1909. | THREAD-WORMS OF THE RED GROUSE. 337 
consequently hard to see and often overlooked. We have found 
them, with hardly an exception, in every one of the hundreds of 
grouse examined. They are rendered opaque and white, and 
hence much more apparent, by shaking up the contents of the cecum 
in 75 per cent. alcohol, to which a few drops of corrosive sub- 
limate have been added. Their presence is also readily detected 
by compressing a drop or two of the cecal contents between two 
microscope-slides and holding them up to the ight. The worms, 
if there be any, then appear as thin, white, transparent lines. 
We owe this method to Dr. Wilson. 
T. pergracilis is an extremely fine worm, measuring in the male 
on the average 6 to 8 mm. and in the female 8 to 10 mm. 
They are very narrow and hair-like and, as a rule, whitish in 
colour, but sometimes have the tinge of blood when seen in a 
very thin layer on a slide through the microscope. ‘They are very 
transparent, readily revealing their internal structure, and they 
are so soft that the pressure of a cover- shp almost always ruptures 
them. The cuticle is very clearly and definitely ringed ( Pl XLT, 
figs. 3 & 5), and the rings are so constituted that whilst the worm 
can easily work its way forward through a tissue, it would have 
difficulty in wriggling backward. The rings give the edge of the 
body a strongly serrated appearance like a saw. ‘This is most 
marked a little way behind the head and extends over about cne- 
third the body length. There is no trace of longitudinal marking 
on the cuticle. 
The genital bursa in the male is well formed, and opens to the 
exterior by an oval opening with its long axis longitudinal. The 
bursa 1s supported by a number of ridges as an umbrella is by its 
ribs, and, using Looss’s nomenclature, these are arranged in three 
groups. The members of each group arise from a common root 
and are recognisable, even when, as in the case of 7’. pergracilis, 
some of them run close to and parallel with members of another 
group. The three groupsare: (1) Dorsal, (2) Lateral, (3) Ventral. 
The Dorsal group consists of a median, posterior dorsal rib, 
which is forked slightly at its free end ; oer the same root with 
it arise a pair of externo-dorsal ribs, one of which passes into the 
side membrane of the bursa (Pl. L. figs. 11 & 12). In front of the 
externo-dorsal, three lateral ribs arise from a common root ; on each 
side of these the most posterior or postero-lateral vib is smaller than 
the others and slightly separated from them; the median-lateral 
rib and the antero-lateral rib are of similar size and lie parallel. 
There are on each side in front of the lateral ribs a pair of ventral 
ribs; of these the hindermost or latero-ventral rib forsakes its 
group and comes to lie close and parallel to the antero-lateral rib, 
which it further resembles in size and shape. The ventro-ventral, 
on the other hand, is simmer and stretches forward to support. 
the anterior edge of the bursa. The whole arrangement of the 
lateral and ventral ribs recalls a slightly flexed hand, with the 
thumb thin and extended and the little finger slightly divaricated. 
The thumb represents the ventro-ventral rib, the first finger the 
