Chap. XIII. 
EMBRYOLOGY. 
441 
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 com¬ 
pound eyes, and extremely complex antennsB ; but they 
have a closed and imperfect mouth, and cannot feed: 
their function at this stage is, to search 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 an¬ 
tennae, 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 larvae become 
developed either into hermaphrodites having the ordi¬ 
nary 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 widely different 
animals within the same class, that we might be led 
to look at these facts as necessarily contingent in some 
manner 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 structoe 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 
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