THE ARACHNIDA. 
427 
times before adult age and all its powers are perfected. The 
moulting may, therefore, be considered in the light of an incom- 
plete metamorphosis. A great deal of careful observation is still 
required before any decided assertions can be made satisfactorily 
concerning even the habitual moultings of all the spiders that do 
not undergo metamorphosis. Some genera do not even appear 
to moult, and the incompleteness and deficiency of any metamor- 
phoses have been stated in others, so that our present knowledge 
permits us to say that the Arachnida very closely allied by habits, 
instincts, and structural peculiarities, differ materially in their 
method of evolution after leaving the egg; moulting and metamor- 
phosis being apparent or absent in spiders which resemble each 
other very much. 
The relation of the metamorphoses to the classification of the 
spiders is, therefore, very interesting to the philosophical naturalist, 
but unsatisfactory to the species maker and to those who only 
classify. 
The Arachnida are divided into two great orders. In the first, 
which includes the true spiders and scorpions, the respiration is 
carried on by means of sac-like depressions in the body, and in the 
second the creatures breathe like the true insects and Myriapoda 
by means of tracheae. The true insects are thus connected with 
the second order of tracheary spiders by a similarity of construc- 
tion ; and the Crustacea or crab tribe, to be noticed hereafter, are 
allied to the pulmonary Arachnida with the respiratory sacs, and 
by the general arrangement of the segments of the body and limbs. 
The Arachnida thus form a link between the insects and the 
Crustacea. 
How the Arachnida are to be distinguished from these classes 
may be understood by examining their peculiar conformation. Thus, 
the Arachnida may be said to have the body divided into a chest 
or thorax and an abdomen. Wings are never present, and not 
even their rudiments. The legs are eight in number in the adult ; 
the eyes are smooth ; and the head is soldered into and continuous 
with the chest. 
If the My gale fodiens — which is represented in the middle of its 
curious nest in the accompanying engraving — be examined, it will 
be found to consist of a swollen abdomen, which is attached to the 
