84 



ZOOLOGY. 



Vertebrates of brain and spinal cord, lies in Arthropods' 

 almost entirely on the ventral side. In the head i^ 

 situated the cerebral ganglion, a large nervous mass 

 resting on the gullet, and giving oflf nerves to the eyes 

 and feelers. Besides this, there is a ventral nerve- 



FiG. 65. — The Large Centipede (^Scolopendra ttuyrsitans). 



cord on the ventral side of the animal, running below 

 the gut, and made up of several pairs of ganglia, 

 united with one another by means of nerve-fibres. 

 The ganglia of the ventral cord send their nerves to 

 the jaws, limbs, muscles, viscera, etc. The cerebral 

 ganglion is connected with the first ventral ganglion 

 by means of a cord on each side of the gullet (Fig. 56), 

 so that a ring-like structure is 

 formed. The breathing organs 

 are gills in some Arthropods 

 (the Crustacea, e.g. crayfish, 

 crabs); insects and centipedes 

 breathe by air-tubes or tracheae, 

 while the respiratory organs of 

 spider-like animals, when these 

 do not breathe entirely by 

 means of the skin, are more or 

 less strongly modified tracheae. 

 The structure of the tracheal 

 system is generally as follows : 

 on each side of the body there 

 is a row of breathing-holes, or 

 stigmata, through which the air can enter the trachese ; 

 these are very much branched, so that they finally 

 become very fine tubes investing the various organs, 



Fig. B6.— Ground Beetle, with 

 nervous system drawn in white. 



