ARACHXIDA. 445 



and are terminated by three short setae ; the anellidous type of the 

 locomotive appendages being still retained. The integument of the 

 abdomen is very minutely annulated. The mouth is a suctorial one, 

 or proboscidiforra, consisting of two small spine-shaped maxillae (b), 

 and an extensile labium, capable of being elongated and retracted ; it 

 is provided on each side with a short and thick maxillary palp 

 (a, a), consisting of two joints, and with a narrow triangular labrum 

 above. Although the structure of the mouth, as described and 

 £gured by Dr. Simon, has much analogy with that of the Acari, like 

 which, also, the follicular parasite in one of its stages of develop- 

 ment is a hexapod, yet it differs from the Acari, and from all other 

 Holeterce of Duges, in the articulations of the thorax'; whilst it 

 equally differs from the Psmdo-scorpionidce, and the Pycnogonidce, 

 which have the thorax articulated, in the rudimental form of the 

 feet, and the structure of the trophi. 



It can hardly be supposed that the changes of form indicated by 

 the figures 8, 1, and 2 of Dr. Simon's Memoir can be acquired with- 

 out ecdysis ; but such a metamorphosis, with the natural divisions of 

 the body and the structure of the oral and thoracic appendages, in- 

 dubitably raise the parasite of the hair-follicle above the Entozoa, to 

 which class Prof. Erichson, in Dr. Simon's Memoir, has correctly 

 stated that the present parasite cannot belong. For the reasons 

 above given, the Arachnid in question cannot be generically asso- 

 ciated with the AcaridcB, and it has been referred to a genus 

 called Demodex, from Ir^fioQ, lard, and Ij)1, the name of a boring 

 worm, indicative of the habitat and vermiform figure of this parasitic 

 Arachnid, which insinuates itself into the hair-follicles and the seba- 

 ceous glands that communicate therewith.* 



* LXXXIV. p. 252. This generic name, and the position of the genus In the 

 class Arachnida, have been accepted by the judicious and experienced naturalist 

 Siebold (xxiv. p. 507.)- The Demodex is not confined to man, as the following 

 paragi-aph shows : — "A communication from A. Tulk, Esq., upon certain para- 

 sites in the dog, was read. These parasites were found by Mr. Topping, on ex- 

 amining, microscopically, the contents of the pustules in a mangy dog. They 

 belong to the genus Demodex (Owen), founded upon parasites, described by 

 Dr. Simon, of Berlin, as inhabiting the sebaceous sacs and hair-follicles of the 

 human skin. The insects now described as existing in the dog, were found in 

 such abundance, that thirty or forty were frequently seen in a single drop of pus. 

 They differ very slightly from the human parasites before referred to; but 

 analogy would lead to the conclusion, that they are of a different species. The 

 discovery of this parasite may throw some light on the cause of the disease called 

 mange, a disease by no means confined to one class of animals, while at the same 

 time it is far from being certain whether this insect is the exciting cause, or is 

 merely developed during the progress of the disorder." — Proceedings of the 

 Microscopical Society, 20th December, 1 84.3. 



