152 
Nematodirus filicollis Rud. 
just in front of the rudimentary bursa measuring 45/x. The head is 
20jU, broad and is provided with a circular mouth and six cephalic 
papillae, the latter now more conspicuous than in the earlier stages. 
The caudal extremity of this specimen bears a striking resemblance to 
that of an early stage of a Strongylid figured by Leuckart (1876) under 
the name of Strongylus polygyrus. 
With the final moult the characteristic tail-fork and caudal process 
are lost in the male specimens, the bodies of the latter now terminating 
in the bursae, at first small and with very faint and inconspicuous rays. 
The smallest specimen in which all the rays as well as the spicules were 
visible measured 5 mm. in length. 
In the young females the caudal fork remains for a longer period 
and seems to pass gradually into the truncated extremity of the adult, 
the rod-like process of the larva is evidently retained as the terminal 
spike so characteristic of the mature female. 
Summary of Life Histoiry. 
1. The eggs of Nemalodirus fUicoUis when laid contain an embryo 
with seven or eight cells, they pass out of the infested host with the 
faeces. 
2. Even under favourable conditions development takes place 
slowly and the embryos are not ready to hatch until 24-28 days have 
elapsed. In their early stages the embryos are not able to withstand 
desiccation and are killed if frozen or subjected to high temperatures. 
3. Whilst still within the egg-shell the embryo undergoes two 
ecdyses and when ready to hatch is enclosed in a tightly fitting sheath 
formed by the incompletely cast skin of the second moult; the larvae 
on liberation from the egg-shell are therefore in a more advanced 
condition than those of most other Strongvlids (e.g. Haemonchus or 
Anchylostoma) and are in a stage comparable with that reached by the 
latter at the end of their period of free existence. 
4. The sheathed larvae are often retained for a long time within 
the egg-shells and both in this position and after hatching can resist 
complete desiccation for considerable periods (twenty months or even 
longer); when dried they are able to withstand freezing as well as 
temperatures much above those likely to be met with in the open. 
5. The free larvae will live for a considerable time in water, they 
possess well-developed migratory instincts and climb vertical surfaces, 
such as grass stems and blades, and the glass walls of the vessels in which 
they are kept. 
