628 



ARACHNIDA. 



The nervous system consists of a small brain, a group of 

 thoracic ganglia and a few abdominal ganglia, which, however. 

 are aborted in the spiders. The cerebral ganglia, or brain, lie 

 just above the oesophagus, and send down two cords embrac- 

 ing the throat, and also distribute nerves to the ocelli and 

 mouth-parts. 



In the mites (Acarina), where the body is oval, and not 

 divided into the two distinct regions, there is no brain, and 

 but a single ganglion lodged in the abdomen, from which are 

 distributed the nerves supplying the head and the peripheral 

 parts. In the spiders the brain is of considerable size, and the 

 thoracic ganglia or " suboesophageal ganglia," are large, send- 

 ing off on each side four large 

 processes from which proceed 

 the nerves supplying the feet. 



In the scorpion (Pedipalpi) 

 the nervous system is still more 

 highly organized. The brain is 

 not large ; it is composed of 

 the two spherical superoesopha- 

 geal ganglia fused together, 

 sending off the usual nerves to 

 the mouth-parts. This brain- 

 like organ is connected b}' two 

 filaments with the ventral gan- 

 glionic mass, formed b}' the 

 probable union of several gan- 

 glia, and situated in the middle 

 of the ftdse cephalothorax. The 

 continuation of the nervous cord consists of seven abdominal 

 ganglia, with the commissures united into a single cord. 



The maxillary palpi, functionally, take the place of antenna, 

 showing how one organ may perform the office of another in a 

 different group of animals. It is also evident that the spider 

 combines in the same organ the senses of taste, smell and feel- 

 ing, which are supposed in insects to reside in the two pairs of 

 palpi and the antennae. 



The alimentary canal is formed, according to Sicbold, on two 

 types. In the mites and spiders, the stomach is produced lat- 



Fig. 622. 



