142 
N ematodirus filicollis Hud. 
within a very short period 1 , thus suggesting the possibility that in this 
species also the liberation of the larvae normally takes place within the 
alimentary canal of the host. Although a quite conceivable solution 
of the difficulty it struck me as a highly improbable one, since the study 
of some liberated larvae had shown that these are admirably adapted 
for leading a free existence and, moreover, that their structure and habits 
are precisely similar to those of other Nematode worms known to have 
free-living stages in their life histories. 
I only accidentally became aware of the true facts of the case; 
during a particularly warm, sunny day in the spring one of my cultures 
was left exposed to the sun’s rays for several hours and examination 
of a small sample showed that a considerable number of larvae had been 
liberated. The temperature in the sun was approximately 25° C. 
Other cultures were then subjected to various temperatures between 
24° C. and 32° C., both in ovens and in the open, and these experiments 
showed that the larvae are quite capable of hatching so long as the 
temperature is sufficiently high. During a warm spell in the early 
summer (24°-28° C. in the shade during the middle of the day) nearly 
all the larvae in my cultures were released. 
Description of the “ Ensheathed ” Larva. 
The newly-hatched larvae of Nematodirus are easily distinguished 
from those of other Nematode parasites of the sheep by their com¬ 
paratively large size and by the great length and tenuity of the tail- 
region. 
It must be noted that these larvae on hatching have already under¬ 
gone two ecdyses, one complete and one partial, within the egg-shell; 
as pointed out by previous observers, they are therefore in a considerably 
more advanced condition than the newly-hatched larvae of the majority 
of Strongylid Nematodes, e.g. Haemonchus contortus or Anchylostoma 
duodenale, in fact they have reached a state of development comparable 
with that of the latter worms at the end of their period of free fife. 
According to Looss’ nomenclature (1911, p. 345) the Nematodirns 
larvae when liberated from the egg are in a state of transition between 
the second and third larval stages; in structure they have reached the 
third stage of development, but are still enclosed in the loose skin 
1 In these experiments the liberation of the larvae took place quite independently 
of the chemical composition of the fluid in which the eggs were kept. The media used 
were those suggested by Martin (1913, p. 64), namely, distilled water, -2 per cent. HC1, 
•2 per cent. NaHC0 3 and -8 per cent. NaCl. 
