Flies can eat toads! (Although astonishment may be lessened in noting that the tiny toads are much smaller than enormous fly larvae.) Unusually large… - Stephen Jay Gould

Gaskell... had to keep his stately soldiers upright and uniformly oriented... by crafting the brain and spinal cord from an arthropod digestive tube, while forming a completely new gut below. ...Gaskell thought that his move would rescue the theory of linear progress, with its necessary transition of arthropod into vertebrate, from the absurdities of the old inversion theory.

If we choose a weak and foolish speculation as a primary textbook illustration (falsely assuming that the tale possesses a weight of history and a sanction of evidence), then we are in for trouble - as critics properly nail the particular weakness, and then assume that the whole theory must be in danger if supporters choose such a fatuous case as a primary illustration.

There is no gene “for” such unambiguous bits of morphology as your left kneecap or your fingernail. […] Hundreds of genes contribute to the building of most body parts and their action is channeled through a kaleidoscopic series of environmental influences: embryonic and postnatal, internal and external. Parts are not translated genes, and selection doesn't even work directly on parts.