Now onto worms.
Worm burrows have been found in rocks dating back to 605 million years ago. 65 million years before the start of the cambrian period.
These could be round worms or flat worms, but worms don't readily fossilize for obvious reasons.
Lets take a look at flatworm anatomy.
Flat worms have no circulatory nor a respiratory system. It is flat enough to allow these functions to occur at the cellular level and perform exchanges directly with its suroundings and gut.
They have a sac like digestive system with only one opening which serves as both mouth and anus. This opening is located in the midsection ventral.
There are two layers of muscles one layer which runs parallel to the length of the specimen and one which forms bands along cross sections. This allows the specimen to move using undulating motions.
The simplest forms have a very primative nerve net. These animals are hermaphroditic meaning both male and female. Planarians can regenerate, if you cut them into eight peices you'll get eight complete individuals! In fact this is one of it's methods of reproduction.
Next we'll discuss round worms.
