XATURE OF THE INVERTEBRATE BRAIN. 



711 



The sub-cesophageal ganglia correspond in the main, as we have 

 already stated, with the medulla ohlongata of vertebrate animals, 

 and their fusion with the thoracic ganglia in the Arachnida, as well 

 as in the Crustacea and Myriapoda, confirms the view held by some 

 anatomists, that the medulla should be regarded as a prolongation of 

 the spinal cord, rather than as an integral j)art of the brain. 



Fig. 6. Nervous System of an Insect (Acrida viridissima). 



The nervous system of insects varies not only among different 

 classes and orders, but even in the same individual, in different stages 

 of its development. The larva, or caterpillar, of a butterfly, for in- 

 stance, presents a nervous system not very different from that met 

 with in the centipede; while in the imago stage, or perfected insect, 

 the same system has undergone some remarkable changes, leading to 



