NEEYOUS SYSTEM AND GENEEAL SENSATION. 



55 



of the animal series. The number of the ganglia in the simpler 



forms of the articulate, corresponds in general to the number of 



the rings of the body: but in the 



higher groups there is often a 



fusion of two or more ganglia 



into one. This change is well 



exemplified in the development 



of insects, spiders, and crus- 

 taceans : the spinal cord of 



the articulate, like that of the 



vertebrate, is composed of 



motory and sensitive columns. 



In insects, a special nervous 



system, the sympathetic, is dis- 

 tributed to the organs of vege- 

 tative life. The annexed figure 



(34) shows the distribution of 



the cerebro-spinal system in a 



beetle, Carabus nemoralis. 

 [ 116. In the moUusca,the 



principal centre of the nervous 



system surrounds the gullet, in 



the form of a gangliated collar; 



but it exhibits many phases of development in the different 

 classes of this sub-kingdom. In the CONCHIFEEA, which are 

 acephalous, as the mussel (Mytilus edulis), distinct organs exist 

 for the ingestion of the food, respiration, and locomotion, and 

 each of these possesses ganglia, in immediate relation with the 

 function over which it presides. Hence we find 



1st. JEsophageal ganglia, which surround the gullet, and re- 

 present the brain. These nerves proceed to the labial pro* 

 cesses, that serve for taste and touch. 



2nd. Branchial ganglia presiding over the respiratory func- 

 tion. From these ganglia, likewise, the muscles concerned in 

 the act of respiration, the adductors of the shell, the folds of the 

 mantle, and the intestine are supplied. 



3rd. Pedal ganglia vary with the presence or absence of a 

 foot for locomotion. The whole of these ganglia are united 

 into a nervous chain by connecting filaments. 



In the GASTEEOPODA we observe a further development o< 

 the nervous system. They possess a head; and the brain 



Fig 34. The nervous system of 

 Carabus nemoralis, a garden beetle. 

 The cephalic ganglia supply nerves 

 to the eyes, antennae, parts of the 

 mouth, &c. ; the thoracic ganglia 

 supply nerves to the thorax, the three 

 pairs of legs and the wings ; the ab- 

 dominal ganglia send branches to the 

 organs contained in the abdomen. 



