460 DEVELOPMENT AND EMBR YOLOG T. 



pilosis; the cepbalopodic character is manifested long be- 

 fore 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 atid often great changes 

 during 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 

 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 embry- 

 ology — namely, the very general, though not universal, 

 difference in structure between the embrvo and the adult: 

 the various parts in the same individual embryo, which 

 ultimately become very unlike, and serve for diverse pur- 

 poses, being at an early period of growth alike; the com- 

 mon, but not invariable, resemblance between the embryos 

 or larvae of the most distinct species in the same class; the 

 embryo often retaining, while within the egg or womb, 

 structures which are of no service to it, either at that or at 

 a later period of life; on the other hand, larvae which have 

 to provide for their own w^ants, being perfectly adapted to 

 the surrounding conditions; and lastly, the fact of certain 

 larvae standing higher in the scale of organization than the 

 mature animal into which they are developed? I believe 

 that all these facts crai be explained as follows. 



