthyes (Chondrichthyes class)

Photograph of a group of manta rays, cartilaginous fish (Image by Mary Gasaway at www.pixabay.com)

They are also called cartilaginous fish. They have jaws, even fins, separate sexes (male and female), they can be oviparous, ovoviviparous or viviparous. Sharks and stingrays belong to this group.

The class is subdivided into two subclasses: the Elasmobranchii subclass and the Holocephali subclass. The first are sharks and rays, characterized by the presence of numerous teeth, 5 to 7 gill slits, scales, a cloaca, breathing spiracles, etc.

Holocephalos, also called "chimeras", are cartilaginous fish that have no scales, no cloaca or spiracles. Their teeth are fused to "bony" plates and they live in temperate marine waters.

Examples of vertebrate species

- Sharks

Photograph of a Carcharodon carcharias specimen (Source: Sharkdiver68 / Public domain, via Wikimedia Commons)

Within the group of fish are sharks, which are important marine predators with surprising hunting abilities. The body of these animals has an aerodynamic design that allows them to reduce the resistance of the water and thus be able to swim at high speeds.

They have thick, triangular, serrated teeth, so they can look quite scary. A popular example of this group is the white shark, whose scientific name is Carcharodon carcharias, widely distributed in the world's oceans, highly threatened or considered vulnerable.

- Amphibians

Photograph of a specimen of Phyllobates terribilis (Source: H. Krisp / CC BY (https://creativecommons.org/licenses/by/3.0) via Wikimedia Commons)

Among amphibians there are very small animals, but extremely dangerous, some species of poisonous frogs are an example. Among these, some representatives of the Dendrobatidae family stand out, specifically the genus Phyllobates.

The golden poison frog, Phyllobates terribilis is an endemic species of the Colombian coast that faces the Pacific and is considered the most toxic animal in the world, despite its beautiful appearance.

- Mammals

Photograph of an Asian elephant taking a bath (Source: Basile Morin / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0) via Wikimedia Commons)

In contrast to the aforementioned little frog, vertebrates also contain the largest and colossal animals in the world, among which we can mention elephants.

Elephas maximus is a species of Asian elephant in the family Elephantidae and is considered the largest mammal in all of Asia. It was domesticated and exploited by man for construction and transportation, as well as for entertainment, but today it is in danger of extinction.

Vertebrate nervous system

Nervous system of the human being, a vertebrate. Medium69, Jmarchn / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0)

The central nervous system of all chordates is found in a dorsal position, in a tubular structure that has an epidermal origin. Sensory structures include paired organs for smell, vision, and hearing, which are accommodated in a well-defined head region, that is, the head.

In the skull that forms the head is one of the most advanced and complex organs of living beings: the brain. This body functions as an information storage center, as a decision-making center and as a center for processing stimuli.

The nasal vesicle is open to the environment, so its sensory cells resemble those that form the taste buds of the mouth. The eyes are highly complex organs and correspond to a lateral "pocket" at the anterior end of the brain tube.

The bony fish lateral line system and sensory organs are unique to vertebrates.

Circulatory system of vertebrates

Closed circulatory system

Most of the animals belonging to this group are characterized by having a closed circulatory system, in which blood is pumped by a well-defined heart through blood vessels.

They have a specialized blood tissue, with cells of different types that are responsible for the transport of oxygen and carbon dioxide as well as nutrients and other important compounds for the defense of the body against infectious or "foreign" agents.

It is in this group that the immune function of blood cells develops, although there are many differences between the groups.

Digestive and excretory systems of vertebrates

Scheme of the human digestive system. By Mariana Ruiz (Spanish version); User: Bibi Saint-Pol, Jmarchn (Spanish version, translation by User: AlvaroRG) - Own work; translated from Image: Digestive system diagram en.svg, Public Domain, The digestive system of vertebrates generally consists of a mouth, an esophagus that extends from the pharynx to a cavity (the stomach), and an intestine that starts from the stomach and ends in the anus.

As we can see, this system has quite complex structures, but it performs the same functions as any digestive system in any animal.

The mouth works in the pre-processing of food, which is transported through the esophagus to the stomach, a more dilated cavity where some acids and digestive enzymes are secreted, which begin with the breakdown and digestion of these foods.

Other organs such as the liver and pancreas are involved in this process, characterized by their secretory functions. What results from stomach digestion then passes into the intestines, where the absorption of nutrients in the form of proteins and lipids, as well as water and salts occurs.

Anything that is not digested or processed is excreted with the feces. Some vertebrates have sewers where solid food waste and liquid waste such as urine converge, while others have different evacuation channels.

Regarding the excretory system

Vertebrates have an excretory system that consists of nephrons, structures capable of filtering blood and removing wastes through secretion and reabsorption processes.

In some cases, the large intestine acts as an auxiliary excretory organ, as well as the gills of fish and the sweat glands of the integument of mammals.

Respiratory system of vertebrates

Human respiratory system

The cells of the body of any vertebrate must replenish the oxygen they extract from the environment that surrounds them and get rid of the secondary products that accumulate during the metabolic reactions they need to live.

For this they need both the circulatory system, which transports different elements through the body, and the respiratory system, which is responsible for the exchange of gases between the body and the environment. We can consider that both systems participate in the "acceleration" of a diffusion process.

Diffusion is defined as the random movement of molecules from a place where they are highly concentrated to another where they are less so. In the case of gases, diffusion is usually studied in terms of pressure and not concentration.

Generally, oxygen is in greater quantity in the environment than in the body of an animal, so it tends to diffuse towards it; while carbon dioxide, one of the products of respiration, is more concentrated in the body than in the environment, so it has to "go out" towards the latter.

Specialized pumps and structures

Respiration works in such a way that gaseous oxygen in the environment (speaking of terrestrial vertebrates) or dissolved in water (for aquatic vertebrates) is transported to the body, specifically to the lungs (there are different devices that participate in different animals).

In terrestrial vertebrates, the "pump" that is responsible for this transport is the rib cage, just as the pump that moves blood is the heart. Both pumps are responsible for maintaining the gas pressure gradients necessary for the exchange with the environment to occur.

Many vertebrates have lungs and those that do not have gills. But there are other animals that use the skin as a gas exchange system.

In these structures, the diffusion of oxygen to the blood and carbon dioxide to the environment is facilitated, whether it is an aquatic or terrestrial animal.

References

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2. Jollie, M. (2019). Encyclopaedia Britannica. Retrieved April 18, 2020, from www.britannica.com/animal/vertebrate
3. Kardong, KV (2006). Vertebrates: comparative anatomy, function, evolution (No. QL805 K35 2006). New York: McGraw-Hill.
4. O'Hare, T. (2005). Amphibians: Amphibians. Carson-Dellosa Publishing.
5. Prasad, SN, & Kashyap, V. (1989). A Textbook of Vertebrate Zoology. New Age International.