LESSON 70.] NERVOUS SYSTEM IN ARACHNIDA. 



251 



the ganglia are remarkably symmetrical, with the exception of the 

 brain, each having to supply precisely the same nerves to the like 

 organic structures. 



In the perfect insect, however, it is found that great absorption 

 has occurred of ganglions no longer required, and Fm S72 



a corresponding deposition of nervous matter, to 

 increase the size of certain ganglia whose influ- 

 ence is imperative. In illustration of this fact, 

 the nervous system (in situ, Fig. 372), of the 

 natural size, is offered from the Blatta Ameri- 

 cana (American Cockroach). Here the abdomi- 

 nal ganglia (d) are minimized, and the thoracic 

 ganglia are largely but unequally developed. 



1134. From these nervous centres it is that 

 the large and powerful superior (wings) and in- 

 ferior locomotive organs (legs) obtain the nervous 

 influence necessary to the performance of their 

 functions. The first thoracic ganglion (a) having 

 only to distribute nerves to the smallest pair of 

 legs, is the least in size ; the second thoracic 

 ganglion (b) has to supply not only the interme- 

 diate pair of legs, which are larger, but the superior organs, which, 

 whilst they afford protection to the true wings, are also engaged to a 

 limited extent in the action of flight. 



The third, and last thoracic ganglion (c), is required to give im- 

 pulsion to the last and very powerful legs, at the same time that it 

 does the like to the real organs of flight the wings. Here, there- 

 fore, we see a greatly increased size in the nervous centre, especially 

 devoted to this function. The abdominal ganglia (d) supply the 

 muscles of the abdomen only, and are minute and uniform in size. 



Nervous system of the 



natural size of B. 



Americana. 



LESSON LXXYI. 



NEEVOUS SYSTEM IN AKACHNIDA. 



1135. The nervous system in one class of articulate animals yet 

 remains to be examined, the Arachnida. The class contains the 

 Mites, Scorpions, and Spiders, and has usually been placed by natu- 

 ralists below the Crustacea, and between it and Insects. 



