MECZNIKOW, ON ASCARIS NIGROVENOSA. 31 
But, from what has been observed, it is manifest that As- 
caris nigrovenosa has two sexual generations, of which one is 
parasitic, whilst the other, which presents the characters of 
the genus Rhabditis, enjoys a free existence. 
This fact shows not only a remarkable mode of propaga- 
tion, but also indicates peculiar relations between the para- 
sitic and free modes of life. The correspondence of certain 
free nematodes with the parasitic has been partially recog- 
nised by many earlier writers. Goeze and Dujardin,* for 
instance, observed that the young larvee of Ascaris acuminata 
are capable of living in water; and Willt has shown that 
Angiostoma limacis occurs, not only in the interior of snails, 
but also free in the water. 
But the genetic relations between the parasitic and free 
nematodes were first made clear by the observation of Pro- 
fessor Leuckart, who watched the growth in the free state of 
the Rhabditis-like larvee of Dockmius trigonocephalus. 
These intimate relations, as well as the circumstance that 
the nematodes possess a much better-developed digestive ap- 
paratus than all the other parasitic helminths, suffice to 
prove that the mode of life of the parasitic nematodes must 
exhibit peculiarities of some kind. It appears more than 
probable that many of the nematodes found in the intes- 
tines of animals are not true parasites, since they feed, not 
upon the living tissues, but upon the excrementitious matters 
of their host. In favour of this view may be adduced the 
observation made by Dujardin fourteen years ago, of the pre- 
sence in the intestine of Ozxyuris curvula of various solid 
vegetable particles. The author has also found in the intestine 
of the Sclerostomum of the sheep abundance of fecal particles 
in great variety belonging to that ruminant. 
Having thus indicated the principal circumstances attending 
the development of A. nigrovenosa, the author feels compelled 
to pass to a more unpleasant and far less scientific task, viz., to 
the assertion of his right to the credit of this discovery, which 
has not been fully admitted by Professor Leuckart. 
The Professor speaks thus: ‘‘ What I have to relate in the 
following pages contains only that part of my observations 
which has been brought to a more or less complete conclu- 
sion. The greater part of my researches were instituted during 
the past winter semestre, and in them I have in almost every 
point enjoyed the assistance and co-operation of Herr Meczni- 
kow” (1. ¢., p. 221). 
* * Hist. des Helminthes,’ 1845, p. 228. 
+ ‘Archiv fir Naturgesch.,’ 1849, p. 179. 
