The word “animal” literally means “animus naturalis”, which means “of or relating to animal”. The other two definitions are more common. Plants are considered animals as well. We use the term “plant” to refer to any of the billions of living things on earth, while the term “animal” is used to refer only to human beings. So, technically, plants and animals are not exactly the same thing, although many people often use the terms interchangeably.
Different animals have different characteristics that allow them to live in their particular environment. Some examples of these characteristics are: lungs for respiration; nervous system, including central nervous system and reproductive system; heart for pumping blood; limbs for grasping, walking, running and climbing; gastro-intestinal tract for digestion; gastrointestinal tract for absorption of food; eyes for vision and hearing; and muscles for locomotion. Virtually all animals have all of these basic features. However, there are several classifications of animals, depending on what their functions are. These are:
Sex Cells Many animals reproduce internally by means of asexes. Males reproduce via asexes, which include sexual organs like the penis, scrotum, testicles, prothymus, prostate, seminal vesicle, and others. Although males and females have the same kind of reproductive organs, they are different in their functions. While males have asexes for reproduction, females have anemia for connection to the egg. The word “sex” refers to the ability of an animal to reproduce by means of sex cells.
Eukaryotic Organisms are single celled organisms. They are so dense that their nucleus is enclosed within a sphere of water, which serves as their shell. Examples of eukaryotic organism are bacteria, algae, protozoa, viruses, and others.
Sensory Organisms An animal’s sense of smell is called olfactory organs. This organ is so sophisticated that animals can detect even the tiniest odors from distances of hundreds of meters! Examples of olfactory organisms are: bees, cats, dogs, and others. The term “sense” refers to the ability of an animal to detect a stimulus. Animals also have other types of senses apart from olfactory.
Hox Genes Animals are composed of genes that control development, growth, and maturity. In humans, the genes involved in metamorphosis are put into effect before the organs develop. Metamorphosis is a process where an animal grows. It is part of biological evolution. For example, during metamorphosis, eggs are produced and the lower half of the animal becomes smaller and shorter, while the upper half of the animal increases in size and stature.