GENERALITIES CONCERNING ARTICULATES. 



129 



FIG. 



stances, which are essentially dissimilar. The substance 

 that forms the skeleton of the verte- 

 brates is bone, while that which makes 

 up the skeleton of the articulates is a 

 rigid matter called chitin, that in most 

 cases becomes incrusted with carbonate 

 of lime. The skeleton of the vertebrates 

 is said to be bony, while the covering of 

 the articulates, and in general that of all 

 invertebrates, is chitinous, 



The articulates possess a distinct diges- 

 tive apparatus, as well as apparatus of 

 circulation and of respiration ; the latter, 

 however, are much simplified. The study 

 of the peculiarities of these apparatus 

 we will take up as we study the types 

 selected for each of the classes of artic- 

 ulates. 



In all articulates the nervous sys- 

 tem has about the same organization, 

 but this is so different from that with 

 which we have become familiar in the 

 vertebrates that it is entirely character- 

 istic. It is composed of a series of gan- 

 glions masses, joined together in a long 

 chain by nerve-filaments or commissures, 

 The largest of these masses are situated 

 in the head, and form what is called the 

 brain or cephalic ganglion. Behind and 

 under the oesophagus we find another 

 ganglion, called the subcesophageal, joined 

 to the cephalic ganglion by two commissures that circle 

 around the oesophagus: altogether this constitutes the 



IV, nervous system 

 of a myriapod; 

 V, nervous sys- 

 tem of a caterpil- 

 lar. The subin- 

 testinal ganglia 

 are far apart, and 

 show distinctly 

 the chains to 

 which they be- 

 long. 



