152 INVERTEBRATE ANIMALS. 
(fig. 72, c); others are permanently parasitic upon other ani- 
mals, such as sheep, dogs, insects, &c.; and others inhabit de- 
caying provisions, as is the case with the well-known “ cheese- 
Fig. 72.—Arachnida. a Pycnogonum littorale; b Tetranychus telarius, one of 
the ‘‘ Sociable” mites ; c Hydrachna globulus, one of the ‘‘ Water-mites.” 
mite” (Acarus domesticus). ‘Two species have a considerable 
medical interest as attacking man. One of these causes the 
skin-disease which is known as the “itch,” and the other is 
found inhabiting certain glandular follicles of the skin, pro- 
bably without an exception even in favour of the most cleanly 
people. 
ORDER III. PEpIPALpr.—In this family are the most formid- 
able of all the Avachuida—namely, the Scorpions. They are all 
Fig. 73.—Scorpion (reduced). 
distinguished by the fact that the abdomen is divided into dis- 
tinct segments, and is continued into the cephalothorax with- 
out any well-marked boundary or constriction. In the true 
