

AUTHOB'S PREFACE. 



INVESTIGATIONS into the natural history of the entozoa, con- 

 tinued for many years, have taught me that it is impossible to 

 obtain a complete view of the different stages of existence through 

 which these parasites pass, if one's observations are restricted to 

 but a few of the localities in which they are found. At an early 

 period of my researches, it became evident that the same entozoon, 

 in its young state, may have a very different habitation from that 

 in which it is found in its adult condition ; for these animals 

 undergo the most remarkable metamorphoses, and their habits 

 varying with their changes in form and age, they are necessitated 

 repeatedly to change their residence. 



These peculiarities in the natural history of the entozoa, often 

 most difficult of investigation, have rendered the task of the 

 helminthologist, in seeking to obtain a just conception of their 

 genera and species, a very difficult one. It has only too frequently 

 happened that the different stages of development of the same 

 species of entozoa, have been described as so many distinct species 

 or genera ; and thus the systematic arrangement of the group has 

 been built upon a faulty foundation. Hence, again, a difficulty 

 has arisen in the way of attaining correct ideas with regard to the 

 modes of propagation of the intestinal worms, and this obstacle 

 could only be removed by determining, in defiance of the authority 

 of the older helminthologists, to give up many genera and species 

 established upon what they supposed to be independent forms. 

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