290 
COMPARATIVE ZOOLOGY. 
union of a thousand or more of these delicate streams of 
silk. 150 These primary threads are drawn out and united 
by the hind legs. 
The mandibles are vertical, and end in a powerful hook, 
in the end of which opens a duct from a poison-gland in 
the head. The maxillae, or “ palpi,” which 
in Scorpions are changed to formidable claws, 
in Spiders resemble the thoracic feet, and are 
often mistaken for a fifth pair. The brain is 
of larger size, and the whole nervous system 
Fig. 261 .—spin- more concentrated than in the preceding or- 
der&!°?a?pai- der. There are generally eight simple eyes, 
piform organs. rare jy g j x> They breathe both by tracheae 
and lung-like sacs, from two to four in number, situated 
under the abdomen. All the species are carnivorous. 
The instincts of Spiders are of a high order. They are, 
perhaps, the most wily of Articulates. They display re¬ 
markable 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 tapes¬ 
try, and close the entrance with a lid which moves upon a 
hinge.’ 51 
Class III.— 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, antennae, mandibles, max¬ 
illae, and labium. They breathe by tracheae, and have two 
antennae and a variable number of eyes. 
