52 LIFE-HISTORY OF PARASITES. 



with certain Nematode larvae, which are present in the same situation 

 in other birds, entirely free from Filaria attenuata or any other 

 Haematozoa. 



The Haematozoa, then, after a longer or shorter sojourn in 

 the blood-vessels, would appear to leave the body of their host in some 

 way or other, and continue their life-history under other conditions. 

 This supposition is strongly supported by what has been observed in 

 human Hsematozoa. According to Lewis, these wormsborethrough the 

 capillaries of the kidney and make their way into the renal tubules, 

 and thence to the exterior. [This emigration takes place so rapidly 

 that, after a day has elapsed, in most cases very few worms, some- 

 times not even one, can be found, unless a fresh introduction have taken 

 place. The significance of this migration to the future development of 

 the worm is still unknown. R L] Up to the present, this observa- 

 tion is certainly unique, and nothing similar has been observed in the 

 Haematozoa of other animals, though investigations have been carried 

 on. 1 If future researches throw no fresh light upon the subject, and 

 it is always possible that the emigration is different and more difficult 

 to observe than in man, whose urine contains in abundance not only 

 the Haematozoa, but also a quantity of blood and albumen mingled 

 with them, which renders their presence obvious there always 

 remains the possibility that the Haematozoa continued to live in the 

 blood, without change, until set free by the death of their host, which 

 enables them to undergo further metamorphosis ; and this is rendered 

 more possible by the fact that no one has succeeded in finding the 

 worms that originate from these Haematozoa in any animals where 

 the latter are present, 2 and it is evident that they must at some time 

 or other have been there. 



We have hitherto been considering those embryos only which, 

 after being hatched, remain for some time in the body of their host ; 

 but these are only a small number of examples. The general rule is, 

 that the eggs, as soon as they are laid, are evacuated from the body 

 of their host togetlier with its excreta, and undergo their further develop- 

 ment in various places and under various conditions, as chance directs. 



1 Borrell (Archiv f. pathd. Anat., Bd. Ixv., p. 399, 1876) shows reason to believe that 

 the Haematozoa of the crow leave the body by the bile-duct, but the above-quoted 

 investigations of Kahane prove that no Filarice are present here, or in the cloaca, 

 ureters, or bronchi, except, of course, there has been some mixture of blood. 



2 Gruby and Delafond only found once, in twenty -four dogs infested with Haematozoa, 

 the Filarice from which these originated. According to Ercolani, Filaria mltis is to be 

 found not only in the heart, but also in the connective tissue under the skin, where it 

 might be easily overlooked (Rivolta, "Studi fatti nel gabinetto di Pisa," 1879). 

 Similarly, among the above-mentioned thirty-eight crows, there were only three in which 

 the presence of Filarice could be proved ; of course it is probable that the sexually mature 

 worms may have escaped observation, here and there, on account of their concealed position. 



