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 



