ANNULOSA. 529 



marked, however, in the middle by a groove. These lobes 

 furnish the optic nerves, and frequently send filaments to 

 the mouth. Near the posterior edge, two cords arise, 

 which, after proceeding backwards, and, in many cases, 

 embracing the gullet, unite to form a ganglion, from which 

 nervous filaments proceed to the neighbouring parts. From 

 this ganglion two cords again proceed and fonn a second 

 ganglion, and the same process is repeated until the cords 

 reach the anal extremity, where they terminate. The num- 

 ber of ganglia which are formed, differ in different genera, 

 according to the number of articulations of the body. The 

 cords, in some species, appear to unite and exhibit only the 

 appearance of one. 



Of the organs of the senses, the eye alone is the most 

 perfect ; and next to it may be classed the organ of touch. 

 The existence of the other senses common to the vertebral 

 animals, is rather inferred, in many cases, from the actions 

 performed than from the structure of their parts. 



The organs of vision are of two kinds, simple and com- 

 pound eyes. The simple eyes are usually in the form of 

 small black circular dots, three in number, situate on 

 the crown of the head towards the neck. They are de- 

 nominated ocelli or stemmata. They are not present in 

 many tribes. The compound eyes are present in all insects 

 and many Crustacea. They are in general two in number, 

 and situate one on each side of the head. In the Gyri- 

 nus, however, each eye appears to be divided by the mar- 

 ginal band of the head, so that there is one eye above and 

 another below. Similar appearances may be observed 

 in the Geotrupes stercorarius. In some Crustacea they are 

 united. In some cases the compound eyes are imbedded in 

 a cavity of the head ; in others they are continuous with 

 its surface, or elevated on a fixed or articulated process 



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