THE NERVOUS SYSTEM 



477 



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 established, 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 dorsal 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. 



-;- GANGLION CELLS. 



NERVE FIBERS. 



'EPITHELIUM. NmUSCLE. 



Fig. 394. — 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 (A) retains its original connexion with the skin. In 

 Allolobophora (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 anneUd (L>) and a vertebrate (£). 

 (Redrawn after Parker.) 



NERVOUS SYSTEM OF CHORDATES 

 Hemichordates. Hemichorda.tes have a dorsal tubular nerve cord 

 limited to the collar region, but a ventral invertebrate type of nerve 

 cord in the gill and trunk region. The tubular portion of the dorsal cord 

 remains open at both ends throughout life. It contains neurosensory 

 cells of a primitive type and some giant ganglion cells, is surrounded by 

 an outer fibrous layer as in vertebrates, and continues forward into the 

 proboscis and backwards into the body as a non-tubular strand of nervous 

 tissue. A circumesophageal ring connects this dorsal cord with a ventral 

 median strand. The ventral nerve strand is simply a local thickening 

 of a layer of nerve fibers which forms continuous network beneath the skin. 



