Origin of Species Variorum

again obtain a well-constructed mouth; but they have no antennæ, and their two eyes are now reconverted into a minute, single, and very 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 larvæ become developed either into hermaphrodites having the ordinary structure, or into what I have called complemental males, and in the latter, the development has assuredly been retrograde; for the male is a mere sack, which lives for a short time, and is destitute of mouth, stomach, or other organ of importance, excepting for reproduction.

We are so much accustomed to see differences in structure between the embryo and the adult, and like-wise a close similarity in the embryos of different animals within the same class, that we might be led to look at these facts as in some manner necessarily contingent on growth. But there is no obvious reason why, for instance, the wing of a bat, or the fin of a porpoise, should not have been sketched out with all the parts in proper proportion, as soon as any structure became visible in the embryo. ↑ And in some whole groups of animals and in certain members of other groups, the embryo does not at any period differ widely from the adult: thus Owen has remarked in regard to cuttle-fish, "there is no metamorphosis; the cephalopodic character is manifested long before the parts of the embryo are completed." .. .. .. .. .. .. .. .. .. .. .. .. .. Land-shells and fresh-water crustaceans are born with their proper forms, whilst the marine members of these two great classes pass through considerable and often great developmental changes. Spiders, again, barely undergo any metamorphosis. With almost all insects, the larvæ, whether adapted to diversified and active habits, or remaining inactive, being placed in the midst of proper nutriment or fed by their parents,