280 COMPARATIVE ZOOLOGY. 
brain is of larger size, and the whole nervous system more 
concentrated than in the preceding order. There are gen- 
erally eight simple eyes, rarely six. They 
breathe both by trachee and lung-like sacs, 
from two to four in number, situated under 
the abdomen. All the species are carnivo- 
rous. 
Fia.253.—Spimer- ‘The instincts of Spiders are of a high 
fe eae order. They are, perhaps, the most wily 
eo of Articulates. They display remarkable 
skill and industry in the construction of their webs; and 
some species (called “ Mason Spiders”) even excavate a 
subterranean pit, line it with their silken tapestry, and 
close the entrance with a lid which moves upon a hinge.” 
Crass [V.—Myriapoda. 
Myriapods differ from Crustaceans and Spiders in hav- 
ing the thorax merged in the abdomen, while the head 
is free. In other words, the body is divided into similar 
segments, so that thorax and abdomen are scarcely distin- 
guishable. They resemble Worms in form and in the 
simplicity of their nervous and circulatory systems; but 
the skin is stiffened with chitine, and the legs (indefinite 
in number) are articulated. The legs resemble those of 
Insects, and the head appendages follow each other in the 
same order as in Insects—eyes, antennze, mandibles, max- 
illae, and palpi. They breathe by trachez, and have two 
antennee and a variable number of eyes. 
There are two orders: 
1. Chilognatha, having a eylindrical body, each segment 
furnished with two pairs of legs. They are of slow loco- 
motion, harmless, and vegetarian. The Thousand-legged 
Worm (/udus) is a common representative. 
2. Chilopoda, characterized by having a flattened body 
composed of about twenty segments, each carrying one 
