98 NERVOUS SYSTEM OF ARTHROPODS. 



many naturalists really to embody three pairs of ganglia, 

 in relation with three pairs of sensory organs, viz., eyes, 

 tactile antennae, and the supposed olfactory antennules. 



The brain is connected, by means of a long cord ou 

 each side (h, h) of the oesophagus, with the anterior ex- 

 tremity of the great ventral ganglion. Nerves in relation 

 with the organs of mastication join the cords about mid- 

 w^ay between the brain and the great abdominal ganglion, 

 and small ganglia are to be found in this situation. Just 

 behind these small ganglia a transverse commissure con- 

 nects the cords with one another. The unusual length of 

 the oesophageal cords is one of the most notable character- 

 istics of the nervous system of the higher Crustacea, and 

 this seems due in part to the fact that the sub- oesophageal 

 ganglia remain separate instead of uniting with one 

 another, as they do in fig. 18. 



The ' stomato-gastric ' system of Crustacea is very 

 similar to that which exists in Centipedes. One part of it 

 is given off from the oesophageal cord on each side, while 

 another median branch proceeds from the posterior part of 

 the united cephalic ganglia, as in lulus (fig. 33, /). Where 

 the main nerve lies on the upper surface of the stomach, 

 in the higher Crustacea, it is connected with one or 

 two ganglia from which branches pass to the walls of this 

 organ. They send filaments also to the right and left, 

 into the liver. This principal visceral nerve is brought 

 into communication with the above-mentioned nerves, 

 going to the organs of mastication, by means of two 

 filaments which join the ganglionic swellings on the 

 oesophageal cord at the part whence they issue. 



Among Arachnida forms of the nervous system exist 

 which agree in many respects with those belonging to 

 members of the class last described — especially where 



