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XI. Researches into the Structure and Development of a newly discovered Parasitic 
Animalcule of the Human Shin — the Entozoon folliculorum. By Erasmus 
Wilson, Esq., Lecturer on Anatomy and Physiology in the Middlesex Hospital. 
Communicated hy Robert Bentley Todd, M.D., F.R.S. 
Received November 23, 1843, — Read December 14, 1843. 
In the June Number of Muller’s Archiv (for 1842) there appeared a paper from 
the pen of Dr. Simon of Berlin, announcing- the discovery of certain animalcules, 
which he regarded as Acari, in the sebaceous substance of the follicles of the human 
skin. After perusing his account of these singular creatures I was induced to 
engage in the inquiry, and was so much interested in what I saw, that I devoted six 
months of exclusive labour to the study of their structure and mode of development. 
The result of my research is at variance with the description of their structure given 
by Dr. Simon ; it corroborates Dr. Simon only in the general fact of the existence of 
these animals and their diversity of form, while it offers several new and original ob- 
servations for consideration. Among the latter I may refer especially to the anatomy 
of the head, which Dr. Simon regards as composed of certain moveable organs, but 
which in reality is distinct from those organs ; the discovery of eyes ; of a power of 
retracting the head within the thorax ; of ova ; of a remarkable embryonic form ; 
and of the progressive development of the young animal. 
The situation of the animalcule in the zoological system is a question still to be 
determined, and it is one on which I have felt unwilling to speculate until a com- 
plete description of the creature shall have been placed before scientific entomologists. 
The name given to the animal by Dr. Erichson of Berlin and adopted by Dr. Simon, 
namely Acarus folliculorum, is founded on the erroneous view of the anatomy of the 
head and mouth, put forth by the latter gentleman. The animal is not an Acarus, 
for it has no haustellum ; nor is it an Arachnidan, for it possesses a distinct head, 
susceptible of retraction within the thorax, and an abdomen most indubitably 
annelidous in structure. Under these circumstances I have followed, in the present 
paper, the suggestion of Dr. Todd, and have made use of the term Entozoon, signifying 
merely an inhabitant of the interior of the body, as a temporary appellation, until a 
better and more suitable one shall have been invented. 
The Entozoon folliculorum is found in the sebaceous substance of the hair-follicles ; 
it feeds upon the secretion contained within the sebaceous cells, and its young are 
produced from ova in the midst of the sebaceous matter. The presence of these ani- 
malcules has no reference to disease of the skin or of the follicles ; they are met with 
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