Graptolithinia.

Graptolithinia is a class of shelled hemichordates that lived from the Cambrian to the Carboniferous. One of the first graptolites was called Chaunograptus, from the Burgess Shale. Chaunograptus made a living by hitching onto other animals like sponges and arthropods. Later graptolites, like Monograptus, had pelagic lifestyles and drifted with the ocean currents in the open sea. But one group of Graptolites, the dendroids, retained a benthic or a parasitic lifestyle.

The name graptolite means "writing on the rocks" in Greek, which refers to the fact that most graptolite fossils look like hieroglyphics. In life, some graptolites, like Monograptus, may have resembled hacksaw blades, where others, like Didymograptus, resembled pinking shears. There were hundreds, probably even thousands, of different forms of graptolites. The morphology of graptolites was very diverse.

The Ordovician graptolite Didymograptus.

Some graptolites were benthic, some were parasitic, some were pelagic, and there were many forms of graptolites living each of those lifestyles. Graptolites were hemichordates. They were not chordates, but they were very important in chordate evolution leading up to humans.

The Ordovician graptolite Phyllograptus. 

Graptolites are index fossils for the Ordovician and Silurian. Some graptolites are very common, like ammonites, which are also good index fossils. Most good index fossils are common, widely distributed, and from a limited time span. This helps scientists date rocks.

The Devonian graptolite Spirograptus. 

References:

http://www.ucmp.berkeley.edu/chordata/hemichordata.html

http://www.asoldasthehills.org/Graptolites.html

http://en.wikipedia.org/wiki/Graptolithinia

http://paleo.cortland.edu/tutorial/misc%20fossils/miscfossils.htm