152 NATURAL HISTORY. [NEW BUILDING. 
raiists to believe that they do not undergo any meta¬ 
morphosis. 
The Arachnida (Cases 11 and 12) differ from the true 
insects in the head and thorax being united into a single 
mass, as well as in having more numerous legs and no wings. 
Some have pulmonary sacs, which fulfil the functions of 
lungs, with a heart, veins, and very distinct vessels ; these 
have six or eight simple eyes, and from the manner in 
which they respire, are called Pulmonaricje , as the various 
kinds of true Spiders. By the side of the kind of Spider 
that forms it, is placed the tubular case of one of the earth 
spiders, which forms a door to its tube that fails down 
to close the cavity when the animal is within. The other 
Arachnida respire by trachese, or air tubes, which traverse 
their body, as is the case with insects in general. The 
organs of circulation are wanting or very incomplete. 
They have two, or at most but four, simple eyes, and have 
been called Trachearice . Among these are the Tarantulw , 
Scorpions, &c. 
The Myriapodes, or Centipedes, are so called on ac¬ 
count of the number of their legs. They have gene¬ 
rally twenty-four or more, arranged along the entire 
length of the body, on a series of rings, each bearing one 
or two pairs. The first, and in several even the second 
pair of legs seem to form a part of the mouth. 
These animals bear some resemblance to little snakes 
and Nereides, from their feet being so closely applied to 
each other through the whole extent of the body. The 
number of the rings and the feet increases with age, a 
character which distinguishes them from the true Insects, 
which always have the same number of rings from their 
being first hatched to their adult state ; all the true legs, 
which are furnished with hooks, are developed at once, 
either when they are hatched or when they pass into the 
Pupa state, while the Myriapodes , when they leave the 
egg, are destitute of these organs. The Myriapodes live 
and increase in size longer than most other terrestrial 
Annulosa, for according to Savi, they require two years 
before the reproductive organs are developed. They are 
divided into two orders, which have very distinct external 
characters. 
