ON THE ANGUILLULIDÆ. 79 
file within the genital tubes, though often occupying the whole width of the body. This 
is a condition of things quite in harmony with the several requirements of animals de- 
pendent upon such totally different conditions. The free Nematodes produce their ova 
or young at once in that environment which they are destined to inhabit, whereas the 
parasitic progeny are subjected to a multiplicity of chances and contingencies before 
they meet with the necessary conditions suitable for their development: there must be 
many blanks in order to ensure a few prizes. It is but another instance of the harmony 
subsisting between the observed biological history of an organized being and the physical 
eonditions to which it is subjected and surrounded; and the difference in this respect 
between the two divisions of the order Nematoidea may not inaptly be compared to that 
existing between the predaceous cartilaginous fishes, on the one hand, and the ordinary 
osseous species on the other. We may note the same limited number of progeny in those 
forms whose young are most likely to survive, owing to their being produced viviparously 
or else with the egg enclosed in a coriaceous envelope, which, for additional security, 
becomes fixed by means of its tendrils to some rock or larger seaweed. Whilst the ova 
or young of such species may be numbered by units, for those of the majority of osseous 
fishes we may substitute, instead of units, millions or even billions. 
Then many of the free Nematoids, more especially of the marine species, are provided 
with such rudimentary sense-organs as would be useless to a parasite. These ‚exist in 
the form of distinct, reddish, conical and circumscribed masses of pigment, with the 
addition occasionally of transparent lens-like bodies, situated on the anterior part of the 
esophagus, which doubtless subserve the purpose of rudimentary visual organs. And, 
lastly, almost all the free Nematodes are furnished with a caudal sucker, most highly 
developed in the marine species, to whom its utility is obvious, by enabling their smooth 
and polished bodies to adhere to the particular weeds which they infest, whilst these 
are swayed to and fro by the currents of the flowing and receding tide. 
These various considerations lead me to believe that the free Nematoids constitute a 
group absolutely distinct from the parasitic forms ; and I have dwelt upon this point, not 
only because it has not been enforced by previous writers, but also. with the view of 
showing the untenability of the opposite hypothesis, advanced, perhaps somewhat hastily, 
by a most accurate observer, and one whose opinions generally are so worthy of credit. 
On this account, too, it does not seem to me desirable to associate with these animals, as 
Dujardin has done in his fifth section, “ Enopliens,” the two parasitic genera, Passalurus 
and Atractis—and this not simply on the arbitrary ground of their being parasitic, but 
because they neither of them comply with those structural conditions which were stated 
to obtain almost universally in the group in question. They appear to have been so 
Placed by Dujardin, from the simple fact of their possessing a mouth armed with three 
teeth or Jaws, which he took to be the typical character of this group, as shown by the 
name he applied to them. But a reference to the figures and descriptions of the species 
discovered by Dr. Eberth and myself will show that this is a structure quite exceptional 
—only met with in one or two genera, and therefore untenable as a family distinction. 
Diesing, also, in his recent communication on the classification of the Nematoids, has 
associated with these animals certain parasitic genera ; and in this paper, as well as in his 
