A REFLEX ACTION 



153 



cord, without leaving the cord at any point. Such cells and 

 fibers, known as co-ordinating cells, extend from somite to 

 somite, and give rise to co-ordinating impulses which cause the 

 entire worm to act as a single unit. 



The ventral cord and ganglia have other cells than those 

 mentioned, which may be grouped together as (i) afferent nerve 

 cells which receive impulses, (2) efferent nerve cells which trans- 



71. C 



in. 



FIG. 61. Portion of a transverse section of the ventral part of the body of 

 Lumbricus, to show the nerve connections, n.c., Ventral ganglion giving off a 

 lateral nerve l.n.; p.c., peritoneal endothelium; Lm., longitudinal muscles; hy, 

 hypodermis; in the nerve l.n. are sensory fibers proceeding inward from the 

 sensory cells (in black) of the hypodermis, and terminating in branching extremi- 

 ties; s, seta. (From Sedgwick and Wilson, after Lenhossek.) 



mit impulses to motor fibers, (3) co-ordinating cells which bring 

 about concerted action of the entire chain of somites, (4) giant 

 fibers, the functions of which are somewhat problematical, 

 but which may have both supporting and co-ordinating func- 

 tions, and (5) glia cells which form the matrix or main body of 

 the chain (Fig. 62). 



Each young nerve cell in development first forms an axial 

 process called the axon, which carries impulses away from the 

 cell. Other processes of the cell are termed dendrites, which 

 are shorter and more branched than the axon, and 'they receive 



