DEVELOPMENT AND EMBRYOLOGY. 431 



of butterflies, they have six pairs of beautifully constructed 

 natatory legs, a pair of magnificent compound eyes, and 

 extremely complex antennae ; 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 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 organized than they were in the larval condition. 

 But in some genera the larvae become developed into hermaph- 

 rodites having the ordinary structure, and into what I have 

 called complemental males ; and in the latter the develop- 

 ment has assuredly been retrograde, for the male is a mere 

 sac, 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 struc- 

 ture 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 porpoise, should 

 not have been sketched out with all their parts in proper 

 porportion, 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 re- 

 marked, in regard to cuttle-fish, "there is no metamor- 

 phosis ; the cephalopodic character is manifested long before 

 the parts of the embryo are completed." Land-shells and 

 fresh-water crustaceans are born having their proper forms, 

 while the marine members of the same two great classes 

 pass through considerable and often great changes dur- 

 ing their development. Spiders, again, barely undergo any 

 metamorphosis. The larvae of most insects pass through 

 a worm-like stage, whether they are active and adapted to 

 diversified habits, or are inactive from being placed in the 

 midst of proper nutriment, or from being fed by their 

 parents ; but in some few cases, as in that of Aphis, if we 

 look to the admirable drawings of the development of this 



