ARACHNIDA. 149 
CE ASP TER. Xv; 
ARACHNIDA. 
Crass II, ARACHNIDA.—This class includes the mites, ticks, 
scorpions, and spiders, and, as a whole, is very nearly related 
to the preceding. The Arachnida, however, are distinguished 
from the Crustacea by being adapted in most cases for a strictly 
terrestrial life, so that when any distinct breathing-organs are 
present these are never in the form of gills, but are always either 
pulmonary sacs or air-tubes (¢vachee@). In none of the Arachnida, 
further, are there ever more than four pairs of legs, and ¢he seg- 
ments of the abdomen never carry limbs of any sort. The eyes 
are always sessile, and never supported upon stalks; if antenne 
exist at all, they are much modified, and the head is always 
amalgamated with the thorax, so as to form a cephalothorax. 
The integument usually produces chitine more or less abun- 
dantly, so as to constitute a resistant shell; but in some cases 
the skin remains permanently soft. The mouth is situated in 
the anterior portion of the body, and in the higher forms is fur- 
nished with a pair of prehensile jaws, called ‘“ mandibles,” a 
pair of chewing-jaws, called “maxillz,” and a lower lip. In 
the scorpions an upper lip is present as well. In the true 
spiders each mandible terminates in a sharp movable hook 
(fig. 71, 4), perforated by a canal which communicates with a 
poison-gland situated near its base. By means of this poisonous 
fluid the spiders kill such animals as they capture. In the 
scorpions (fig. 71, 1) the mandibles are short, and terminate 
in strong pincers. In them, too, the maxille are furnished 
with enormously-developed nipping-claws or chele.* In all 
the Arachnida the mandibles are believed to correspond to 
* These nipping-claws in the scorpions are produced not by the maxillz them- 
selves, but by two appendages to the maxilla, which are known as ‘‘ maxillary 
palpi.” 
