Animals with spine

The numerous invertebrates’ phyla are characterized by a wide range of shapes and way of living. All the vertebrates (Vertebrata), instead, have a similar body. Despite their limited differences and the modifications in time, the vertebrates have conquered on just emerged lands, but also the sky. Vertebrates include some of the biggest organisms that have ever lived on Earth and our species also belongs to them
The vertebrates are characterized by a spine, or backbone, consisting of vertebra that surround the nerve cord. Between the vertebra are the cartilage disks, that make the spine a flexible bone structure. Connected to the vertebra are the muscles that allow the movement of the different parts of the spine. These animals have a bone internal skeleton, mainly made up of living material that grows with the animal until it reaches its definitive dimensions.
Fish
Fish have been the first vertebrates to have appeared on the Earth. Their appearances can be the most diverse, depending on how they have adapted to the environments in which they live. A typical organ of most, but not all, fish is the air bladder, which is filled with gas and allows therefore the fish to change its specific gravity (i.e. the body weight/volume ratio) to swim up and down without moving its fins. Fish produce a huge number of eggs, since they have to go through many dangers and few can survive: cods lay up to six million eggs at a time. Fertilisation is mostly external: the female lays its eggs and the male fertilises them by coating them with its sperms. In this case, the fish are called “oviparous”, which means the embryo develops inside the egg after having been laid by the female. If instead the eggs are held inside the female’s body until the hatching, the fish are called “ovoviviparous”.
Amphibians
The word “amphibian” comes from the Greek and means “double life”, since these animals live partly in water and partly on land. Before becoming adults, they undergo changes that completely transform their appearance. From the eggs wriggle out the young animals, called tadpoles and looking like small fish: later on, the tadpole from a larva becomes a fully formed adult. During the metamorphosis, the tadpole begins to grow legs and its gills become smaller and smaller until they disappear: they are replaced by lungs, that are necessary to breath air. Finally, the tail is completely swallowed up into the body.
Reptiles
The word “reptile” comes from the Latin reptilis, which means “creeping”. Actually, the animals that best represent the entire class are snakes, but Reptiles also include such quadrupeds as tortoises and crocodiles. Reptiles look quite like Amphibians, but have something more: they can also live in dry areas, far from water. They have lungs to breathe and are commonly called “cold-blood vertebrates”; in fact the temperature of their blood depends on that of the environment, and is slightly higher than that, so it would be more correct to call them heterothermal animals, i.e. “whose body temperature varies”. Their body is coated in scales and they reproduce by laying eggs. Reptile reproduction is sexed, with internal fertilisation; their eggs, protected by a waterproof shell, are always laid on land (sometimes the eggs develop inside the oviduct).
Birds
Birds are perfectly formed for flying; their skeleton is actually very light because their bones are pneumatized, i.e. contain air. The upper limbs of Birds are wings, while their lower limbs are retractile; their body is coated in feathers and plumage, that offer excellent protection with minimum weight. They also have an exceptional prehensile organ: the beak, whose proper name is “rhamphotheca” and consists of two corneal cases. The class Birds is composed of approximately 9,000 species that are very different from each other in both their physical appearance and living habits: some live in water, where they find plenty of food, while others, instead of using their wings, climb trees with their claws and beak, others can no longer fly. Obviously, each species has all it needs to live in its surroundings: webbed feet to swim, strong feet to run and scratch about, claws to grasp their preys. Fertilisation is external and the female lays its eggs on the ground, coated in a protective shell.
Mammals
Mammals are homoeothermic animals, i.e. they maintain a body temperature of around 37°C. The name “mammal” (which literally means “that bears mammary glands”) refers to one of their distinguishing features, i.e. for some time they feed their offspring with the milk secreted by their mammary glands. Their body is covered in hair, which are reduced or missing in those species that have adapted to living in water (Cetaceans, such as dolphins and whales) or that have scales (such as armadillos and pangolins). Apart from Cetaceans (such as dolphins and whales), Sirenians (such as dugongs) and Pinnipedia (such as seals and sea lions;), whose limbs have turned into flippers, all Mammals have four limbs and are therefore called quadrupeds. Quadruped Mammals are divided into Plantigrades (such as bears), Digitigrades (for instance dogs and cats) and Unguligrades (such as horses), depending if they walk by resting all the sole, fingers only, or the last phalanxes on the ground. Mammals can be divided into three groups depending on their offspring: Monotremata, Marsupials and Placentalia. Monotremata are oviparous, their females lay eggs and their offspring develop inside them, such as the platypus and the echidna. Marsupials are viviparous, which means their offspring are incomplete at birth and complete their growth inside their mother’s marsupium, a pouch located in the abdomen, where the new-born animals move (for instance the kangaroo and the opossum). The offspring of the Placentalia develop, instead, inside their mother’s uterus and are very well developed at birth.

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