Animals are multicellular, single-celled organisms in the unicellular biological kingdom Animalia. With the exception of the jelly-fish, all other animals are completely water-bound. They breathe air, can multiply, reproduce, and eat food. They can even digest solid matter. Since they are so water-bound, the majority of an animal’s body is made up of water: muscular tissues, blood, and organs are all primarily water.
All animals, even fish, are multicellular organisms, with all of their cells being composed of a complex network of plasma membranes (plasma membranes are like cellular walls that protect the internal organs from chemicals, toxic gases, or infectious diseases). In fact, only some animals have a completely enclosed biological kingdom; insects, which have no vascular system, and amphibians, which lack a tail, are therefore considered to be members of the unicellular organisms. The majority of animals that live in this world are either bony (such as amphibians and reptiles) or fleshy (such as mammals). All animals have evolved from a single celled ancestor that lived unicellular until the arrival of a multicellular world, and since then all members of the animal kingdom have multiplied into less complex forms as a result of descent from animals.
The body of an animal is composed of several different types of cells. Multicellular organisms may have a single cell or a “least cell.” All cells in an animal body are together, however, because they are tightly coupled by proteins. Most animals possess both an animal cell receptor and a corresponding protein, although most animals do not have both at the same time. A receptor for a specific animal cell will respond to that animal cell’s gene in order to form a specialized form of DNA, called a somatic nucleus.
The other part of an animal’s body is composed of organs, which are either very complex (e.g., kidneys and stomach’s) or relatively simple (e.g., lungs and eyes). Organelles are present in all animals and belong to the class of living things called “organisms.” An animal’s digestive system consists of tubes, intestines and glands. The digestive system of an animal is a complex system that includes many specialized components. In comparison to the highly diversified systems of bacteria and other unicellular organisms, the systems of animals appear to be highly organized.
All forms of animals have sexual and asexual reproduction capacities. Sexual reproduction involves the process by which an individual spermatozoa fertilizes an egg. Male spermatozoa are responsible for the development of the male gametes, while female spermatozoa are responsible for the development of the female eggs. Sexual reproduction often involves a process called fertilization, in which an embryo is made during a woman’s ovulation. In an asexual reproduction process, eggs and sperm remain in a woman’s body until she experiences ovulation, which is often brief for women.
The majority of bacteria and some other unicellular organisms reproduce by means of asexual reproduction; however, other forms of multicellular organisms reproduce asexually, through a process called metallogenesis. In this case, reproductive cells (e.g., eggs) are formed in the ovary of a woman. Some animals do not undergo metallogenesis, including all forms of eusocial insects, arachnids, ciliates, chiroptogs, mites, spirochetes, myxomatous protozoans, tunicates and certain cormorantails. The reason for this may be that the requirements of their life style (e.g., eating a diet rich in protein and other nutrients) prevent them from metallogenesis.