All of the remaining animals which will be discussed are bilaterally symmetrical, including Phylum Platyhelminthes, Pseudocoelomates, Phylum Mollusca, and Phylum Annelida, which will all be discussed in separate sections.  This section will provide a general overview of the common physical and physiological characteristics shared by bilaterally symmetrical animals.

    In bilaterally symmetrical animals, the body is organized along a longitudinal axis, with the right half an approximate mirror image of the left half.  Bilateral symmetry makes possible more efficient movement than does radial symmetry, which is typically found in animals that move slowly or are sedentary.
    A bilaterally symmetrical animal also has a dorsal (top) and a ventral (bottom) surface.  These terms apply even when the organism is turned upside, or, as with humans, stands upright, in which case dorsal means back and ventral means front.  Most bilateral organisms also have distinct anterior (head) and posterior (tail) ends.  Having one end that goes first is characteristic of actively moving animals.  In such animals, many of the sensory cells are collected into the anterior end, enabling the animals to assess an area before entering it.  This collection of sensory cells brought a gathering of nerve cells that is the forerunner of the brain.  Structures useful in capturing and consuming prey are also generally located in the anterior region of the animal, while digestive, excretory, and reproductive structures tend to be located toward the posterior.

Bilateral animals are all triploblastic, meaning they have three embryonic tissue layers that give rise to the various specialized tissues of the adult animal.  These layers are known as the "germ layers".  The nervous system and the skin develop from the ectoderm germ layer; the digestive structures are derived from the endoderm.  Muscles and most other parts of the body develop from the mesoderm, a layer of cells located between the ectoderm and the endoderm layers.  As animals became more complex and additional structures evolved for locomotion, excretion, reproduction, etc., they did so from the mesoderm layer.
 
 
 
 

    The triploblastic animals can be grouped in three categories, according to the presence or absence of a body cavity--a coelom (pronounced "see-loam")--in addition to the digestive cavity.  The coelom is a body cavity formed between layers of mesoderm and in which the digestive tract and other internal organs are suspended.

    In the simplest arrangement, tissues derived from the three germ layers are packed together and there is no body cavity other than the digestive cavity.  Animals of this type are known as acoelomates.

    A more complex arrangement is found in the pseudocoelomates, which have an additional cavity that develops between the endoderm and the mesoderm.  This cavity is known as a pseudocoelom (fake coelom), because of its location and because it lacks the epithelial lining characteristic of a coelom.

    The coelomates (mollusks, annelids, and all of the more complex animals) have bodies that are three layered, with a true coelom, which is a fluid-filled cavity that develops within the mesoderm.  Within the coelom, the digestive tract ("gut") and other internal organs are suspended by the mesodermal mesenteries.

  The advantages of these increasingly complex arrangements will become apparent in the following web pages, which go more in depth into specific animals.
 
 

Thanks to this site for the first "germ layers" picture:
http://www.uoguelph.ca/zoology/devobio/splab3/sld013.htm

Thanks also to  http://www.utm.edu/~rirwin/symmetry.htm  for the possum symmetry picture.