LESSON 43.] 



NUTRITION IN ARACHNIDA. 



151 



the cephalo-thorax ; the remainder is the body, so that it exhibits 

 but two divisions ; and this is equally true of all the members of 

 this class. 



672. Some of the lower Arachnida, so minute as to be quite 

 microscopical, are parasitic on man, and the history of one of these 

 parasites is not a little curious. It has already been observed that 

 the human hair grows out of a bag, or follicle ; this follicle re- 

 ceives the ducts of two fat secreting glands, called sebaceous glands. 



673. Within the ducts of these glands, the parasite in question, 

 Demodex folliculorum (demos, lard; and dex, a boring worm), is 

 found (Fig. 245) ; in length, they range from the ^ th to the T th 

 of an inch. The figure gives a magnified view of a hair-follicle (a), 

 containing the bulb of the hair (b d) } the sebaceous gland (c), and the 



FIG. 247. 



FIG. 245. 



Fro 246. 



Demodex folliculorum. 



Demodex from the scalp. 



Demodex folliculorum ; 

 the perfect animal mag- 

 nified. 



duct in which is the parasite (d). In adults, these creatures usually 

 abound on either side of the nose ; in young men, red pimples, of a 

 painful character, appear on the forehead, face, neck, &c., which, 

 when the inflammation subsides, is distinguished by a black spot. If 

 this spot be tightly squeezed, a white, tortuous, somewhat spiral 

 mass will appear spiral, because a cast in solid fat of the duct 

 placed in oil, the fatty or sebaceous substance will separate and 

 leave a specimen or more of the Demodex. 



Another figure (246) is given, taken fVom the human scalp ; a, 

 the sebaceous glands ; b, a hair ; c, the ducts ; and d, d, the para- 

 sites. 



