RELATIONS AND DIVISIONS OP THE CLASS IN8ECTA. 5 
louse are well-known members of this class, which has 
inhabitants of the sea, fresh water, and dry land. 
The Arachnida, which include the Spiders Scorpions, 
and Mites, are very closely allied to the Crustacea, and 
possess most of their characters, but differ in not being 
provided with branchiae, and in the fact that the number 
of locomotive limbs is only eight ; the body is com- 
posed of two distinct parts, the head (or cephalo-thorax) 
and abdomen, in some cases even these being joined so 
closely together as scarcely to admit of distinction. 
In others, as for example the Scorpions, the abdomen 
is composed of many rings, and the palpi are de- 
veloped so as to look like two additional legs. They 
are all without antennae and wingless, and do not 
undergo the complete metamorphoses of insects, being 
mostly hatched at once from the egg, and growing 
afterwards only in size ; they breathe either through 
internal air-gilis (pulmonary sacs), or by tracheae, or 
by both combined : the tracheae vary from two to eight 
in number, and open into spiracles (or breath-holes) 
on the lower part of the abdomen or sides of the head ; 
the covering of the body is mostly leathery (but harder 
in the Scorpions), and the eyes vary in number from 
two to eight, being placed in different positions 
on the head or cephalo-thorax ; the sexes are always 
distinct, as in the insects ; the spiders are all predaceous 
animals, and in most cases are furnished with special 
glands that secrete a viscid fluid, which hardens 
rapidly on exposure to the air, and which is cast into 
a proper thread-like shape by being passed through 
certain conical or cylindrical organs called “spin- 
nerets by means of this property they are enabled to 
construct webs for the capture of their prey. 
