NERVOUS SYSTEM OF ARTICULATA. 3S'l 



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, aflbrds 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 oesophogcal, 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 2;ani2;lia as subordinate bi-ains. 

 This large ganglion, whicli supplies an abundance of ner- 

 . vous filaments to every part of the heacl, 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 

 th.eir 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 

 */innelida: but in the higher orders of Jirticulata 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 locust a, or Sand- 

 ^oo hopper, (Fig. 43S.) The central parts 



y^^M^j^fe^ of its nervous system are seen in Fig. 



f f^^S|^^ 439, wiiich 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 II. M. Ed- 

 wards, on this subject, given in the Ann. dcs Sc. Nat. xix. 181. 



