Cases can even be given of the larvae of allied species, or groups of species, differing more from each other than do the adults.

In most cases, however, the larvae, though active, still obey, more or less closely, the law of common embryonic resemblance.

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.

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