Page - 481 - in The Origin of Species

DEVELOPMENT AND EMBRYOLOGY 481 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 larvx, though active, still obey, more or less closely, the law of common embryonic resem- blance. Cirripedes aft'ord a good instance of this; even the illitstrious Cuvier did not perceive that a barnacle was a crustacean : but a glance at the larva shows this in an un- mistakable manner. So again the two main divisions of cirripedes, the pedunculated and sessile, though differing widely in external appearance, have larva; 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 the organisation being higher or lower. But no one probably will dispute that the butterfly is higher than the caterpillar. In some cases, however, the mature animal must be consid- ered as lower in the scale than the larva, as with certain parasitic crustaceans. To refer once again to cirripedes : the larvae in the first stage have three pairs of locomotive organs, a simple single eye, and a probosciformed mouth, with which they feed largely, for they increase much in size. In the second stage, answering to the chrysalis stage of butterflies, they have six pairs of beautifully constructed natatory legs, a pair of magnificent compound eyes, and extremely complex antennas ; but they have a closed and imperfect mouth, and cannot feed : their function at this stage is, to search out by their well-developed organs of sense, and to reach by their active powers of swimming, a proper place on which to be- come attached and to undergo their final metamorphosis. When this is completed they are fixed for life: their legs are now converted into prehensile organs; they again obtain a well-constructed mouth ; but they have no antennae, and their two eyes are now reconverted into a minute, single, simple eye-spot. In this last and complete state, cirripedes may be considered as either more highly or more lowly organised than they were in the larval condition. But in some genera the larvae become developed into hermaphrodites having the ordinary structure, and into what I have called complemcntal males; and in the latter the development has assuredly been p—HC XI