CHAPTER IV. 



LIFE-HISTOEY OF PAEASITES. 



All that has been hitherto said about the origin, metamorphosis, 

 and migTation of parasites demonstrates plainly that the older 

 observers, who denied any remarkable changes in their life-history, 

 were entirely wrong. We now know that parasitism only repre- 

 sents a single phase in the life of an animal, which, in spite of its 

 importance and extent in many cases, always presupposes another 

 stage. In fact, if we only know concerning a certain animal that 

 it is a parasite, we know but little; thoroughly to understand its 

 history, we must follow out all the separate stages and conditions of 

 its existence, and especially the circumstances under which it becomes 

 a parasite. 



However varied and numerous these may be, they are contained 

 within fixed boundaries. There are certain standards, or rather 

 certain types, of parasitic life, under which the individual cases are 

 more or less definitely grouped. The knowledge of these conditions 

 not only renders the individual cases intelligible, but it also enables 

 us to cast a comprehensive glance over the whole field of parasitism, 

 and therefore we may be thoroughly justified in prefacing the detailed 

 study of individual types by a general sketch of their life-history. 



We commence with the period of sexual maturity, since this leads 

 to the beginning of a new life-cycle. Between different parasites 

 there is a striking difference with respect to the sexual maturity ; for, 

 in agreement with what has already been stated concerning parasitism 

 — that it is sometimes perpetual, and sometimes only temporary — we 

 find some parasites whose period of sexual maturity coincides with the 

 parasitic period, and others that do not attain to sexual maturity until 

 they have commenced to lead a free existence. 



On the whole, however, the last-mentioned class is but small, and 

 contains only the larvae of parasitic insects and the Gordiacese and 

 Mermithidse, so that it may be confidently asserted as a law, that 

 parasites, and especially the Helminths, attain sexical maturity while 

 in the parasitic stage, and therefore reproduce themselves in the body 

 of their host. A closer examination shows that this fact is entirely in 

 harmony with the conditions of parasitic life. The position of a 

 parasite is — economically considered — most fortunate ; its expenditure, 



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