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Show 440 EMBRYOLOGY. CHAP. XUI. tw. ns of ex1.s t ence. We cannot' for instance, suppo.s e that in the embryos of the vertebrata the P.ecuh.ar I oop-1I.1 r e course of the ar.t e.r ies ne.a r the branchial shts are re1 a t ed t o s1· m1'Iar conditions,-In th.e young mam. mal w lu ·c h I·S nour1· shed in the womb o.f Its mother, I.n the egg of the bird which is hatched In a nest, and In the f frog under water. We have no more reason ~~noa . to b eI 1. eve I·n such a relation, than we have to .b eheve that the same bones in the hand of a m~n,. wing of ~ b at, an d fin of a porpoise, are related to shi milar. condi-tions of life. No one will suppose that t e stnpes. on the whelp of a lion, or the spots on the young blackbird, are of any use to these animals, or are related to the conditions to which they are exposed. . . . The case however, is different when an ammal durmg a ny part of its embryonic career is a.c.t ive, and has to provide for itself. The period of actlvi.ty may come on earlier or later in life; but when~~er It c~me~ o~, the adaptation of the larva to its conditions of l~fe IS JUSt as perfect and as beautiful as in t~e. ad~lt animal. From such special adaptati?ns, t~e sim_Ilanty o~ the larvre or active embryos of allied an1mals IS sometimes much obscured . and cases could be given of the larvre o~ two . s' or of two groups of species, differing qmte as speme , d h · d It much, or even more, from each other than o t eir a u parents. In most cases, however, the larvre, though active still obey more or less closely the law of common ~mbryonic resemblance. Cirripede~ aff?rd a goo~ in tance of this: even the illustriou~ Cu;Ier did not p:~ ceive that a barnacle was, as it certmnly Is, a crustac.ea ' but a glance at the larva shows this to be the ca~e md.a~ unmistakeable manner. So agai.n the two m. ain IVI- 1 . 1 sions of cirrip Jes, the pedunculated and sessile, :u~li differ widely in xternal appearance, have larvro their several stages barely distinguishable. CHAP. XIII. EMBRYOLOGY. 441 . Th~ embryo. in ~he course of development generally nses In organisation : I use this expression, though I am a':are 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 is generally considered as lower in the scale than the larva, as with certain parasitic crusta.ceans. To refer once again to cirripedes : the larvre m ~he first stage have three pairs of legs, a very 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 antennre; but they have a closed and imperfect mouth, and cannot feed: their function at this stage is, to search by their welldeveloped organs of sense, and to reach by their active powers of swimming, a proper place on which to become 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 antenure, 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 larvre 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. u 3 |