Here's the original (Shel Silverstein) version of...

BOA CONSTRICTOR
Shel Silverstein -
Tro-Hollis Music, Inc. BMI

I'm being swallowed by a boa constrictor
I'm being swallowed by a boa constrictor
I'm being swallowed by a boa constrictor
And I don't like it very much!

Oh no, oh no,
He swallowed my toe, he swallowed my toe
Oh gee, oh gee,
He's up to my knee, he's up to my knee
Oh fiddle, oh fiddle,
He's reached my middle, he's reached my middle
Oh heck, oh heck,
He's up to my neck, he's up to my neck
Oh dread, oh dread,
He swallowed my.... schlirppp!

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Note: the version I learned from my older brothers included a bit of the "folk process"; they added:

Oh my, oh my, he swallowed my thigh, he swallowed my thigh

and

Oh darn, oh darn, he swallowed my arm, he swallowed my arm