ARACHNIDA, 151 
pulmonary sacs. The trachez are essentially similar in struc- 
ture and function to the breathing-tubes of the /yrzapoda 
and Jusecta, and consist of tubes, which open on the surface of 
the body by distinct apertures called “‘spiracles,” and ramify 
through the tissues internally. The walls of the tube are pre- 
vented from collapsing by means of a spirally-coiled thread or 
filament of chitine, which is wound round their walls within 
their inner lining. ‘The pulmonary sacs which occur in the 
Arachnida are simple chambers formed by an inversion of the 
skin, which constitutes a number of closely-set plates or folds. 
The whole of the interior of the pulmonary sacs is richly sup- 
plied with blood, and air is admitted by means of minute ex- 
ternal openings (fig. 71, 3). 
The nervous system is of the regular articulate type, but the 
ganglia of the ventral chain are often massed together in par- 
ticular situations. In no case are compound eyes present; and 
when distinct organs of vision exist, these are in the form of 
from two to eight or more simple eyes. 
ORDERS OF THE ARACHNIDA. 
ORDER I. PoposomatTa.—In this order are included the 
“ Sea-spiders,” which are wholly marine, and were long be- 
lieved to be referable to the Crustacea on this account. As they 
have no respiratory organs of any kind, the question cannot be 
definitely settled, but they have no more than four pairs of legs, 
and would therefore seem to be properly referable to the Arach- 
nida. In some forms the legs attain an extraordinary length, 
and contain prolongations from the stomach. They are all 
grotesque-looking animals, found at low water upon stones or 
marine plants, or parasitically attached to marine animals. One 
of the commonest forms is figured below (fig. 72, a). ° 
ORDER II. Acartna.—The most familiar members of this 
order are the Mites and Ticks (fig. 72, 4, c). They are distin- 
suished by the fact that the abdomen is amalgamated with the 
cephalothorax to form a single mass. Respiration is effected 
by the general surface of the body or by air-tubes (trachez). 
The habits of the mites are extremely varied. Some are 
found upon different plants (fig. 72, 4); others are parasitic upon 
water-insects when young, but swim about freely when adult 
