What's one of these? Please

[steveharriss]

gold_dotted_flatworm.JPG

The first multicellular animals to appear were the sponges, which are the ancestors of all other complex animals. Sponges have no definite shape and cannot move around. They filter a ton of sea water for each ounce of nutrients they extract.

The next development was jellyfish-type animals which could move about randomly by drifting or by pulsing.

Finally, flatworms developed. Flatworms were the first hunters: they had sensory sections (not true eyes--but eyespots that were sensitive to light), and nervous systems. More than this, flatworms were the first animal to be bilaterally symmetric (ie: they had front and back ends and a left and right side that were mirror images of one another) and they could move forward rather than randomly.

Flatworms remained primitive compared to animals that followed. They had no circulatory system and only a single opening that served as both mouth and anus (ughh!). But for millions of years they were the most advanced animals on earth