animal kingdom

1. Characteristics of Animals Animals belong to the Eukarya domain and the Animalia kingdom. They evolved from protozoa about 650 million years ago through the development of multicellular organisms. 2. Vital functions in animals. 2.1.Nutrition function Nutrition of animals is heterotrophic, so they eat other living things. According to their nutrition, animals can be divided into:  Herbivores: plant eaters  Carnivores: animal eaters.  Omnivores: eaters of plants and animals.  Detritivores: they eat the remains of dead animals and plants, as well as feces. To perform nutrition, animals use the following organ systems: -Digestive system: Depending on what they eat, animals have different types of digestive system, but most of them share the same digestive process: Animals eat through the mouth, where the food is smashed and passes through the esophagus to the stomach, where it is sterilized by stomach acids. Then the food goes to the intestines, where some juices secreted by glands such as the liver or pancreas and some bacteria help to break down the food and obtain the nutrients. Nutrients go to the circulatory system where they are distributed throughout the body. Food scraps that cannot be digested are excreted through the anus. -Respiratory system: All animals need oxygen to burn nutrients and obtain energy. They obtain oxygen from air or water through different structures such as:  Lungs: They are found in terrestrial vertebrate animals such as amphibians, reptiles and mammals. Some fish and invertebrates have lung-like structures.  Gills: They are found in fish, amphibian larvae and aquatic invertebrates.  Spiracles: They are small holes in the exoskeleton of insects and spiders that allow them to breathe.  Skin: Some underground animals can obtain oxygen through their wet skin, such as amphibians when they hibernate or invertebrates such as earthworms. – Circulatory system: It serves to transport water, nutrients and oxygen to cells. In addition, it carries waste substances to the excretory system. The circulatory system is made up of blood that travels throughout the body in some animals within veins and arteries and in others freely between cells. Animals have one or more hearts to pump blood. – Excretory system: It serves to eliminate waste substances produced by the body. The main ones are carbon dioxide that is expelled through the lungs, and urine that is filtered from the blood by the kidneys and expelled through the urethra. 2.2.Relationship function Another basic function of animals is relationship, performed by the following systems: – Senses: The senses are receptors that detect changes both in the environment and within the body itself. The main types are:  Mechanoreception: Perception of mechanical stimuli such as touch, pressure or sound. It allows the echolocation of some cetaceans and bats.  Vision: Perception of light. Some animals have evolved to perceive different wavelengths allowing them to see at night or to see ultraviolet colors.  Chemoreception: Perception of chemical compounds being responsible for both taste and smell. Some insect perform it through their antennas or even through their legs.  Thermoreception: Perception of internal or external temperature. It allows snakes to “see” the heat.  Equilibrioception: It is the perception of balance and spatial orientation.  Proprioception: It is the sense of self-movement and body position.  Nociception: Perception of pain.  Electroreception: Perception of electrical signals that allows electrolocation, it is found for example in sharks.  Magnetoreception: Perception of the magnetic field that allows to perceive direction, altitude or location. It is found for example in pigeons. -Nervous system: It is formed by the nerves and in some animals by the brain. It serves to analyze the signals received by the sensory organs, being responsible for different behaviors or even in some animals for the ability to feel or think. -Endocrine system: It is made up of glands that secrete hormones into the blood. Hormones are chemicals that regulate growth, sexual maturity, behavior, or metamorphosis in some animals. The hormonal response is slower than the nervous one but its effects last longer. -Locomotor system: It is responsible for the mobility of animals. It is formed by muscles, internal or external skeletons or other propulsion mechanisms such as the tube feet of echinoderms. – Immune system: It is responsible for the defense of the body against both infections or poisonous substances. 2.3.Reproductive function Animals can reproduce in two ways: – Asexual reproduction: Happens when the offspring inherits the genes of a single parent. In animals it could be:  Budding: A new organism develops from a bud formed in its body. (Ex: hydras)  Fragmentation: A new organism grows from a fragment of the parent. (Ex: planarians, starfish)  Parthenogenesis: A new organism grows from an unfertilized egg. It happens in many invertebrates and in some vertebrates such as some species of reptiles, amphibians, and fish. -Sexual reproduction: Happens when the offspring inherit a mixture of genes from both parents.  Sexual differentiation: To perform sexual reproduction, organisms should be male, female or hermaphrodite. Hermaphrodites are found in species with no separate sexes and in others in which an organism could change its sex during its lifespan such as clownfish, or be male and female at the same time.  Gamete production: In order to perform sexual reproduction, males and females develop gametes. These are called ovules in case of females and sperm in case of males.  Fertilization: It is the fusion of gametes to initiate the development of a new individual. It could be external, when it happens outside the body, usually in the water, for example in some fishes and amphibians or internal, when it happens inside the female’s body.  Types of sexual reproduction: In relation to the fertilization process and the embryo development there are three main types of sexual reproduction: 1. Ovuliparity, in which fertilisation is external, so the female releases the eggs and the male fertilises them outside her body. This happens in most fish and amphibians. 2. Oviparity, in which after an internal fertilisation the female releases the eggs to the environment. This happens in most reptiles and birds and some other species 3. Viviparity: in which after an internal fertilization, the embryo develops connected to the mother’s body. This happens in most mammals and some other species.