P. A. Buxton 
131 
strengthened by a U-shaped, band-like thickening of its integument, the base 
of this U articulating with the fork at the distal extremity of the epimere, and 
the tip of each of its limbs with the second joint. On the ventral side of the 
joint a long seta (P 1) arises and passes backwards over the ventral surface 
of the body. The line of articulation between the first and second joints is 
oblique, and is apparent only on the under surface of the leg. On the upper 
surface the joints appear to be completely united, and it may be supposed 
that movement of the second article on the first is extremely limited; we must 
presume that there is some movement because as I have said there are definite 
articular surfaces between the ends of the U band of the first joint and the 
base of the second joint. On the mesial side of the leg there is an indentation, 
conspicuous from above and below, marking the line of the articulation 
between the first and second joints. On the under surface of the second joint 
(Fig. 11 a) is a T-shaped piece of chifcin, the base of which articulates with the 
first joint; the head of the T lies upon the lateral margin of the leg; from it 
arise two structures, the seta P 2 which reaches as far as the tip of the ambula¬ 
crum, and the spur (sr), first described by Megnin. This is a point of some 
importance for no writer since Megnin’s time, with the sole exception of 
Canestrini, has figured or described this structure, and some doubt has arisen 
as to whether it actually exists. Megnin first observed it in Sarcoptes taken 
from the horse, and on this character and others he founded a new species 
S. uncinatus; when, however, he looked for the spur in Sarcoptes from other 
hosts he invariably succeeded in finding it. It is probably present throughout 
the genus, for Canestrini figures it (1894) in the species dromedarii , leonis, and 
precox. 
The third joint is short, and on each side of it there is a longitudinal band¬ 
like thickening of the chitin. From each of these thickenings springs a seta; 
on the median side P 3 arises from a pit in a small chitinous plate, and on the 
lateral side P 4, a very short, stiff seta; both of these are on the upper side 
of the leg. The fourth joint is longer than the third, which it resembles in being 
stiffened by a mesial and a lateral region of relatively thick chitin. The mesial 
side of the joint is noticeably straighter than that of any other joint; from its 
base on the ventral side arises the minute seta P 5; the extremity of the joint 
is hollowed out; from the excavation arises P 6, a short stiff seta which does 
not taper to a point but is equally thick throughout its length. The fifth joint 
is short and may barely be seen among the multitude of structures which 
arise from it. The largest of these is the ambulacrum arising directly from the 
ventral side of the joint. This ambulacrum consists of a cylindrical chitinous 
stem (as), which suddenly narrows at its distal extremity where a cup-shaped 
sucker (ask) is attached to it by a narrow base. Fiirstenberg figured a fine 
tube passing through the stem and opening in the concavity of the sucker. 
I believe that no such tube exists and that the error is due to an optical 
illusion such as is easily produced when one looks at a solid transparent 
cylinder, and I have confirmed my opinion by examining transverse sections 
9—2 
