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 aphids, roundworms or lice.
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 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 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: