ARTICULATA. QT 
Crass II].—Arachnida. 
The Arachnids are closely related to the Crustaceans, 
having the body divided into a cephalo-thorax and abdo- 
men.” To the former are attached eight legs of sey- 
en joints each; the latter has no locomotive appendages. 
The head carries two, six, or eight eyes, smooth and sessile 
(7. é., not faceted and stalked, as in the Lobster), and ap- 
proaching the eye of the Vertebrates in the completeness 
and perfection of their apparatus. The antenns, if pres- 
ent, are only two, and these are not “ feelers,” but modi- 
fied to serve for the prehension of food. They are all air- 
breathers, having spiracles which open either into air-sacs 
or trachee. The young of the higher forms undergo no 
metamorphosis after leaving the ege. 
Arachnids number nearly 5000 species. The typical 
forms are 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 no brain, only a single gan- 
elion lodged in the abdomen. 
They breathe by trachee. 
The mouth is formed for 
suction, and they ihoh oie Fie. 250.—A Mite (Demodex folliculo- 
ally parasitic. The Mites — rum), one of the lowest Arachnids ; 
iS a parasite on human hair; X 125. 
(Acarus) are among the low- 
est of Articulates. The body is soft and minute. The 
Ticks (Jvodes) have a leathery skin, and are sometimes 
half an inch long. The mouth is furnished with a beak 
for piercing the animal it infests. 
2. Pedipalpi, 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 
