THE CCELOMATA INVERTEBRATA 187 



the ganglionic enlargements than in the connectives, although they 

 are also present in these. Their axon leaves the cord as a non- 

 medullated afferent or motor nerve fibre, andruns outwards, frequently 

 terminating in a muscle. They may, however be acceleratory, 

 inhibitory, or excitatory in function when they are distributed to 

 other parts than the actual muscle elements. 2. The commissural 

 or association neurons are only to be found in the cord itself and are 

 bipolar. The axon and single dendron are very similar to look at, 

 and they function in placing the various other nerve elements in 

 communication with one another. A sense-bud, which causes a 

 .slight projection on the surface of the epidermis, is composed of a 

 number of elongated rod-like sensory cells. Each cell bears at its 



,s.c. 



M.F 



N.C. . 



FIG. 59. Diagram to show the relations of the nerve cells in Lumbricus, 

 adapted and modified from Lenhossek and Retzius. 



A.N., association neuron ; C., cuticle ; E., epidermis ; M.F., motor fibre ; M.N., motor neuron ; 

 N., nerve ; N.C., nerve cord ; S.C., sensory cell or sensory neuron ; S.F., sensory fibre ; S.P., 

 sensory process. 



external end a short stiff sensory or receptive process, and the cuticle 

 overlying it is pierced with a small hole to allow it to project freely 

 to the exterior. At its inner end the sensory cell is continued on as 

 an afferent or sensory fibre, whose terminal dendrite arborises in 

 connection with the association neurons. The whole structure is a 

 typical neuro-epithelial cell, whose receptive process corresponds with 

 the dendron and the fibre with the axon, and is comparable with the 

 olfactory cell in the frog. It is also probable that certain sensory 

 neurons are to be found in the cord, and in this case their dendrons, 

 which terminate by branching round a modified epidermal cell, 

 would form the afferent fibre, but such cells are apparently not very 

 numerous. Through these various sensory cells the earthworm is 

 put in communication with the outer world, and so can appreciate 



