Chap, xii.] NERVOUS SYSTEM OF CRAYFISH. 413 



or that already mentioned ; from this there arise two 

 trunks which give ofi' nerve fibres to the anterior por- 

 tions of the digestive tract. "While the median 

 ganglia and nerves form the unpaired system, the two 

 lateral ganglia are the most anteriorly placed repre- 

 sentatives of a paired system of stomato - gastric 

 nerves and ganglia. 



Other nerve cords connected with the sympathetic 

 system supply especially the air tubes (tracheae), and 

 the muscles of their orifices (stigmata) ; from the fact 

 that the nerve which runs above the ventral gangli- 







omc chain gives off lateral branches which pass out- 

 wards, the system is known as that of the iiervi 

 transversi accessor!!. In the crayfish the ter- 



/ 



minal ganglion of the ventral chain gives off nerve 

 fibres which innervate the hinder portion of the 

 digestive tract. 



The function of the several parts of the nervous 

 system have been investigated in so few of the Inver- 

 tebrata, that it will be well to state at some length 

 what is definitely known as to the physiology of the 

 nervous system of the crayfish or the lobster. 



We note in the first place that the presence of a 

 comparatively large cerebral mass is associated with a 

 lar^e amount of influence over the rest of the ganglia ; 



. O O 



thus, the limbs, which in ordinary circumstances move 

 in due order in such a way as not to oppose, but rather 

 to assist one another, cease to exhibit this harmonious 

 activity when the cerebral ganglia are removed ; in 

 other words, they are no longer co-ordinated ; but this 

 is not all ; the cerebrum appears to be the centre of 

 what, in our ignorance of all the circumstances of the 

 case, we call spontaneous activity, and this is very 

 pointedly spoken to by the loss of power in the selec- 

 tion of food, which follows on a removal of the cerebral 

 centres. The separate condition of the cesophageal 

 commissures which unite the brain with the chain of 



