306 
MR. WILSON ON THE STRUCTURE AND DEVELOPMENT OF 
in almost every person, but are most numerous in those in whom the skin is naturally 
torpid, in invalids, and in the sick. Their numbers are various; in some persons 
not more than two or three can be found in a follicle, while in others I have seen 
upwards of fifteen, indeed the follicle has been completely distended by them. Their 
position within the follicle is peculiar, the head is always directed inwards, while the 
tail is directed in the opposite sense ; and when a number are present, they have the 
appearance of being collected into a conical bundle, the large end of the cone being 
formed by the heads. The situation in which these little animals are most commonly 
found is the integument of the face, and more particularly that of the nose, but I 
have also met with them in the follicles of the back, and of the breast and abdomen ; 
I have not yet found them on the limbs. 
As regards the size and form of the animalcules there is considerable variety, some 
of the differences being referrible to growth and development, and others amounting 
to a specific distinction. They pass their whole existence in the sebaceous substance, 
undergoing ecdysis repeatedly during their development and growth ; and are finally 
expelled from the follicles with the secretion of those organs. For examination, they 
are obtained by compressing a follicle and placing the small mass which escapes 
from its aperture in a drop of olive oil between two plates of thin glass. By a slight 
movement of the glasses the sebaceous matter may be broken up and distributed 
without injury to the animalcules, but if much pressure be employed, they will be in 
danger of being crushed. When prepared in this manner, they may be seen to move 
with activity for many hours, and they are so tenacious of vitality that I have found 
them alive in a subject in the dissecting room that had been dead for fourteen days. 
They are seen very distinctly with an object-glass magnifying fifty diameters. 
The perfect animal may be known by the following typical characters : it is elon- 
gated in figure, being several times longer than the breadth of its body ; and is divi- 
sible into a head, thorax and abdomen ; the head being more distinct from the thorax 
than the latter from the abdomen. 
The head is subcoriical in form, obtusely truncated anteriorly and depressed. It 
is connected with the thorax by means of a narrow segment formed by a fold of 
integument, and can be withdrawn within the thorax so as to be completely buried 
from sight. When the head is protruded the outline of the narrow segment is di- 
stinguished by two slight grooves, which are seen with the microscope as boundary 
lines; but when the head is retracted the segment is seen as a thick folded border, 
which constitutes the anterior extremity of the animal. 
The truncated extremity of the head is composed of two moveable, arm-like organs, 
which lie side by side, and together with two intermediate antennse-like organs, con- 
stitute the entire breadth of the snout. The cephalic arm-like organs or palpi are 
cylindrical at their bases, where they are continuous with the general tegument of the 
head, and become somewhat smaller and prismoid towards their distal end. In the 
latter situation they are truncated and present a flattened extremity. They appear 
