Chap. XTII. EMBRYOLOGY. 441 



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 but- 

 terfly 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 para- 

 sitic crustaceans. To refer once again to cirripedes : 

 the larvae in the 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 com- 

 pound eyes, and extremely complex antennae ; 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 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 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. Bat 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. 



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