The diversity of animal forms is truly amazing. Estimates offer the possibility of 10 to 30 million different species (forms) on this planet with many more awaiting discovery. Is there any commonality to this chaos of animal life? While no one criterion fits all types, several characteristics are common to all animals.
Cellular Characteristics
Multicellularity. All higher animals are multicellular beings. While the protozoa exhibit most animal characteristics, most of their species are single-celled. In regards to the animal characteristic of multicellularity, protozoa are most aptly described as animal-like.
Lack Cell Walls. Unlike algae and vascular plants, the cells of animals lack rigid outer cell walls. Instead, the outer boundary of animal cells is marked by a thin, flexible cell membrane.
Metabolic Characteristics
Heterotrophy.All animals are heterotrophs. As such, they obtain energy and necessary organic molecules (food) by ingesting other organisms. Plants on the other hand are autotrophs which make their own food through the process of photosynthesis.
High Activity Level. While plants are capable of self-movement, animals move more rapidly and in more complex ways than do plants. Plants move by growth and changes in water pressure while animals employ muscles and nerve cells. One form of movement unique to animals is the ability to fly.
Developmental Characteristics
Embryonic Development. Most animals have similar patterns of embryonic development.
As the fertilized egg of an animal develops, different layers of cells form. Known as germ layers, these cells eventually develop into the external covering and internal structures of the animal
There are three germ layers in animals: ectoderm, mesoderm, and endoderm. The ectoderm develops into the external ectoderm which becomes the skin and skin glands, hair, and nails and the neural tube which becomes the brain, the spinal cord, the retina, and the posterior pituitary gland.
The mesoderm layer develops into bones, connective tissue, most of the circulatory system, gastrointestinal tract, muscles, peritoneum (lining of the coelom), urinary system, and reproductive system. The key to the development of complex animal bodies lies with the mesoderm. Besides forming various internal body structures, the mesoderm also allows for the formation of a coelom (Gr. koiloma, cavity + koilos, hollow) or body cavity where organs may be housed.
The endoderm layer develops into the epithelial lining of the entire digestive tract save for part of the mouth and the terminal part of the rectum which form from involutions of the ectoderm, the epithelial lining cells of all glands that open into the digestive tract, and the epithelium of the auditory tube and tympanic cavity, of the trachea, bronchi, and alveoli of the lungs, of the urinary bladder and part of the urethra and that which lines the follicles of the thyroid gland and thymus gland.
Symmetrical Characteristics
Symmetry refers to the geometric design of the parts comprising animal bodies.A very few types are said to be asymmetrical because the body lacks any definite form or geometry. Sponges and some protozoan display asymmetrical body plans.
Spherical symmetry describes a body that is basically round with its parts concentrically arranged around a central point. Spherical symmetry is usually found in animals that float such as some protozoa.
Animal bodies in which the body parts are arranged around and radiate outward from a central axis that is shaped like a pie, wheel, or column are said possess radial symmetry. Cnidarians, such as the jellyfish and sea anemone, and the echinoderms, such as the starfish demonstrate radial symmetry.
Most animals exhibit bilateral symmetry (two-sided), a type of symmetry in which one side of the body is a mirror image of the other. Bilateral symmetry is correlated with motility (mobility) and cephalization (development of a head end). Bilateral animals are much better suited for directional movement and the concentration of brain and sense organs in a head bestows great advantages to an animal moving through its environment head first.
The Terminology of Zoology
To describe the regions of animal bodies, zoologist use certain terms. These terms, what we might call the compass points of animal morphology. Anterior designates the head end and posterior the rear or tail end. Dorsal refers to the top or back side and ventral to the bottom or belly side. Distal parts are farther from the body; proximal parts are nearer. A frontal plane passes from anterior to posterior and from left to right sides. It is perpendicular to both the transverse and sagittal planes. The sagittal plane divides the animal into right and left halves while the transverse plane (also called a cross section) divides the animal into anterior and posterior portions.
Whatever size or form an animal species takes, it shares certain commonalities with the millions of other animal types that inhabit the animal realm.
 |
