FUEE KEMATOIDS. 
171 
I Lave found no representatives of these particular types in salt 
water ; and, as far as my experience goes, the marine species 
are all incapable of being revived after having remained with- 
out water on a slip of glass for a few minutes. 
I have been able to verify the observations of Spallanzani, . 
Dujardin, and others, regarding the tenacity of life evinced by 
the Nematoids found in tufts of moss. These animals do not, 
however, belong to the genus Rhabditis, as reported by Du- 
jardin ; they are distinct types which I have included under the 
genera Plectus and AphelenchiLs. Similar forms are met with 
in many of our lichens, and notably so in the fine orange- 
coloured Parmelia paidetina, from old houses and walls. In both 
lichen and moss the Nematoids are, as first observed by Spallan- 
zani, almost invariably associated with certain Rotifera, and 
curious slow-moving arachnidal animals, called by him ^‘sloths.” 
These three different types of animals all possess the same kind 
of tenacity of life. Very many of the species of the four genera 
of Nematoids I have mentioned are found in earth, lichen, moss, 
or other situations in which they are exposed to constant vicis- 
situdes of drought and moisture, according to ever-changing 
meteorological conditions, and in the possession of this power of 
resisting the effects of desiccation, they are endowed with the 
only means of neutralising what would otherwise be the fatal 
influence of the varying conditions of their environment. The 
life-history of these species must be a strange one, made up of 
periods of life and activity alternating with others of apparent 
death — the two states bearing no definite relation to one another 
as regards duration, being altogether inconstant and variable, 
and succeeding one another under the influence of laws so re- 
mote as to make their successions of active and passive existence 
seem almost a matter of chance. Doubtless they have a definite 
span of active existence in which to go through their stages of 
growth, development, and reproduction. But whilst the sum- 
total of these periods of active life peculiar to the species may 
be pretty definite, the duration of time over which their frag- 
mentary existence may be extended, is altogether variable and 
indefinite, owing to the uncertain length and number of the 
interpolated periods of apparent death. An admirable exem- 
plification of this is seen in Tylenchus tritici ; the duration of 
the active life of this species is probably about nine or ten 
months, but individuals have been known to retain their life 
for a period of twenty-seven years. Thus, Baker ascertained 
that some of the animals contained in diseased wheat, given to 
him by Needham in 1744, still possessed the power of resuming 
all their vital manifestations in 1771, after immersion for a 
time in water. 
Davaine has made a most elaborate series of experiments upon 
VOL. VII. NO. XXVII. N 
