Cirripedes afford a good instance of this: even the illustrious Cuvier did not perceive that a barnacle was a crustacean: but a glance at the larva shows this in an unmistakable manner.

So again the two main divisions of cirripedes, the pedunculated and sessile, though differing widely in external appearance, have larvae in all their stages barely distinguishable.

The embryo in the course of development generally rises in organisation.

I use this expression, though I am aware that it is hardly possible to define clearly what is meant by organisation being higher or lower.

But no one probably will dispute that the butterfly is higher than the caterpillar.