NERVOUS SYSTEM OF AUTICULATA. 487 



In this division of the animal kingdom, the pri- 

 mary nervous cords always pass along the middle 

 of the lower surface of the body, this being the 

 situation which, in the absence of a vertebral bony 

 column, affords them the best protection. They 

 may be considered as analogous to the spinal cord 

 of the vertebrata, and as serving to unite the series 

 of ganglia through which they pass into one con- 

 nected system. They form round the oesophagus 

 a circle, or collar, studded with ganglia, of which 

 the uppermost, or that nearest the head, is termed 

 the supra-oesophageal, cephalic, or cerebral ganglion ; 

 being regarded as the immediate organ of the senso- 

 rial faculties, and consequently as being analogous to 

 the brain of larger animals. This ganglion, which 

 supplies an abundance of nervous filaments to 

 every part of the head, seems to be the chief organ 

 of the higher senses of vision, of hearing, of taste, 

 and of smell, and to be instrumental in combining 

 their impressions, so as to constitute an individual 

 percipient animal, endowed with those active 

 powers which are suited to its rank in the scale 

 of being. 



Such is the general form of the nervous system 

 in all the Annelida; but in the higher orders of 

 Articulata we find it exhibiting various degrees of 

 concentration. The progress of this concentration 

 is most distinctly traced in the Crustacea* One of 

 the simplest forms of these organs occurs in a little 

 animal of this class, which is often found in im- 

 mense numbers, spread over tracts of sand on the 



* See the account of the researches of Victor Audouin, and Milne 

 Edwards, on this subject, given in the Annales des Sciences Natu- 

 relles ; xix. 181. 



