CHARACTERS DRAWN FROM NERVOUS SYSTEM. 



75 



includes. But it will be advantageous here to show, how they 

 may be distinguished by the examination of a single portion 

 only of their structure j because we shall be thus led to see, how 

 single and easily-recognised charasters become of great importance 

 in classification, provided they are found to be constantly associ- 

 ated with certain general types or plans of structure. A little 

 consideration will show, that the Nervous System, being more 

 or less concerned with all the functions of the body, but particu- 

 larly with those of animal life, will probably exhibit, in the 

 arrangements of its parts, very distinctive peculiarities in the 

 four sub-kingdoms ; and this is, in fact, the case. 



53. In the Protozoa, as we have seen, no trace of a nervpus sys- 

 tem can be discovered. This is also the case with many of the Radi- 

 ata, but in those members of this division in which it can be traced, 

 it is arranged on the same plan with the other organs ; its centres 

 being disposed in a circle around the mouth, and connected by a 

 cord that forms a complete ring, as shown in Fig. 33. Each of these 



Fio. 33. NERVOUS SYSTEM 

 of STAR-FISH. 



Fro. 34. NERVOUS SYSTEM OF AN INSECT. 



centres or ganglia supplies that part of the body, with which 

 it is immediately connected ; and seems to have precisely the 

 same functions with the rest. In the Articulated animals, we 

 find a series of ganglia arranged along the central line of the 

 body, and connected by a cord, which is always either evidently 

 double (as in Fig. 34), or which may be easily separated into 

 two equal tracts, one belonging to either side. And at the front 

 extremity of this cord, situated in the head, there is a pair of 

 ganglia larger than the rest, connected with the organs of special 



