Animals Make the World Go Round

Do you know what animal skin really is? You should if you’re interested in knowing more about our planet and the types of creatures that live on it. A lot of people might think that “skin” refers only to animal fur or feathers, but this is not entirely true. In fact, the skin is more accurately referred to as exoskeletal tissue, including cartilage, epidermis, blood vessels, nerves, and other connective tissues.


An animal is multicellular, meaning that it has multiple cells that make up its body. Humans, as well as all other mammals and fishes are part of an animal species. With the exception of some marsupials, all animals including humans are composed of at least two major cell types: an egg cell and a sperm cell. The latter has a definite nucleus (a.k.a. cell nucleus).

In contrast to the single-celled organisms such as plants and animals, all vertebrates, including humans, have an exoskeleton (the uppermost layer of the animal’s body). The major parts of this skeleton are limbs, the muscular system, and internal organs. The major difference between an animal’s skeleton and that of a vertebrate is that the former is made up of soft tissues whereas the latter are composed primarily of bone. The exception are certain very small amphibians and reptiles, which are composed entirely of bony materials.

Besides having a skeleton, an animal also possesses a series of features that differentiate it from other vertebrates and molluscs. One such feature is the presence of external genitals, which are present in both sexes of most animals. In fact, the only known animal with a fully developed sex organ is the leopard gecko, which is the only member of its genus that does not have a genital opening. In most animals, the genitals are paired, but in some taxonomic studies, some have been found to be unimpaired. Another distinction that has been made is between animal sexes that have both sexes present at once. For instance, both genders of the tiger are present in nature, while in other instances, one sex is consistently paired, while the other is either unsexed or subspecies specific.

All vertebrates, although unlike the unicellular algae and bacteria that make up the protozoa, belong to a class of organisms called vertebrates. The class that most people are familiar with, and the one that bears most resemblance to the true amphibians and true reptiles (which do belong in the vertebrate phylum), are amphibians. The widest recognized taxonomic classifications of amphibians and reptilians are the iguanodon (including the common iguana) and crocophorozoan. The iguana and its close relatives (such as the tuatara) are among the oldest known terrestrial organisms. All other known terrestrial vertebrates, including all known reptilians and all chordates, belong in the reptile phylum.

A few examples of the diversity within the animal kingdom are seen in Earth’s own ecosystems. The microbial kingdom, composed of all types of microbial entities, including every single organism known to science today, is vastly diverse, containing perhaps hundreds of thousands of different classes. The arthropods (which include insects, spiders, and other crustaceans) are a diverse group of organisms with very complex social structures, ranging from simple solitary units to complex mating groups. Theplant kingdom consists of plants, with each class consisting of unique life cycles and characteristics. Insects, with their varied arrangement of legs and wings, make up the Kingdom Protista, while theacteria are classified bacteria, a group that includes nearly every single type of living thing on earth.

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