174 
POPULAK SCIENCE BEVIEW. 
transition forms which tend to bi-eak down the artificial barriers 
between these two groups of the order Nematoidea. As before 
stated, many of the free Nematoids live for the most part as 
parasites upon plants ; and two or three of the most highly 
organized and typical species — animals provided with the most 
distinct ocelli — habitually live as pseudo-parasites within the 
substance of some of the softer marine S'pongiadce, Although 
the animal nature of sponges is undoubted, still these differ so 
much from the higher animals that Nematoids existing within 
them are subjected to conditions very slightly differing from 
what they would have experienced amidst the corallines found 
in the same tide pools ; and consequently, even the same or most 
nearly allied species may be met with indifferently in either 
habitat. Elsewhere, too, I have shown* there are strong grounds 
for believing that the Gruinea-worm, so well known as a human 
parasite, is a parasite only accidentally, and that it and its 
parents were originally free Nematoids. So that we have free 
Nematoids, so to speak, parasitic upon plants ; others parasitic 
upon lower animal organisms, such as the Spongiadce ; and one 
even accidentally parasitic upon man himself, in the case of the 
Gruinea-worm. But, most marvellous of all, we now know of an 
animal which in succeeding generations leads alternately the 
life of a free and of a parasitic Nematoid. This animal is 
Ascaris nigrovenosa, which has long been known in its parasitic 
condition as it exists within the lung chambers of the frog. 
But it has been shown by Leuckart and Mecznikow that the 
voung of this animal become real free Nematoids, for, after 
passing from the intestine of the frog into damp earth or mud, 
they grow rapidly, and actually develope in the course of a few 
days, whilst still in this external medium, into sexually mature 
animals. Young, differing somewhat in external characters 
from their parents, are soon produced by them, and these attain 
merely a certain stage of development whilst in the moist earth, 
arriving at sexual maturity only after they have become para- 
sites and are ensconced in the lung of the frog. This is a most 
marvellous life-history. Such a regular alternation between the 
parasitic and the non-parasitic state by succeeding generations, 
each produced from ova, exhibits an instance unparalleled, so 
far as our present knowledge goes, throughout the whole animal 
kingdom.! 
* On the Structure and Nature of the Bracunculus, or Guinea- worm. 
Trans, of Linn. Soc. vol. xxiv., p. 130, also Phil. Trans. 1866, p. 609. 
t The history of this animal will he more marvellous still, if Professor 
Leuckart’s supposition he correct (and it seems the most prohahle one), that 
the young of the parasitic forms are produced hy a process of agamogeneds. 
No parasitic males of this species have ever yet heen met with. The same 
mode of generation seems to exist in the parasitic Guinea-worm, as I have 
