Invertebrates are important organisms.  97 percent of all animal species are invertebrates.  These animals are the link between single cellular organisms and many of the vertebrate species that people are most familiar with.  Many of the advanced features that we see in vertebrates, such as organ systems, first appear in these invertebrates.  On this web site we will focus on the lower invertebrates: sponges, cnidarians, flatworms, roundworms, mollusk and annelids.

Before learning about each group of organisms, lets first define some words used to characterize them, namely the types of symetry they posses:

Similarity or balance between parts of an organism so that when a straight cut is made through a point or along a line, equal, mirror-image halves are formed. Symmetry in body shapes is related to the lifestyles of organisms. Asymmetry, or the absence of symmetry, most often occurs in sessile organisms or in slow-moving forms such as amebas . Most other organisms can generally be classified in three groups with respect to symmetry type.     In spherical, or point, symmetry, any straight cut through the central point of a sphere divides it into mirror-image halves. Point symmetry, often called universal symmetry by biologists, is seen in some floating animals with radiating parts, such as the single-celled protozoans of the order Radiolaria.

    Radial, or line, symmetry, as exemplified by a cone or a disk that is symmetrical about a central axis, is especially suitable for sessile or floating animals. Most radially symmetrical animals are symmetrical about an axis extending from the center of the oral surface, which contains the mouth, to the center of the opposite, or aboral, end. Radial symmetry is seen in sessile organisms such as the sea anemone, floating organisms such as jellyfish, and slow-moving organisms such as sea stars, or starfish. Many jellyfish have four radial canals and are said to have tetramerous radial symmetry; sea stars, with five arms, have pentamerous radial symmetry.

    In plane, or bilateral, symmetry, one particular plane, termed the sagittal plane, divides the body into two equal halves, usually right and left halves that are mirror images of each other.  Bilateral symmetry is most suitable for actively moving organisms, as it permits streamlining, and is the most common symmetry among animals. In animals this symmetry type also favors the formation of main nerve centers and special sense organs and contributes to cephalization, or the evolutionary development of a head.

SPONGES
Phylum name: Porifera

These are the first animals to appear on the earth.  They are simple, creatures that live in the oceans.  They resemble the sponges you probably have sitting on your kitchen sink at home.  Sponges and sessile creatures that come in many shapes, colors and sizes.

CNIDARIA
Phylum name: Cnidaria

Cnidaria is the classification of invertebrates that include hydra and jelly fish.  They get their name from cnidocytes, which are the singing cells that all organisms in this phylum posses.

FLATWORMS
Phylum name:  Platyhelminthes

These worms are the first organisms to posses bilateral symmetry and cephalization.  This pattern of having most sense receptors at the anterior end (head) and having a central line of symmetry, is present in almost all organisms that evolved after the flatworms.

ROUNDWORMS
Phylum name:  Nematoda

Roundworms are a highly studied group of animals in science.  They are the first organisms to posses a digestive system.

SEGMENTED WORMS
Phylum Name: Annelida

The type of worm most people are familiar with, including the earth worm.  The segments in this phylum are repeating, revealing a pattern that is common in many later organisms, such as the backbone.

MOLLUSKS
Phylum name: Mollusca

Mollusks are possibly the most familiar group we look at with species ranging from oysters and clams to octupi and snails.