102 GENERAL STRUCTURE OF ARTICULATA. | 
mens of these divisions ; and it will become more apparent 
as we proceed. 
93. In the second division, that of ArTIcunATA, or Articu- 
lated (jointed) animals, we find a conformation very different 
from that which has been just described. The 
exterior of the body is. still perfectly symme- 
trical, as in the Vertebrata ; and the interior is 
even more symmetrical ; for the organs that 
represent the heart and lungs are equally dis- 
posed on the two sides of the central line of the 
body. But the skeleton, instead of being internal, 
is external ; and is composed of a series of pieces 
jointed together, which form a casing that in- 
cludes the whole body. In general, these pieces 
are very similar to each other ; so that the whole 
body appears like the repetition of a number of 
similar parts, as we see in the Centipede (fig. 42). 
- The limbs are usually very numerous, where 
they exist at all; and they have a jointed cover- 
ing, like that of the body. But in the lower 
tribes of this group, such as Leeches and Worms, 
the limbs or members are but slightly developed, 
or are altogether absent; and in the highest, 
which approach most nearly to the Vertebrata 
in their general organization, the number of 
members is much reduced,—although it is never 
less than six. The hard matter of which the 
“Saag external skeleton is composed, undergoes little 
* or no change when it is once fully formed ; and, 
in order to accommodate it to the increasing size of the 
animal, this covering is thrown off and renewed at intervals 
during the period of growth. 
94, The nervous system consists of a series of separate 
ganglia, which are arranged in a cord or chain along the 
central line of the body. There is usually a pair of large 
ganglia in the head, bearing a resemblance (in their peculiar 
connexion with the eyes) to the ganglionic centres of the 
optic nerves in Vertebrata; and there is commonly one for 
each segment or division of the body, from which the nerves 
pass to supply its muscles, as they do from the spinal cord of 
Vertebrata. The cord which connects these ganglia is double, 
