252 



LECTURE XIX. 



the head and mouthy which are confluent with the thorax, and from 

 the undivided abdomen. The thoracic appendages (^fig. 107. c, c), 

 eight in number, as in the Arachnida, are however of 

 the simplest and most rudimental kind, 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 proboscidiform, 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 (o, o), consisting 

 of two joints, and with a narrow triangular labrum above. 

 Although the structure of the mouth, as described and 

 figured by Dr. Simon, has much analogy with that of the 

 Acari, like which, also, the follicular parasite in one of 

 its stages of development is a hexapod, yet it differs 

 Demodex foi. from the Acari, and from all other Holeterce of Duges, in 

 magnified. the articulations of the thorax ; whilst it equally differs 

 from the Pseudo-scorpionidce, and the Pycnogonidce, which have the 

 thorax articulated, in the rudimental form of the feet, and the struc- 

 ture of the tropin. 



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 without 

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

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

 bitably 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, I cannot assent to the place which that accomplished 

 naturalist has assigned to the Arachnidan in question among the 

 Acaridce, much less to the genus Acarus. Of the generic distinction 

 of the parasite there can be no doubt, and I therefore propose to call 

 it Demodex foUiculorum, from drij^wc, lard, and dr]E, the name of a 

 boring worm, indicative of the habitat and vermiform figure of 

 this parasitic arachnidan, which insinuates itself into the hair-follicles 

 and the sebaceous glands that communicate therewith. 



In some of the small and parasitic tracheary Arachnida, or mites, 

 certain pairs of legs are terminated by adhesive suckers, and others 

 are occasionally terminated by setas, as in the itch-mite (Sarcoptes 

 Galei, Jig. 108.). 



The mouth, in all Arachnidans, is situated on the anterior segment 

 and is provided with instruments adapted either for suction or mas- 

 ication. In the parasitic mites the rudiments of the jaws are more 



