NERVOUS SYSTEM. 185 



habits, but is much more lengthened and vermiform in out- 

 ward shape, two of the minute longitudinal nervous filaments 

 extending backwards from the cesophageal ring are developed 

 only on one side of the body, like the abdominal nerves in 

 the helminthoid articulata. Thus, in all the typical forms of 

 this extensive class the nervous columns of motion and sen- 

 sation are seen to form a collar around the entrance of the 

 alimentary canal, which corresponds with the inverted posi- 

 tion of the mouth, and the expanded, globular, or radiated 

 form of the body around a short vertical axis. The length- 

 ening of the axis in the soft naked apodal vermiform species 

 of echinoderma necessitates a horizontal position of the 

 trunk, and the nerves, yet unprotected by an osseous sheath, 

 are, for safety, continued only along the inferior surface of 

 the body. This introduces the distinction of dorsal and 

 ventral surface of the trunk, unknown in the inferior tribes 

 of radiata, where the parts are equally developed around a 

 central axis, and where the nervous system presents the 

 same peripheral development. 



THIRD SECTION. 



Nervous System of the Diplo-Neurose or Articulated 

 Classes. 



In the long cylindrical trunks of the helminthoid and en- 

 tomoid classes, the nervous system partakes of the same 

 lengthened form, and its motor and sensitive columns are ex- 

 tended for protection along the ventral or under surface of 

 the body. This system is still contained in the same rings 

 or segments which envelop the other viscera of the trunk in 

 all the vermiform articulata, where the articulated members 

 are scarcely developed from the sides ; but in the highest 

 entomoid animals of this great division the nervous columns 

 are inclosed in a distinct thoracic osseous canal, separate from 

 that which contains the other organs. And in all these classes 

 the motor and sensitive columns appear to me to occupy the 

 same inverted position, with relation to the spino-cerebral 



