Chap. XIV.] DEVELOPMENT AND EMBRYOLOGY. 245 



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 

 an 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 develop- 

 ment 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 remark- 

 able discovery that certain shrimp-like crustaceans (al- 

 lied to Penoeus) first appear under the simple nauplius- 

 f orm, 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 mem- 

 ber is as yet known to be first developed under the nau- 

 plius-form, though many appear as zoeas; nevertheless 

 Miiller assigns reasons for his belief, that if there had 

 been no suppression of development, all these crusta- 

 ceans would have appeared as nauplii. 



How, then, can we explain these several facts in 

 embryology, — namely, the very general, though not uni- 

 versal, difference in structure between the embryo and 

 the adult; — the various parts in the same individual 

 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 larvse of the most distinct spe- 

 cies 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 period of 



