226 EISTTO-PAKASITES AND HYGIENE. 



ogy of these creatures; we shall find a number of most 

 remarkable facts which owe their appearance largely to 

 parasitic life or to certain features thereof. 



The sexual relations of parasites are peculiar indeed. 

 It may be stated at first that all parasites have a distinct 

 sexual character and reproduce their species through 

 eggs. The latter are either deposited outside or developed 

 inside of the mother animal, so that we call such forms 

 viviparous, the young being fully alive when they begin 

 their existence away from the mother. As to the sepa- 

 ration of the sexes, we find upon investigation among 

 the parasites, especially among the more sessile ones, 

 very few with sepii rated sexes, but in most cases herma- 

 phroditic forms, /. e., such as produce in their bodies at 

 the same time male and female materials of reproduction 

 and are therefore able alone to bring forth a new genera- 

 tion. Thus among the worms the most degenerated 

 tape-worms, nearly all trematods or flukes, and a great 

 number of round-worms are such hermaphroditic indi- 

 viduals. 



According to all modern scientific views this latter form 

 is the original one; it is more advantageous, especially for 

 the sessile parasites, because only then will they have a 

 chance to get their eggs fructified; in the other case, 

 when the sexes are separate, the fulfillment of this 

 condition is often left to chance. 



At all events we can distinguish four different ways of 

 reaching the aforesaid end, the fructification of the eggs: 



1. The sexes are separate, both sessile, the products, 

 however, more or less movable ; large quantities of the 

 latter are deposited in the surrounding mediums and the 

 meeting of egg and sperma is left to chance. In spite of 

 this the probability is no small one here, since the num- 

 ber and movability of the germs is rather great. Such a 

 case is possible in water only. 



2. The sexes are separate, only one of them sessile. 



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