THE NERVOUS SYSTEM 



347 



The nerve cord of the primitive annelids consists of a chain of paired 

 ganglia Hnked together both by longitudinal connectives and by trans- 

 verse commissures. The longitudinal connectives pass around the eso- 

 phagus to connect the supra- and sub-esophageal ganglia. In the higher 

 annehds the paired ganglia tend to unite in the mid-ventral line and to 

 lose the primitive rope-ladder arrangement. Concentration and fusion 

 greatly reduce the number of ganglia, especially in arthropods. (Fig. 308 ) 



These profound changes in the form of the nervous system of articu- 

 lates are accompanied by histological and physiological differences. Most 

 of the nerve cells become definitely polarized to transmit impulses in one 

 direction only, either towards or away from the central nerve cord. Thus 



; GANGLION CELLS. 



^EPITHELIUM 

 MOTOR NEURONE. 



NERVE FIBERS. 



/SENSORY NEURONE 



GANGLION 

 SENSORY /CELL 



NEURONE 



~ MUSCLE . 

 'EPITHELIUM. ^MUSCLE. 



Fig. 308. — Invertebrate (annelid) and vertebrate nervous systems compared. In 

 both sub-kingdoms the nervous system is derived from the ectoderm. The central 

 nervous system of Sigalion (.4) retains its original connexion with the skin. In AUo- 

 lobophora (B), however, as in most annelids and in vertebrates (C) the nerve cord 

 separates from the skin. In figure C the vertebrate spinal cord is shown in reversed 

 position with dorsal side down. In A, B, and C the axon processes of the ganglion 

 cells within the nerve cord are similarly directed away from the surface. D and E 

 show the relations of afferent and efferent neurons in an annelid (D) and a vertebrate 

 (E). (Redrawn after Parker.) 



the neurons become in all essentials like those of vertebrates, being 

 differentiated either as sensory or motor, while those within the nerve cord 

 become association cells. A reflex-arc pattern is thus estabUshed in 

 which the motor and sensory cells involved in the reflex may be either 

 homolateral or heterolateral according as the neurons involved belong to 

 one or to both sides of the body. In insects separate motor and sensory 

 nerve roots appear, reversed in position as compared with vertebrates. 

 Motor roots are dorsal and the sensory roots ventral, while in vertebrates 

 motor roots are ventral and sensory roots dorsal. It will be noted, how- 

 ever, that if the insect were turned over on its back so as to bring the nerve 

 cord on the upper side of the body as in vertebrates, the relations of the 

 nerves would be similar to those of vertebrates. Giant ganglion cells and 

 fibers, resembling those of fishes, appear in the nerve cord of annelids. 



