288 
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.j 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 46 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 
merged into one. They have 
. .... . no brain; only a single gan- 
Fig. 258. — A Mite (Demodex folliculo - e . o o 
rum), one of the lowest Arachnids; glion lodged in the abdomen, 
a parasite in human hair-sacs; X 125. m . . . . . 
I hey breathe by tracheae. I he 
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 
