366 DEVELOPMENT AND EMBRYOLOGY. [CHAP. XIV. 



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 

 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 retro- 

 grade, for the male is a mere sack, which lives for a short time and 

 is destitute of mouth, stomach, and every other organ of im- 

 portance, 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 development. Spiders, again, barely undergo any metamor- 

 phosis. 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 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 

 structure between the embryo and the adult ; the various parts 



