390 DEVELOPMENT AND EMBRYOLOGY. [Chap. XIV. 



be considered as either more highly or more lowly organised than 

 they were in the larval condition. But in some genera the larva 

 become developed into hermaphrodites having the ordinary struc- 

 ture, 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 be- 

 tween 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 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 their proper 

 forms, whilst the marine members of the same two great classes pass 

 through considerable and often great changes during their develop- 

 ment. Spiders, again, barely undergo any metamorphosis. The 

 larva) 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 insect, by 

 Professor Huxley, we see hardly any trace of the vermiform stage. 



Sometimes it is only the earlier developmental stages which fail. 

 Thus Fritz Miiller has made the remarkable discovery that certain 

 shrimp-like crustaceans (allied to Penoeus) first appear under the 

 simple nauplius-form, and after passing through two or more zoea- 

 stages, and then through the mysis-stage, finally acquire their 

 mature structure : now in the whole great malacostracan order, to 

 which these crustaceans belong, no other member is as yet known 

 to be first developed under the nauplius-form, though many appear 

 as zoeas ; nevertheless Miiller assigns reasons for his belief, that if 

 there had been no suppression of development, all these crustaceans 

 would have appeared as nauplii. 



How, then, can we explain these several facts in embryology, — 

 namely, the very general, though not universal, difference in struc- 

 ture between the embryo and the adult ; — the various parts in the 



