NERVOUS SYSTEM OF INVERTEBRATES. 



87 



of vertebrates. The oesophageal ganglion presides over 

 the gathering and mastication of food, and, judging 

 from experiments on crustaceans, it seems also to co- 

 ordinate the motions of the „-„ 

 whole body. It may be said ....„ § f^f, 

 to correspond in function V - y '"-0 1 ~-'&L- / '-~-2 

 to the cerebellum of verte- 



L--\ 



Fig. 58. — Diagram of nervous sys- 

 tem of a leech, seen from above. 



Fig. 59. — Nervous system of a cray- 

 fish, seen from above. 



brates. The other ganglia preside each over its own 

 body-segment and corresponding limbs automatically, 

 but all under the conscio-voluntary control of the oeso- 

 phageal collar, and especially of the cephalic ganglion. 

 Modifications going down and up the Scale.— 

 Taking the above as the type, as we go down the scale 

 of articulates, the cephalic ganglion becomes smaller in 

 comparison with the others, and loses more and more its 

 dominance over them. The other ganglia become 

 more and more independent, and the movements more 

 and more automatic or reflex, until finally, in the lowest 



