244 DEVELOPMENT AXD EMBEYOLOGY. [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 antennae, 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 

 certain 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 cephalopodic 

 character is manifested long before the parts of the 

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

 crustaceans are born having then proper forms, w T hilst 

 the marine members of the same two great classes pass 

 through considerable and often great changes during 



