CHAPTER I. 



INTRODUCTION. 



By W. K. BROOKS. 



No great group of animals is more favorable than the Crustacea for the study of the history 

 and significance and origin of larval forms, for these animals possess a number of peculiarities 

 which serve to render the problem of their life history both unusually interesting and significant, 

 and at the same time unusually intelligible; nor are these peculiar features exhibited, to the 

 same degree, by any other great group of animals. 



The body of an arthropod is completely covered, down to the tip of each microscopic hair, 

 by a continuous shell of excreted matter, and as this chitinous shell is not cellular it can not grow 

 by the interpolation of new cells, nor can it, like the excreted shell of a mollusk grow by the dep- 

 osition of new matter around its edges, for there are no such growing edges, except in a few 

 exceptional cases, such as the barnacles. Once formed and hardened the cuticle of an arthropod 

 admits no increase in size, and as soon as it is outgrown it must be discarded and replaced by a 

 new and larger one. The new shell is gradually excreted, in a soft condition, under the old one, 

 and as soon as this is thrown off the new one quickly becomes fully distended and solid. As a 

 result, from the very nature of the chitiuous shell and the method of renewal which its structure 

 entails, the growth of an arthropod, from infancy to the adult condition, takes place by a series of 

 well-marked steps or stages, each one characterized by the formation of a new cuticle and by a 

 sudden increase in size. 



In most arthropods the newly-born young are very different in structure from the adults, and 

 growth is accompanied by metamorphosis. As the changes of structure are necessarily confined 

 to the moulting periods, the stages of growth coincide with the stages of change in organization, and 

 there is none of the indefiniteness which often characterizes the different larval stages of animals 

 with a more continuous metamorphosis. On the contrary the nature of each change is as sharply 

 defined and as characteristic as the structure of the adult itself. As the moulting period is fre- 

 quently a time of inactivity the animal may then undergo profound changes without inconvenience, 

 and the successive steps in the metamorphosis of an arthropod are not only well marked, but often 

 very profound as well. 



In these features all the other arthropods are like the Crustacea, but another consideration, 

 the fact that, with few exceptions, the higher Crustacea are marine, renders the problem of their 

 life history much more intelligible than that of any other class of animals. 



So far as the ontogenetic history of the metamorphosis of a larva is a recapitulation of ances- 

 tral stages in the evolution of the species its retention at the present day must depend to a great 

 degree upon the persistency of those external conditions to which the larval stages were originally 

 adapted. 



This is true at least of all free larvse, which have their own battle to fight and their own living 

 to get, and while a larva inside an egg or within a brood pouch may possibly recapitulate obsolete 

 ancestral stages, the survival of a free larva depends upon its adaptation to its present environ- 

 ment. 



As compared with the ocean the inorganic environment of terrestrial or fresh-water animals 

 is extremely variable, and changes in climate, elevation, and continental configuration are accom- 

 panied by corresponding changes in enemies, competitors, and food, so that the conditions which 



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