PARASITES AFFECTING THE SKIN 37 
sericeum). It occasionally attacks horses and dogs, 
producing a pustular eruption and a considerable 
amount of itching. The pustules are at first small, but 
may reach the diameter of a shilling, and are seen on the 
back, on the muzzle, and between the thighs. The con- 
dition is quite amenable to treatment with parasiticides. 
PHTHIRIASIS (LOUSINESS). 
The lice include all the Aptera, with the exception 
of the fleas. The Pediculine are divided into Pedicu- 
lide and Ricinide. The two chief types of louse are 
Haematopinus and Trichodectes, the former being a 
division of the Pediculide, and the latter of the 
Ricinide. 
The true lice (Hematopinus) have a projecting 
rostrum, with barbed hooks, and in addition a hollow 
extensile sucker. The legs are provided with hooked 
terminal joints. They have simple eyes, are wingless, 
and pass through none of the ordinary intermediate 
stages between the egg and the perfect parasite, as in 
the Acari. The eggs are oval in shape, are attached 
to the hairs, and are commonly known as “nits.” In 
Heematopinus the shape of the body is generally said 
to resemble that of a Dutch doll—that is to say, the 
thorax is larger than the head, and the abdomen larger 
than the thorax. With the exception of the sheep, all 
animals may be affected with one or more varieties of 
Hematopinus. 
The Hzmatopinus forms are blood-suckers. 
Trichodectes.—These are epidermis-eaters, and the 
head is modified for this purpose. Instead of being 
terminal, as in Hematopinus, the mouth is situated 
on the under surface of the head, which is broad and 
shield-shaped. In these the thorax is the smallest 
segment of the body. 
