NERVOUS SYSTEM OF MOLLUSCA. 



The nervous system consists of little knots, or gan- 

 glia, variously distributed, but connected together by 

 nervous threads, a large central 

 ganglion being situated on the 

 gullet, or oesophagus, near the 

 stomach, and to it the rest ap- 

 pear subordinate. This chief 

 ganglion has been termed the 

 brain ; a term, however, by no 

 means appropriate. With I'e- 

 spect to the senses, that of feel- 

 ing is at a low ebb, though in 

 most the irritability of the sur- 

 face, or muscular tissue, is ex- 

 treme, and continues long even 

 after the animals have been di- 

 vided. Some possess organs of 

 taste and sight; in others they are not to be discovered. 

 One family only, that of the cephalopods, (of which the 

 common cuttle-fish is one,) is provided with organs of 

 hearing ; but these animals, it may be observed, rise 

 so high in structure above the ordinary mollusca, that 

 it is doubtful whether naturalists are not warranted in 

 removing them altogether from that sub-kingdom. 



The blood of the mollusca is white, or bluish, and 



I a a a, ganglia, or knots, 

 joined by nervous tiireads. 



