1,52 PHILOSOPHY OF ZOOLOGY. 



ture of the nervous system, in the different genera of invcr- 

 tebral animals, there may still be perceived two models, ac- 

 cording to which, the organs belonging to it are arranged. 



In the first, the brain is situated upon the oesophagus. 

 It presents different forms, according to the species. It 

 appears more like a ganglion, than, the brain of the verte- 

 bral animals. It sends off several nerves to the mouth, 

 eyes, and feelers. One on each side passes round the oeso- 

 phagus ; these uniting below, form a ganglion ; in some cases 

 larger than what is considered the true brain. From this 

 ganglion, nerves are likewise sent off to different parts of 

 the body. The animals in which this nervous system pre- 

 vails, belong to the great division termed MOLLUSCA. 



In the second, the brain is situated as in the mollusca, 

 sends out nerves to the surrounding parts, and likewise one 

 nerve on each side, which, by their union, form a ganglion, 

 from which other nerves issue. This ganglion produces 

 likewise a nervous cord, which proceeds towards the ex- 

 tremity of the body, forming throughout its length ganglia, 

 from which small nerves proceed ; this cord, at its commence- 

 ment, is, in some cases, double for a short distance. It has 

 been compared to the medulla oblongata and spinal mar- 

 row of the vertebral animals. This kind of nervous system 

 is peculiar to the Annulose Animals. There are usually 

 ganglia on the nervous cord, corresponding with the num- 

 ber of rings of which the body consists. 



III. On the Nervous System considered in Action, 



This is a subject unquestionably the most interesting in 

 the whole range of zoological science. Yet it is still in- 

 volved in much obscurity, and will probably continue to 

 be so, unless new methods of observation shall be devised, 

 and more rigorous induction practised. 



