44 ' EVOLUTION OR 7۳ 
dermata from the Soft Worms, with which they are most 
closely allied, is in harmony with the views of the most 
eminent naturalists of the present day. 
ARTICULATA. 
We turn now to a consideration of the Articulata, so 
called from their bodies being composed of distinct pieces 
jointed or articulated together. They include the Centi- 
pedes, known as the Myriapoda, from their numerous feet, 
the Spiders, or Arachnida, with eight feet, and the Insects, 
which have only six feet. We will pass over for the 
present the Crustacea, or last order of Articulata, and con- 
fine ourselves for the moment to the Centipedes, Insects, 
and Spiders (Figs. 46, 48, 49), which agree in breathing by 
means of fine tubes opening externally. These tubes pass 
from the walls of the body, getting smaller and smaller, until 
they are lost in a net-work. In the inside of these tubes, 
arranged in the form of a spiral, is found a delicate wire, 
which serves to keep the tubes expanded. This respiratory 
system is known as the Tracheate: hence the Centipedes, 
Insects, and Spiders are joined in one division, the Trache- 
ata. A Myriapod, or Centipede (Fig. 46), is composed of 
numerous segments resembling a Nereid; in fact, it has 
been observed that the Myriapoda are to the land what 
the Annelida are to the water. In the Insect (Fig. 48) we 
can distinguish only three segments, known as head, thorax, 
and abdomen. There are seen usually in Insects two pairs 
of wings, less often one pair, and in some cases none are 
apparent. In the Spider (Fig. 49) the head and breast are 
soldered into one piece, known as cephalo-thorax. So in 
the Arachnida we find only two segments. While the 
Myriapoda, Insecta, and Arachnida breathe by trachee, 
the Crustacea, including the Crabs, Lobsters, etc., breathe 
by gills. They live in the water, and are of every size, 
