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 ARTICULATA, 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 Yertebrata 

 in their general organization, the number of 

 members is much reduced, although it is never 

 less than six. The hard matter of which the 

 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 

 Yertebrata. The cord which connects these ganglia is double, 



