An animal is any living creature with a skeleton, including all fishes, amphibians, reptiles, mammals, birds, and insects. Animals are multicellular, meaning that they are made up of many cell types. In comparison to plants, animals have a very high metabolic rate and their cells are held together by a loose system of connective tissue and a thin layer of internal protectory membrane. Animals breathe air, eat food, require oxygen for respiration, are capable of moving, can reproduce by means of a placenta, develop internally through a gestation period, and secrete chemicals as part of their waste material as well as excrete waste materials.
An animal’s reproductive process is divided into two different stages: a prostrate or ovulation stage, and a post-ovulation stage. The egg is fertilized by a sperm cell found in an appropriate place in the female’s vagina, and the newly formed zygote is then swallowed or excreted by the female. In most animals, this zygote grows and develops inside the uterus where it eventually implants in a fallopian tube and becomes a new life. At this point in an animal’s life cycle, sexual reproduction occurs. Female animals may undergo a brief metamorphosis before reaching the age of puberty, while male animals may undergo puberty and reproduction all throughout their lives.
Recent years have revealed a surprising amount about some invertebrate animals. Researchers now know that all vertebrates, which includes all birds, amphibians, reptile species, and insects are members of a class of animals known as Plankton. Many marine invertebrates, including sea crabs and certain mollusks, belong to this same group.
Animals have inhabited the earth’s surface from the very beginning. There is evidence in fossilized fossils of animals that lived in prehistoric times. Some groups of animals such as dinosaurs have remained firmly entrenched as members of a single kingdom, while others such as ants and fish have split into two or three subkingdom levels. It is interesting to note that the word “animal” actually refers to the class of creatures that we recognize today, whereas “protein” refers to the classes or kinds of tissues and cells that make up all animal systems. All modern-day living systems, including animals and plants, belong to a single kingdom of living organisms.
Insects, amphibians, and reptiles have evolved independently over the course of millions of years. The most diverse group of animals is the vertebrates, which include all forms of fully grown animals with joints and muscles. The major class of vertebrates is the chordates, which also includes all the amphibians and reptiles. All chordates are social insects that live in groups of several hundred together.
Many invertebrates are important to human life because they provide food, shelter, and breeding grounds. Many invertebrates are also useful to humanity because they form a part of our health and well being. For example, humans depend on insects for food, but many insects (e.g., mosquitoes, ticks, fleas, chiggers) are harmful to humans and can even cause disease. As such, understanding more about invertebrates will help us better care for the planet and prevent the extinction of many species.