Chap. XIII. AND EilBEYOLOGY. 397 



lower in the scale than the larva, as witli certain parasitic 

 crustaceans. To refer once a<?ain to cirripcdes : the larvas in 

 the lirst stage have three pairs of legs, a very simple single 

 eye, and a proboscifonned mouth, with which they feed largely, 

 for they increase much in size. In the second stage, answer- 

 ing to the chrysalis stage of butterflies, they have six pairs of 

 beautifully-constructed natatory legs, a pair of magnificent 

 comi)Ound eyes, and extremely complex antenmxj ; but they 

 have a close and imperfect mouth, and cannot feed : their func- 

 tion at this stage is, to search by their well-developed organs 

 of sense, and to reach, by their active powers of swimming, a 

 projier j)lacc on which to become attached and to undergo their 

 final metamorphosis. When this is completed, they are fixed 

 for life : tlicir l(\gs arc now converted into prehensile organs ; 

 they again obtain a well-constructed mouth ; but they have no 

 antenna', and their two eyes are now reconverted into a mi- 

 nute, single, and very simple e^e-spot. In this last and com- 

 plete state, cirripedes may be considered as either more highly 

 or more lowly organized than they were in the larval condi- 

 tion. But in some genera the larva? become developed either 

 into hermaphrochtes having the ordinary structure, or into 

 what I have called comj)lemcntal males, and, in the latter, the 

 development has assuredly been retiograde ; for the male is a 

 mere sack, which lives for a short time, and is destitute of 

 mouth, stomach, or other organs of importance, excepting 

 those for reproduction. 



AVe are so much accustomed to see a difference in structure 

 between the embryo and the adult, that we are tempted to 

 look at this diflerence as in some manner necessarily contin- 

 gent on growth. But there is no reason why, for instance, the 

 wings of a bat, or the fins c^f a porpoise, should not have been 

 sketched t)ut with all their parts in projier proportion, as soon 

 as an}' structure became visible. In some whole groups of 

 animals and in certain inemliers of other groups tin's is the 

 case, and the cmljryo does not at any period difler widely from 

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

 " there is no metamorphosis ; the ceplialopodic character is 

 manifested long before the parts of the embryo are com- 

 pleted." Land-shells and fresh-Avater crustaceans are born 

 having their proper form, while the mafine members of the 

 same two great classes pass through considerable and often 

 great changes during their develoj^ment. Spiders, again, 

 barely undergo any metamoqihosis. The larvae of most insects 



