8. Yokocawa 
135 
the former stage. Therefore, when these structural changes have been 
accomplished the larvae themselves have decreased considerably in length. 
It is very difficult to determine exactly when the first moulting begins 
and when the sheathed larvae cast their old skins, since the structural changes 
proceed very slowly, and the ecdysis usually does not occur in liquid media. 
The casting of the old cuticula is a purely mechanical process and is effected 
by the activity of the larvae at the beginning of the second stage. The old 
cuticula, however, is so thick and rigid that the larvae cannot easily rid them¬ 
selves of it without mechanical assistance. Accordingly the larvae after the 
completion of the structural changes described above prefer to penetrate into 
the filter paper or into pieces of faeces or to climb up the wall of the container 
in order to bring themselves in contact with some surface which will offer re¬ 
sistance. At this time the posterior end of the old cuticula usually becomes 
bent in the form of a hook (Fig. 5, Plate VIII). Occasionally a larva can be 
seen to be firmly attached by means of this hook to a piece of faeces, as it 
begins to free itself from the sheath. 
In solid cultures, the structural changes of the larvae can be noted in 
60 to 80 hours after the beginning of development, and some mature larvae 
(in the second stage) are found at the edge of the filter paper or on the sides 
of the container in 100 to 120 hours. 
Measurements of the larvae toward the end of the moulting period show 
that while the length of the sheath is 0*77 mm. to 0-82 mm., the larvae within 
the sheaths are only 0-62 mm. to 0*75 mm. in length. In the moulting period, 
the movements of the larvae are slow and sometimes they are very quiet. 
They always react, however, to any stimulus. If they are removed from the 
culture into water about 5 mm. in depth, they will become quiet within several 
hours and finally die. 
(e) The Second Larval Stage. (The infective stage.) 
The larvae after the first moult are usually found at the edge of the filter 
paper or on the wall of the container. This stage is reached under favourable 
conditions five days after the cultures are made. Since the structural changes 
which occur during the first moult are very pronounced, the structure of the 
larvae during the second stage differs fundamentally from that of the first 
stage. Those larvae (Fig. 6, Plate VIII) are now ready to infect the host and 
differ from the larvae of other known Strongylidae in not having any sheath. 
Shajpe and size. In shape, they do not differ greatly from the first stage 
except that the tail is short and blunt, having various shapes as shown in 
Text-fig. 4. These variations are nothing but individual differences. The larvae 
in this stage do not vary much in size, averaging 0-69 mm. by 0*027 mm. 
Body-cavity. The body-cavity is so narrow that it can only be detected 
with difficulty. 
The cuticula. The cuticula shows faint transverse striations. On each side 
of the body there is a sharp narrow projection of the cuticula extending from 
