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7.3.4. Metazoans

Kingdom Metazoos or Kingdom Animalia or Kingdom Animals

The Kingdom of animals or Metazoa or Animalia is a large group of eukaryotic beings (cells with a nucleus), heterotrophs, multicellular. Its cells are capable of forming tissues (except porifera).

Its cells do not have a cell wall and differentiate and specialize to form tissues which are grouped together to form organs, apparatus and systems. In addition, animals are characterized by their great capacity for movement, and because their cells do not have chloroplasts.

Humans belong to this kingdom.

Varias imágenes de animales

By creator of composite, User:Medeis; original authors, as credited individually above:User:Bkmiles, User:Panda3, Hans Hillewaert (Lycaon), User:Anilocra, Rob Hanson from Welland, Ontario, Canada, User:Nhobgood Nick Hobgood, Richard Ling, A. Slotwinski as credited by uploader user:Australianplankton, User:Lviatour, User:Kevincollins123, S. Taheri, edited by Fir0002, User:Opoterser, DBCLS, TheAlphaWolf, Philippe Guillaume [CC BY-SA 2.5 , GFDL, CC-BY-SA-3.0, FAL, CC BY-SA 3.0 , CC BY-SA 3.0 , Public domain, Public domain, CC BY 2.0 , CC BY-SA 3.0 , GFDL, CC BY-SA 2.0 , CC BY-SA 3.0 , CC BY-SA 3.0 , GFDL, CC BY-SA 3.0 , CC BY-SA 3.0 , CC BY-SA 2.5 , CC BY-SA 3.0 , GFDL, CC BY 3.0 , CC BY 2.1 jp, GFDL, CC-BY-SA-3.0, CC BY 2.0 or CC BY-SA 3.0 ], via Wikimedia Commons

Vital functions of animals

Animal nutrition

Animals are heterotrophs, they feed on organic matter made by other living things. They take food by ingestion, being able to be herbivores, carnivores, omnivores and saprophytes. At the cellular level, your cells obtain food by phagocytosis and pinocytosis. Fungi, on the other hand, although they are also heterotrophs, absorb nutrients after digesting them externally.

Some animals can also be parasites of other living beings, both plants and animals, such as aphidsroundworms or lice.

They need oxygen to live, and they take it from the air or water, use it for cellular respiration, and give off carbon dioxide.

To perform the function of nutrition, animals have four devices:

En la función de nutrición intervienen los aparatos digestivo, respiratorio, circulatorio y excretor.

  • Digestive system. Through digestion, food is broken down so that the nutrients that make it up can be used by cells. Food is digested throughout its journey through the digestive tract that begins in the mouth, due to the mechanical action of some structures such as teeth, tongue, etc. and other movements, in addition to the chemical action of juices discharged into the tube from glands such as the pancreas and liver. 
  • Respiratory system. It allows capturing oxygen (O2) from the medium (air or water) and expelling CO2 from cellular respiration. Depending on the type of living being, there are several different specialized organs:
  • Circulatory system. The nutrients resulting from digestion and oxygen from the respiratory system are transported to the cells to carry out cellular respiration. The waste will be taken from the cells to the apparatus in charge of its expulsion to the outside. There are two types of circulation:
    • Open circulatory system. The circulating fluid leaves the blood vessels surrounding the cells. It is characteristic of many invertebrates.
    • Closed circulatory system. The circulating fluid never leaves the blood vessels, as it does in vertebrates.
  • Excretory apparatus. The waste products from metabolism have to be eliminated. The circulatory system is responsible for taking them to the systems responsible for their elimination, such as CO2 expelled by the respiratory system or other substances that go to specialized excretory organs (kidneys in vertebrates, nephridia in annelids, or Malpighian tubes in insects).

Relationship in animals

Almost all animals have the ability to move, although some are very slow. Others, like corals or sponges, live attached to the substrate.

They have a  nervous system in which some cells, called receptors, can perceive changes in their environment (sounds, light, etc.) and other nerve cells process that information (in the brain and spinal cord) and respond appropriately to these changes.

They have muscle tissues that shorten to allow movement, as well as to move, to feed or carry out their vital activities.

In addition, the endocrine system is made up of glands that release hormones into the blood in response to certain stimuli. These hormones cause responses in some cells.

Reproduction in animals

Although most animals reproduce by sexual reproduction, some do so by asexual reproduction.

  • The sexual reproduction is carried more evolved animals. The gonads (ovaries and testicles) produce haploid sex cells called gametes (eggs or sperm) that unite to give rise to a zygote that will later give rise to a new individual. Sexual reproduction allows genetic diversity to exist in a population, which favors evolution according to changes that occur in the environment.
  • The most primitive organisms, such as some invertebrates, can reproduce asexually from any part of their body. Their offspring are genetically identical to their parents. This facilitates rapid reproduction of organisms. Asexual reproduction usually takes two forms:
    • By budding, through the formation of buds.
    • By splitting and fragmentation, dividing the animal into two parts (splitting) or several (fragmentation), generating new individuals.

Most animals are unisexual (either male or female), but some are hermaphroditic (an individual with both male and female reproductive systems).

All animals follow an embryonic development from when they are inside an egg or inside the mother until they reach the adult state, some presenting metamorphosis.

In animals, embryonic development can be:

  • Viviparous: they develop inside the mother's body.
  • Oviparous: they develop inside eggs that are deposited outside, outside the mother's body.
  • Ovoviviparous: they develop inside eggs that hatch inside the mother's body.

Curiosity: Embryonic development of chameleons

Although most chameleons are oviparous, there are also ovoviviparous species. In this video we have the Cape dwarf chameleon (Bradypodion pumilum), an ovoviviparous species, whose embryo is surrounded by a very fine membrane that is discarded immediately after birth.


Types of animals

Given the importance of this kingdom, we will dedicate a topic to each of the two groups in which animals are classified, depending on whether or not they have a backbone and a bony skeleton:

  • Invertebrates
    • Some animals, such as earthworms or jellyfish, do not have a skeleton. Others may have an exoskeleton or external skeleton.
    • Some invertebrate groups exhibit bilateral symmetry, with an axis dividing the animal into two symmetrical parts. Other invertebrates exhibit radiate symmetry.
  • Vertebrates
    • Animals have an internal skeleton or endoskeleton, which can be made of bone or cartilaginous tissue, which protects the soft parts of the body.
    • Vertebrates have bilateral symmetry , that is, their body could be divided by an imaginary axis into two symmetrical parts.
    • The body is divided into three distinct regions: headtrunk, and limbs.
    • They have highly developed sense organs, with a nervous system in which the brain and spinal cord stand out.

Rebaño de ovejasRebaño de vertebrados ovejas