FREE NEMATOIDS. 
165 
The discovery of so many free Nematoids is likely to suggest 
to many the possibility that these may after all represent only 
certain stages in the life-history of parasitic forms hitherto un- 
I known^ and may cause the belief that the former have no claim 
j to be considered as distinct and independent species. Latterly, 
' however, through the researches of Professor Leuckart more 
especially, the complete history of many of the parasitic species 
has been revealed ; and though the suspicion has been confirmed 
that certain stages of the life of many of these are passed whilst 
they are in a free condition in external media, it has also been 
shown that, with one remarkable exception, to which I shall 
again allude, all of them when thus “ free ” are in an undeve- 
loped, asexual condition. For this reason, although it is perfectly 
true that many of the parasitic forms lead a temporary free 
existence at certain stages of their history, it is no less true that 
there are other Nematoids which are always “ free,” which pass 
their whole cycle of existence in external media, and which 
never become parasites at all. Such are the animals of which 
I am now writing. These are the real Free Nematoids, whose 
title to an independent existence is based upon the following 
considerations. In their various habitats, individuals of all 
ages may be seen from the young, immature, and non-sexual 
embryo just emerged from its egg or its parent, up to others in 
the adult condition ; and frequently the ova of species infesting 
a particular sea-weed may be seen attached to the branches 
amongst which the parent worms are gliding. This fact alone 
might induce one to believe that these animals are never para- 
sitic — seeing that they may be observed at all stages of develop- 
ment in their external habitats — even if no other evidence were 
forthcoming. But I have shown,* also, that they do possess 
almost invariably certain anatomical combinations which are 
very rarely encountered amongst the parasitic forms ; and that, 
in addition, some of their anatomical and physiological peculi- 
arities, which are perfectly in harmony with a non-parasitic 
existence, would be useless or defective were they destined to 
live within other animals. Thus, 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. 
They consist of ocelli or rudimentary visual organs, in the form 
of reddish or crimson circumscribed masses of pigment, in 
which are situated, occasionally, transparent refractive lens-like 
bodies. These when present are found on the anterior part of 
the oesophagus (figs. 6 and 7e). Almost all the free Nematoids 
also are furnished with a terminal caudal sucker — most highly 
developed in the marine species — and to these its utility 
Loc, cit p. 76. 
