NERVOUS SYSTEM OP ARTICULATA. 381 



In this division of the animal kingdom, the primary ner- 

 vous cords always pass along the middle of the lower sur- 

 face of the body, this being the situation which, in the ab- 

 sence of a vertebral bony column, affords them the best pro- 

 tection. They may be considered as analogous to the spi- 

 nal marrow, and as serving to unite the series of ganglia^ 

 through which they pass, into one connected system. On 

 arriving at the oesophagus, they form round it a circle, or 

 collar, studded with ganglia, of which the uppermost, or that 

 nearest the head, is generally of greater size than the rest, 

 and is termed the cesophageal, cephalic, or cerebral gan- 

 glion, being usually regarded as analogous to the brain of 

 larger animals. Perhaps a more correct view of its func- 

 tions would be conveyed by calling it the principal brain, 

 and considering the other ganglia as subordinate brains; 

 This large ganglion, which supplies an abundance of ner- 

 vous 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 Jlrticulata, we find 

 it exhibiting various degrees of concentration. The pro- 

 gress of this concentration is most distinctly traced in the 

 Crustacea* One of the simplest forms of these organs oc- 

 curs in a little animal of this class, which is often found in 

 immense numbers, spread over tracts of sand on the sea 

 shore, and which is called the Talitrus locusta, or Sand- 

 hopper, (Fig. 438.) The central parts 

 of its nervous system are seen in Fig. 

 439, which represents the abdominal 

 side of this animal laid open, and mag- 

 nified to twice the natural size. The two primary nervous- 



* See the account of the researches of Victor Audouin, and H.. &L Ed- 

 wards, on this subject, given in the Ann. des Sc. Nat. xlx. 181.. 



