Chap. xiv. Development and Embryology. 389 



it comes on, the adaptation of the larva to its conditions of life is 

 just as perfect and as beautiful as in the adult animal. In how 

 important a manner this has acted, has recently been well shown 

 by Sir J. Lubbock in his remarks on the close similarity of the 

 larva? of some insects belonging to very different orders, and on 

 the dissimilarity of the larva? of other insects within the same 

 order, according to their habits of life. Owing to such adaptations, 

 the similarity of the larva? of allied animals is sometimes greatly 

 obscured ; especially when there is a division of labour during the 

 different stages of development, as when the same larva has during 

 one stage to search for food, and during another stage has to search 

 for a place of attachment. Cases can even be given of the larva? of 

 allied species, or groups of species, differing more from each other 

 than do the adults. In most cases, however, the larva?, 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 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 considered as lower in the scale than 

 the larva, as with certain parasitic crustaceans. To refer once 

 again to cirripedes : the larva? 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 antenna? ; 

 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 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 antenna?, 

 and their two eyes are now reconverted into a minute, single, 

 simple eye-spot. In this last and complete state, cirripedes may 



