244 Development and embryology, [Chap. Xiv. 



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 antennas, and 

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

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

 pedes 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 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, and every other 

 organ of importance, excepting those for reproduction. 



We are so much accustomed to see a difference in 

 structure between the embryo and the adult, that we are 

 tempted to look at this difference as in some necessary 

 manner contingent on growth. But there is no reason 

 why, for instance, the wing of a bat, or the fin of a por- 

 poise, should not have been sketched out with all their 

 parts in proper proportion, as soon as any part became 

 visible. In some whole groups of animals and in cer- 

 tain members of other groups this is the case, and the 

 embryo does not at any period differ widely from the 

 adult: thus Owen has remarked in regard to cuttle-fish, 

 "there is no metamorphosis; the cephalopodie char- 

 acter is manifested long before the parts of the embryo 

 are completed." Land-shells and fresh-water crusta- 

 ceans are born having their proper forms, whilst the 

 marine members of the same two great classes pass 

 through considerable and often great changes during 



