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COMPARATIVE ZOOLOGY. 
Class III.— Arachnida. 
The Arachnids are closely related to the Crustaceans, 
having the body divided into a cephalo-thorax and abdo¬ 
men. 148 To the former are attached eight legs of seven 
joints each; the latter has no locomotive appendages. 
The head carries two, six, or eight eyes, smooth and ses¬ 
sile {i. e., not faceted and stalked, as in the Lobster), and 
approaching the eye of the Vertebrates in the complete¬ 
ness and perfection of their apparatus. The antennae, if 
present, are only two, and these are not “ feelers,” but 
modified to serve for the prehension of food. 149 They are 
all air-breathers, having spiracles which open either into 
air-sacs or tracheae. The young of the higher forms un¬ 
dergo no metamorphosis after leaving the egg. 
Arachnids number nearly five thousand species. The 
typical forms may be divided into three groups: 
1. Acarina , represented by the Mites and Ticks. They 
have an oval or rounded body, without any marked artic¬ 
ulations, the head, thorax, and. 
abdomen being apparently 
merged into one. They have 
Pig. 25S.— A Mite (Demodex folliculo- U ° ^ ra ^ n ? 0I1 ty a single gail- 
rum ), one of the lowest Arachnids; ghoil lodged in the abdomen, 
a parasite in human hair-sacs; X 125. mi , , , 
They breathe by tracheae. The 
mouth is formed for suction, and they are generally para¬ 
sitic. The Mites ( Acarus ) are among the lowest of Ar¬ 
ticulates. The body is soft and minute. The Ticks 
{Ixodes) have a leathery skin, and are sometimes half an 
inch long. The mouth is furnished with a beak for pierc¬ 
ing the animal it infests. 
2. Pedijpalpi , or Scorpions, characterized by very large 
maxillary palpi ending in forceps, and a prolonged, joint¬ 
ed abdomen. The nervous and circulatory systems are 
more highly organized than those of Spiders; but the 
