Caterpillar identification

[Smithmel]

I live in a slightly wooded suburban area outside of St. Louis,
Missouri, and today my brother and his friend found a caterpillar that
I have never seen before.  The friend accidentally brushed up against
the caterpillar and immediately experienced a painful tingling in his
arm and got a red rash with several large, raised bumps that subsided
after about 15 minutes.  The caterpillar is approximately one inch long
with no obvious segmentation (I'm not sure if it is a caterpillar or
some kind of slug).  The body is new-leaf green with an oval shape on
its back ringed with white and grey-brown in the middle.  The head,
abdomen and tail is the same grey-brown color.  On each side is six
tufts of hair.  The head has tufted antennae, and various tufts around
the face.  The tail is a mimic of the head with two bright yellow spots
that resemble eyes.  It was consuming a leaf when the boy brushed  up
against it.

Can anyone tell me if this is a caterpillar and what kind it might be? 
Any help would be greatly appreciated as I am very curious.
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