G. H. F. Nuttall, W. F. Cooper and L. E. Robinson 173 
proper consists of a long cylindrical tube, the shaft (s.ch.), upon the 
distal and free extremity of which is borne the chelate digit (d.ch.) 
which constitutes the cutting apparatus by means of which the animal 
penetrates the skin of its host. The sheath is covered on its dorsal and 
lateral surfaces, excepting its proximal third, with numerous minute 
reflexed denticles, regularly arranged in oblique rows and in close 
contact with one another, giving it an appearance which has suggested 
the term “shagreened sheath”: it is firmly fused with the basis capituli 
at its proximal end. At its free extremity, the sheath is invaginated 
and becomes membranous and flexible, and this thin membranous 
portion is attached to the shaft some little distance posterior to the 
articulation of the terminal digit, by which arrangement the shaft 
draws in the extremity of the sheath over the delicate digital structures 
when fully retracted. The shaft of the ehelicera is very long, so long 
in fact as to project backwards beyond the basis capituli into the 
body-cavity: it is divided into two portions, differing in diameter. 
The proximal portion, comprising about one-third of the entire length, 
has thinner walls than the distal portion and is dilated: the edges of the 
proximal extremity are irregularly incised and it is to this extremity 
that the retractor muscles of the ehelicera are attached (Text Fig. 6): 
this dilated portion is occupied by muscles attached to its internal 
surface and from which proceed the tendons which are inserted into the 
base of the digit. The distal two-thirds of the shaft is roughly cylindri¬ 
cal with a slight ridge along its supra-internal margin, the ridge being 
longitudinally traversed by a minute canal (Plate XVI, Figs. 2—9): the 
chitinous walls are thick and show a delicate transverse striation : the 
distal extremity is thickened internally and presents a surface for the 
articulation of the base of the digit. At the junction of the proximal 
and distal portions of the shaft is a somewhat rectangular area of thinned 
and membranous chitin, in length about equal to the diameter of the 
shaft and extending round the dorso-lateral surface of the latter for 
about half its circumference. The digit consists of two movable articles, 
a larger internal article (i.a.), with which is articulated a smaller external 
article (e.a.). The internal article is expanded at its base and articulates 
directly with the shaft; it is prolonged forwards on the internal side 
in the form of a long cutting lancet with a slightly hooked extremity, 
the point of which is directed laterally; it is hollow throughout its 
length, the cavity in the expanded base being of considerable size. 
Near the distal extremity of this article, attached to its dorsal surface, 
is a crescentic chitinous structure with laterally directed points which is 
