Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 2i5 



their development. Spiders, again, barely undergo any 

 metamorphosis. The larvse 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 ; neverthe- 

 less 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 in the same in- 

 dividual embryo, which ultimately become very unlike 

 and serve for diverse purposes, being at an early period 

 of growth alike; — the common, but not invariable, 

 resemblance between the embryos or larvae of the most 

 distinct species in the same class ; — the embryo often 

 retaining whilst within the egg or womb, structures 

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



