"Oh it's nice to have a spine!
It really is so great!
It helps me keep in line,"
Said the happy vertebrate.
"A central nervous system
Is the boon of our subphylum.
Oysters clearly missed 'em
Which is why they are not smilum.
"No, you never see an oyster smile,
But those of us with backs
(Like the knobbly crocodile)
Are grinning stacks and stacks.
Perhaps we're rather boist'rous,
We who are so dorsal,
But better thus than oysterous -
Him's a squidgy spineless morsel."